MD Camille Le Ray
email: camille.le-ray@cch.aphp.fr

William Fraser

Patrick Rozenberg

Bruno Langer

Damien Subtil

François Goffinet

Duration of passive and active phases of the second stage of labour and risk of severe postpartum haemorrhage in

Keywords: passive second stage, active second stage, nullipara, postpartum haemorrhage

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Many authors report that a prolonged second stage is associated with maternal complications, especially postpartum haemorrhage (PPH) related to uterine atony [START_REF] Cheng | How long is too long: Does a prolonged second stage of labor in nulliparous women affect maternal and neonatal outcomes?[END_REF][START_REF] Janni | The prognostic impact of a prolonged second stage of labor on maternal and fetal outcome[END_REF][START_REF] Menticoglou | Perinatal outcome in relation to second-stage duration[END_REF][START_REF] Moon | Perinatal outcome after a prolonged second stage of labor[END_REF][START_REF] Myles | Maternal and neonatal outcomes in patients with a prolonged second stage of labor[END_REF][START_REF] Saunders | Neonatal and maternal morbidity in relation to the length of the second stage of labour[END_REF][START_REF] Rouse | Secondstage labor duration in nulliparous women: relationship to maternal and perinatal outcomes[END_REF]. They studied the second stage of labour, beginning at full dilatation and finishing at birth, as an only one continuous phase. However, the second stage is divided into two phases: the passive second stage, during which the fetal head descends passively down the maternal pelvis, and the active second stage, which corresponds to the phase of pushing expulsive efforts. The physiology of the two phases differs, as demonstrated by Buhimschi et al., who found that women in labour increase their intrauterine pressure 62% by actively pushing with a contraction during the second stage [START_REF] Buhimschi | Pushing in labor: performance and not endurance[END_REF]. Studying the two phases separately might make it possible to determine which-passive or active or bothincreases maternal complications, especially the risk of severe PPH. This finding could have important clinical implications, favouring the practice of either early or delayed pushing, or limiting the duration of expulsive efforts. In a recent publication from a secondary analysis of the PEOPLE trial, we found an increased maternal risk of PPH after 2 hours of active second-stage labour [START_REF] Ray | When to stop pushing: effects of duration of second-stage expulsion efforts on maternal and neonatal outcomes in nulliparous women with epidural analgesia[END_REF]. That study did not, however, take in account the relation between PPH and duration of the passive second stage.

The objective of our study was to analyse maternal consequences, especially severe PPH, according to durations of the passive and active phases of the second stage of labour, in lowrisk nulliparous women.

Methods

The PREMODA study was a prospective observational study of the delivery of term breech infants, conducted, between 1 June 2001 and 31 May 2002, in 138 French maternity units (more than one fifth of all French maternity units) [START_REF] Goffinet | Is planned vaginal delivery for breech presentation at term still an option? Results of an observational prospective survey in France and Belgium[END_REF]. The study was approved by the National Data Protection Authority in Paris (on 9 May 2001). The control group included 9962 singleton pregnancies in cephalic presentation at more than 37 weeks, specifically, those in the same units over the same period who were assigned a delivery record number that was a multiple of 20. This procedure provided a random selection of 5% of all the singleton cephalic term births. This study was based on prospective data rigorously collected from a questionnaire specifically designed to assess management of labour and delivery, neonatal health status and maternal complications.

For our study, we conducted a secondary analysis of a population of low-risk nulliparas from the PREMODA study control group. This population of low-risk nulliparas was defined by the following criteria: nulliparous women with a singleton fetus in cephalic presentation at term (gestational age 37 weeks) (n=4218), as used in other secondary analysis of PREMODA study [START_REF] Ray | Level of perinatal care of the maternity unit and rate of cesarean in low-risk nulliparas[END_REF][START_REF] Ray | The influence of level of care on admission to neonatal care for babies of low-risk nullipara[END_REF]. Women with caesarean delivery before (n=101) and during labour (n=460), induction of labour for maternal (n=159) or fetal (n=116) disorders, and congenital malformations (n=60) were excluded.

The duration of each stage of labour was prospectively collected, together with maternal (age and place of birth) and neonatal (gestational age and birth weight) characteristics and obstetric practices (elective induction of labour, oxytocin during labour and operative vaginal delivery).

The main outcome criterion was severe PPH, defined by an estimated blood loss exceeding 1000 ml or blood transfusion in the first 24 hours after delivery. Because measurement of moderate PPH is so subjective, we did not record this outcome. Other maternal outcomes studied were third-and fourth-degree perineal tears and postpartum maternal fever > 48 hours or endometritis before discharge. Neonatal outcomes studied were: neonatal asphyxia defined by pH ≤ 7.10 or 5-minute Apgar score ≤7 and neonatal morbidity defined by any adverse neonatal outcome (arterial pH at birth≤ 7.10 or 5-minute Apgar score≤7 or neonatal trauma or NICU admission).

We analysed risk of severe PPH according to duration of each stage or phase of labour, beginning with the active phase of the first stage (excluding the early labour phase), and including the passive and active second stages. About duration of the active second stage, the French College of Obstetricians and Gynaecologists (CNGOF) recommends considering an assisted delivery after 30 minutes of expulsive efforts [START_REF]Modalités de surveillance foetale pendant le travail. Texte des recommandations[END_REF]. Although these guidelines date only from 2007 and the level of evidence for this recommendation is low (grade C) [START_REF] Ray | Duration of pushing in labor: literature review[END_REF], this has been standard practice in France for a long time [START_REF] Naime-Alix | How long can we wait at full dilatation. A study of maternal and neonatal morbidity related to the duration of the second stage of labour in nulliparous women[END_REF]. There is no specific recommendation about optimal duration of the passive second stage.

The duration of the active first stage, i.e., from 5 to 10 cm dilatation, was categorised in twohour intervals. The duration of the passive second stage, i.e., duration of labour between 10 cm (full dilatation) and the beginning of expulsive efforts, was divided into three classes (less than 1 h, from 1 to 2 h and more than 2 h). The duration of the active second stage, i.e., of pushing, was categorised into six classes: 0-9 minutes (min), 10-19 min, 20-29 min, 30-39 min, 40-49 min and ≥50 min. We determined the factors associated with severe PPH, the duration of each stage of labour, neonatal and obstetric factors (gestational age, birth weight, mode of onset of labour and mode of delivery), and their associations with one another.

proportions, and Fisher's exact test when the population size was small (n<5). We used the Mann-Whitney test to compare median durations for each stage of labour. When a variable was missing more than 5% of data we created a specific class for missing data.

For the univariable analysis, we calculated the crude odds ratios and 95% confidence intervals of neonatal and obstetric factors that might be associated with severe PPH, including the duration of each stage of labour. To adjust for confounding factors in the multivariable analysis, we used an unconditional logistic regression model that included factors significant in the univariable analysis with p<0.2. Classes with the lowest severe PPH rate were chosen as the references.

Results

Nulliparas accounted for 42.3% (n=4218/9962) of the PREMODA study control group. Among them, 78.9% (n=3330/4218) were at low risk according to the inclusion and exclusion criteria. Table 1 describes the characteristics, obstetric practices and maternal and neonatal outcomes for this study population. In all, 69 (2.1%) of them had severe PPH, but only two were admitted to a maternal intensive care unit.

Data about duration of expulsive efforts were available in 3258 (97.8%) cases. Consistent with standard French practices and guidelines, the median duration of pushing was 15 minutes (interquartile interval: 10-25 minutes). However, 602 (18.5%) women had an active second stage longer than 30 minutes. Table 2 compares the duration of each stage of labour for the groups with and without severe PPH. The median duration of the active first stage and the passive second stage were similar in the severe PPH and control groups, respectively, 2.5 hours versus 2.7 hours (p=0.279) and 30 minutes for both groups (p=0.391). The median duration of the active second stage, however, was significantly longer in the severe PPH group compared with the control group (20 minutes versus 15 minutes, p<0.001).

The rate of assisted vaginal delivery was 24.0% (n=801) and increased with duration of expulsive efforts (<10 min: 12.4%, 10-19 min: 18.1%; 20-29 min: 26.2%, 30-39 min: 39.4%, 40-49 min: 52.2%, ≥50 min: 50%, p<0.001). The relationship between assisted vaginal delivery and severe PPH was not significant (1.9% of the spontaneous vaginal deliveries had severe PPH versus 2.8% for assisted deliveries, p=0.124). Table 3 summarises the results for the association between severe PPH and duration of the active second stage according to type of vaginal delivery. The association between severe PPH and the duration of the active second stage then remained significant only among women with assisted vaginal deliveries (p=0.006); it was no longer significant for spontaneous deliveries (p=0.093) (Table 3).

In the multivariable analysis, the duration of the active first stage was associated with the risk of severe PPH (active first stage > 6 hours: adjusted OR=2.5 [1.0-6.1], reference: interval 2-4 h) (Table 4). We found no association between the duration of the passive second stage and the risk of PPH (for passive second stage longer than 2 hours: crude OR=1.2, 95% CI [0.5- 4). When the duration of the active second stage was treated dichotomously in two classes (up to and after 30 minutes), the risk of severe PPH was significantly higher after 30 minutes (crude OR=2. The risks of third or fourth degree perineal tears, of postpartum maternal fever >48 hours and of endometritis were not associated with the duration of active pushing in univariate analysis.

We found no association between the duration of the active second stage and the risk of neonatal asphyxia or neonatal morbidity (data not shown).

Discussion

Our results show an association between the duration of the active phase of the second stage of labour and the risk of severe PPH in nulliparous women at low risk. On the contrary, however, severe PPH was not associated with the duration of the passive phase of the second stage.

Most authors who have studied the consequences of a prolonged second stage of labour have not differentiated between the passive and active phases of the second stage (1-7). Only a recent secondary analysis of the Canadian PEOPLE trial specifically assessed the impact of the duration of the active second stage on PPH risk: it reported that the risk of PPH increased among nulliparous women after 2 hours of pushing [START_REF] Ray | When to stop pushing: effects of duration of second-stage expulsion efforts on maternal and neonatal outcomes in nulliparous women with epidural analgesia[END_REF]. That analysis, however, unlike this one, did not assess the impact of the duration of the passive second stage on PPH risk.

Because the two phases of the second stage differ physiologically, prolongation of the duration of each could have different consequences. Thus, the passive and active phases should be analysed separately for assessing management of the second stage. Confirmation of our finding that PPH is associated with the duration of the active but not the passive second stage might allow obstetricians to adapt their management of low-risk nulliparous women, maybe to encourage delayed pushing and limit expulsive efforts. Some randomised trials have assessed management of the passive second stage by measuring the impact of delayed pushing on the risk of PPH and found no difference between -early pushing‖ and -delayed pushing‖ groups, i.e., between women with shorter and longer periods of passive descent [START_REF] Fraser | Multicenter, randomized, controlled trial of delayed pushing for nulliparous women in the second stage of labor with continuous epidural analgesia. The PEOPLE (Pushing Early or Pushing Late with Epidural) Study Group[END_REF][START_REF] Plunkett | Management of the second stage of labor in nulliparas with continuous epidural analgesia[END_REF][START_REF] Vause | Immediate and delayed pushing in the second stage of labour for nulliparous women with epidural analgesia: a randomised controlled trial[END_REF][START_REF] Roberts | Delayed versus early pushing in women with epidural analgesia: a systematic review and meta-analysis[END_REF]. In accordance with these findings, we confirmed that the duration of the passive phase of the second stage did not appear to be associated with severe PPH.

Because French management of the active second stage is particularly short in comparison with other countries, our results can not be extrapolated. However, our results about management of the active phase of the second stage of labour can be compared with those from the PEOPLE trial [START_REF] Ray | When to stop pushing: effects of duration of second-stage expulsion efforts on maternal and neonatal outcomes in nulliparous women with epidural analgesia[END_REF]. Despite these differences between French and Canadian obstetrical practices, we also found an association between the duration of the active second stage and PPH risk under French practices. This increases the external validity of the relation observed between the duration of the active second stage and the risk of PPH.

Because the PREMODA study was not initially designed to assess PPH [START_REF] Goffinet | Is planned vaginal delivery for breech presentation at term still an option? Results of an observational prospective survey in France and Belgium[END_REF], data were not collected about management of the third stage of labour, from delivery until expulsion of the placenta. The systematic use of uterotonic agents and the use of controlled cord traction may be confounding factors for the risk of severe PPH. Similarly, blood loss was not objectively measured in each delivery. Because moderate PPH, i.e., blood loss between 500 and 1000 ml, is so subjective, this outcome was not recorded in the PREMODA study. In view of the large sample of women included and the high number of participating maternity units, the PREMODA study scientific committee considered that severe PPH was an adequate criterion for assessing maternal morbidity. An objective measurement of blood loss with the use of blood collecting bags would certainly have been more accurate.

Moreover, analgesia used during labour was unfortunately not collected in the PREMODA study. Thus, we cannot assess relationship between epidural and severe PPH in our study. We agree that epidural analgesia could be also a confounding factor limiting interpretation of our results. Indeed this practice increases duration of labour and its association with PPH remains unclear in the literature. In France, the rate of delivery with loco-regional analgesia was very high during the study period, around 85% in nulliparous women.

Caesarean section during the second stage of labour is associated with an increased risk of PPH [START_REF] Allen | Maternal and perinatal morbidity of caesarean delivery at full cervical dilatation compared with caesarean delivery in the first stage of labour[END_REF][START_REF] Alexander | Comparison of maternal and infant outcomes from primary cesarean delivery during the second compared with first stage of labor[END_REF]. The caesarean rate during the active phase of the second stage varies according to the management of this phase. The shorter the phase, the lower the risk of caesarean delivery. Delayed pushing is routine practice in France. As recommended by CNGOF to prevent expulsive efforts from exceeding 30 minutes, women do not usually begin to push until the fetal station reaches +2 or more, when fetal heart monitoring is normal. Thus, very few women had caesarean deliveries after beginning to push. Among all the PREMODA lowrisk nulliparous women, only 58 (1.5%) women had a caesarean during the second stage and only 19 (0.5%) during the active phase of that stage. Only one of them had a severe PPH; she had pushed 25 minutes. The difficulty of analysing the impact of the duration of each phase of the second stage on the risk of severe PPH among women with caesarean sections during the passive or active phase led us to exclude women with caesareans during the second stage.

Moreover, in view of the very few patients who had severe PPH after a caesarean during the active second stage, we do not think that this choice modifies our results.

In accordance with national guidelines, most French obstetricians performed operative vaginal deliveries after 30 minutes of pushing, even when the fetal heart rate (FHR) monitoring was reassuring. This probably explains at least in part the higher rate of such deliveries in France (11.1% in 2003) compared with the US (6.6% in 2006) [START_REF] Blondel | Trends in perinatal health in France between 1995 and 2003: results from the National Perinatal Surveys[END_REF][START_REF] Menacker | BirthStats: rates of cesarean delivery, and unassisted and assisted vaginal delivery, United States[END_REF]. Previous studies have reported an association between PPH and operative vaginal delivery [START_REF] Combs | Factors associated with postpartum hemorrhage with vaginal birth[END_REF]. We did not observe such an association, perhaps due to a lack of power. Cervical and vaginal lacerations due to instrumental extractions are a potential cause of PPH. Because the rate of operative vaginal delivery increased with the duration of expulsive efforts and this type of delivery may be a confounding factor, we secondly decided to stratify for it. After stratification, the risk of severe PPH among women with operative vaginal deliveries still increased with the duration of the active second stage. The increase in operative vaginal deliveries with duration of pushing therefore does not explain the association between severe PPH and the duration of expulsive efforts. Nonetheless, our results do not allow us to conclude that severe PPH is reduced by the current French recommendations to limit pushing to 30 minutes. A policy of operative vaginal delivery after 30 minutes of pushing may indeed decrease severe PPH related to uterine atony.

However, this policy may also increase the risk of PPH related to cervical and vaginal lacerations, secondary to instrumental extractions.

Conclusion

Prolongation of the active but not the passive phase of the second stage of labour could be associated with an increased risk of severe PPH. These results may have important implications for second-stage management and may lead obstetricians to encourage delay in pushing and to limit the duration of expulsive efforts. The optimal duration of these phases remains to be defined, however. -3000-3499 g (n=1567)

-3500-3999 g (n=834)

-≥ 4000 g (n=159)

2. 9 ]

 9 , reference: passive second stage less than 1 hour). The risk of severe PPH increased significantly with the duration of expulsive efforts. Odds ratios of severe PPH reached a significant level after 40 minutes of pushing (interval 40-49 min: adjusted OR=3.5 [1.0-12.3], ≥50 min: adjusted OR=10.6 [2.8-40.3], reference: <10 min) (Table

  3 [1.4-3.8] and adjusted OR=2.0 [1.1-3.5]).

-

  Interval 40-49 min (n=134) -More than 50 min (n=42)

Table 1 :

 1 Characteristics, obstetric practices, and maternal and neonatal outcomes for the

	study population
	Characteristics	Total population (N=3330)
	Maternal age in years (mean+/-SD)	27.3 +/-4.7
	-< 25 years (n, %)	994 (30.1)
	-25-35 years	2107 (63.9)
	-35 years	197 (6.0)
	Maternal birth place (n, %)
	-France	2577 (82.9)
	-Abroad	531 (17.1)
	Gestational age in weeks (mean+/-SD)	39.5 +/-1.2
	-37-38weeks (n, %)	660 (19.8)
	-39-40 weeks	1996 (59.0)
	-≥41weeks	702 (21.1)
	Onset of labour (n, %)
	-spontaneous	2815 (84.5)
	-induction	515 (15.5)
	Oxytocin during labour (n, %)
	-yes	2074 (62.3)
	-no	635 (19.1)
	-missing	621 (18.6)
	Mode of delivery (n, %)
	-spontaneous vaginal delivery	2529 (76.0)
	-assisted vaginal delivery	801 (24.0)
	Severe postpartum haemorrhage (n, %)	69 (2.1)
	Postpartum maternal fever>48 h or endometritis (n, %)	22 (0.7)
	3rd-4th degree perineal tear (n, %)	52 (1.6)
	Birth weight in grams (mean+/-SD)	3289 +/-406
	-< 3000 g (n, %)	761 (22.9)
	-3000-3499 g	1567 (47.2)
	-3500-3999 g	834 (25.1)
	-≥ 4000 g	159 (4.8)

Table 2 :

 2 Association between the duration of different phases and stages of labour and severe

	PPH
	Duration	Severe PPH	Control	p
		N=69	N=3261
	Between 5 and 10 cm, in hours (median	2.5 [1.5-3.9]	2.7 [1.4-4.7]	0.279 †
	[IQ])
	-Less than 2 h (n,%)	19 (31.7)	876 (29.6)
	-Interval 2-4 h	19 (31.7)	1345 (45.5)
	-Interval 4-6 h	15 (25.0)	563 (19.0)
	-More than 6 h	7 (11.7)	175 (5.9)	0.076 ‡
	Passive second stage, in minutes (median	30 [10-60]	30 [10-60]	0.391 †
	[IQ])
	-Less than 60 min(n,%)	44 (66.7)	2234 (71.3)
	-Interval 60-120 min	16 (24.2)	647 (20.7)
	-More than 120 min	6 (9.1)	252 (8.0)	0.710 ‡
	Active second stage, in minutes (median	20 [15-31]	15 [10-25]	<0.001 †
	[IQ])
	-Less than 10 min(n,%)	6 (8.8)	477 (15.0)
	-Interval 10-19 min	23 (33.8)	1378 (43.2)
	-Interval 20-29 min	16 (23.5)	756 (23.7)
	-Interval 30-39 min	11 (16.2)	415 (13.0)
	-Interval 40-49 min	6 (8.8)	128 (4.0)
	-More than 50 min	6 (8.8)	36 (1.1)	<0.001 ‡
	IQ: interquartile
	† Mann-Whitney test
	‡ Chi2 test

Table 3 :

 3 Rates of severe PPH according to mode of delivery and duration of the active second

	stage
		Spontaneous Vaginal Delivery	Operative Vaginal Delivery
		Total (n)	PPH (n, %)	Total (n)	PPH (n, %)
	Active second stage
	-Less than 10 min	423	6 (1.4)	60	0
	-Interval 10-19 min	1147	17 (1.5)	254	6 (2.4)
	-Interval 20-29 min	570	12 (2.1)	202	4 (2.0)
	-Interval 30-39 min	258	7 (2.7)	168	4 (2.4)
	-Interval 40-49 min	64	2 (3.1)	70	4 (5.7)
	-More than 50 min	21	2 (9.5)	21	4 (19.1)
	p		0.093		0.006

Table 4 :

 4 Factors associated with severe PPHunivariable and multivariable analysis

	Severe PPH	Crude OR	Adjusted OR
	N=69 n (%))	[95%CI]	[95%CI]

Acknowledgments : None Disclosure of interest There are no conflicts of interest Contribution to authorship CLR conducted the analyses. FG coordinated and supervised the study. CLR, WF and FG wrote the paper. All authors have seen and approved the final version of the manuscript. Details of ethics approval : Procedures of the study received ethics approval from the National Data Protection Authority in Paris (on 9 May 2001). Scientific committee F. Goffinet (INSERM U953-exU149, Port Royal Maternity, Paris, France), S. Alexander (School of Public Health School, Brussels, Belgium), J.M. Foidart (La Citadelle Hospital, Liège, Belgium), S. Uzan (Tenon Hospital, Paris, France), D. Subtil (Jeanne de Flandre Hospital, Lille, France), G. Bréart (INSERM U953-exU149, Paris France) List of 138 maternity units participating in PREMODA Study Région Alsace: Coordinator: Pr Langer: CHU de Strasbourg (Pr Langer), CMCO de Schiltigheim (Dr Vayssiere), CHR de Haguenau (Dr Lehmann),CMC de Colmar (Dr Kutnahorsky), CHR de Mulhouse (Dr Wiedemann), Clinique Sainte Anne, Strasbourg (Dr Jeanmougin), Clinique Diaconat, Mulhouse (Dr Blum) -Basse-Normandie: Coordinator: Pr Dreyfus: CHU de Caen (Pr Dreyfus/Dr Denoual-Ziad), CH de Cherbourg (Dr Ulmann), CH de Lisieux (Dr Zerger), CH de Saint Lo (Dr Refahi), CH de Flers (Dr André), CH de la Ferté Macé (Dr Nelle) -Région Bretagne: Coordinator: Pr Grall: CHU de Brest (Pr Collet), CHU Hôtel Dieu, Rennes (Pr Poulain), CHU Hôpital Sud, Rennes (Mme Pérrigot), CH de Lorient (Dr Getin), Clinique mutualiste de la Sagesse, Rennes (Dr Aussel), CH de Saint Brieuc (Dr Giono-Renaud), CH de Saint Malo (Dr Weyl), CH de Vannes (Mme Pierson), CH de Cornouaille (Dr Germain) -Région Centre: Coordinator: Pr Perrotin: CHU Bretonneau, Tours (Dr Alonso), CHU Beffroi, Tours (Dr Rapp), CH de Blois (Dr Montmasson), CHR d'Orléans (Dr Ceccaldi), CHG de Chartres (Dr Guilbaud) -Région Franche-Comté: Coordinator: Pr Schaal: CHU de Besançon (Dr Riethmuller), CHG de Montbéliard (Dr Zurlinden), CHG de Belfort (Dr Terzibachian) -Région Haute-Normandie: Coordinator: Pr Verspyck: CHU de Rouen (Pr Verspyck), CH de Mont Saint Aignan (Dr Fournet), CH du Havre (Dr Degré), CH d'Elbeuf (Dr Paquet), CH de Dieppe (Dr Gandour), Clinique Saint-Romain, Rouen (Dr Thobois) -Région Ile de France: Réseau Sud Ouest Francilien: Coordinator: Dr Audibert: CHU de Clamart (Dr Audibert), Clinique des Vallées, Châtenay-Malabry (Dr Proust), Clinique de Meudon la Forêt (Dr Chene), CH de Dourdan (Dr Lambert), CH Sud Francilien site d'Evry (Mme Lose), CH de Fontainebleau (Dr Fillippini), CH d'Orsay (Dr Devianne); Réseau Ile de France Port-Royal: Coordinator: Dr Harvey: CHU Cochin, Paris (Dr Kayem), CHU Saint Vincent de Paul, Paris (Pr Lepercq), CHU Saint Antoine, Paris (Pr Carbonne), CH Notre Dame de Bonsecours, Paris (Dr Grovangrandi), CHU Beaujon, Paris (Mme Grapin), CHU de Colombes (Dr Crenn-Hebert), CH de Neuilly sur Seine (Dr Galimard), CH de Saint Cloud (Mme Pecourt), Hôpital Militaire Begin, Saint Mandé (Dr Ponties), Clinique Armand Brillard, Nogent sur Marne (Dr Helvin), CH Les Diaconesses, Paris (Dr Harvey); Réseau de Poissy: Coordinator: Dr Rozenberg: CHI de Poissy-Saint Germain (Mme Bertaud); Réseau inter-maternités de Saint-Denis: Coordinator: Pr Uzan: CH de Bondy (Dr Seince), CHI de Montreuil (Dr Chitrit), CHI de Villepinte (Dr Debièvre), Clinique Vauban, Livry-Gardan (Dr Kamoun), Clinique du bois d'amour, Drancy (Dr Masson), CH de Montfermeil (Dr Ropert), CHG de Saint-Denis (Dr Ekoukou), Clinique de l'Estrée, Stains (Dr Franche); Réseau Tenon: Coordinators: Pr Uzan and Dr Berkane: CHU Tenon, Paris (Dr Berkane), CHU Bichat, Paris (Pr Mandelbrot); CHI de Créteil (Pr Haddad et Dr Touboul); CH de Saint Maurice (Dr Bardou) -Région Limousin: Coordinator: Pr Philippe: CH de Brive (Mme Peron), CH de Tulle (Mme Barbé), CHU de Limoges (Dr Eyraud), CH d'Ussel (Mme Leclerc) -Région Lorraine: Coordinator: Pr Boutroy, Dr Thiebaugeorges: CHU de Nancy (Dr Thiebaugeorges), CH d'Epinal (Dr Scotton), CHR Bonsecours, Metz (Dr Lemarié), CH de Thionville (Dr Szwarcberg), CH Sainte Croix, Metz (Dr Ragage), Polyclinique Majorelle, Nancy (Dr Bey), Clinique Arc en Ciel, Epinal (Dr Gaillet-Schiochet), Clinique Claude Bernard, Metz (Dr Adami) -Nord Pas de Calais: Coordinator: Pr Subtil: CH d'Arras (Mme Finet), CH de Béthune (Dr Hay), CH de Boulogne (Dr Churlet), Clinique Côte d'Opale, Saint Martin les Boulognes (Dr Renault), CH de Douai (Dr Dognin), Clinique Saint Amé, Lambre-lez-Douai (Dr Doutrelant), Clinique Villette, Dunkerque (Mme Gosselin, Mme Deroose), CH de Maubeuge (Dr Hubert), CH de Roubaix (Dr Le Goueff), CH de Seclin (Dr Biausque), CH de Valenciennes (Dr Massoni), CHU de Lille (Pr Subtil), Clinique Cotteel, Villeneuve d'Ascq (Mme Dumon) -Région Pays de Loire: Coordinator: Dr Winer: CHD La Roche sur Yon (Dr Barreteau), CH de Saint Nazaire (Dr Gerard), Clinique du jardin des plantes, Saint Nazaire (Dr Rousseau), CH de Cholet (Dr Aireau), CHU de Nantes (Dr Winer), Maison de naissance, St Sébastien/Loire (Dr Berlivet),

Funding This study was mainly supported by two grants from the Ministry of Health (AOM01123 (PH-RC 2001) and AOM03040 (PH-RC 2003)). It was also partly funded by the French College of Gynecologists and Obstetricians, the French Society of Perinatal Medicine and the Belgian National Funds for Scientific Research. The funding sources had no role in the study design, data collection, data interpretation or the writing of the report.


Lila Mouffok

Lana Damaj

Xavier Begaud

Anne Claire Lepage

Hubert Diez

Mutual coupling reduction between dual polarized microstrip patch antennas using compact spiral artificial magnetic conductor

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Mutual coupling reduction between dual polarized microstrip patch antennas using compact spiral artificial magnetic conductor L. Mouffok 1 , L. Damaj 1 , X. Begaud 1 , A.C Lepage 1 , H. Diez 2 I. INTRODUCTION Microstrip antenna arrays are commonly used for their low profile and light weight. A key requirement for the arrays design is to reduce the coupling between each element. This mutual coupling is due to the excitation of space and surface waves. These surface waves can create very critical problem known as scan blindness [START_REF] Pozar | Scan blindness in infinite phased arrays of printed dipoles[END_REF].

In the recent years, new solutions to suppress surface waves have been proposed using an Artificial Magnetic Conductor (AMC) or Electromagnetic Band Gap (EBG) materials. These AMC, also known as High Impedance Surfaces (HIS), consist of periodic metallic or dielectric arrangements [START_REF] Sievenpiper | High-Impedance Electromagnetic Surfaces[END_REF]. These structures can be used to reduce the mutual coupling between the radiating elements [START_REF] Yang | Mutual coupling reduction of microstrip antennas using electromagnetic band-gap structure[END_REF].

Objective of this paper is to demonstrate that thanks to these artificial structures, two dual polarized microstrip antennas can be placed very close to each other with a low mutual coupling and also low insertion loss between two polarization ports.

II. ANTENNA DESIGN

The final objective of this work is relative to spatial applications. So in order to realize a compact structure operating at 1.62 GHz, a dual polarized square microstrip patch antenna is printed on a Rogers TMM 100 substrate with a thickness of 0.762 mm and a dielectric permittivity ε r of 9.82. This antenna has to be compact (maximum authorized surface of 115x115 mm 2 ) and must retain a directivity no lower than 6 dBi to be compliant with our specifications.

The mutual coupling between two microstrip antennas printed on a high permittivity substrate is high [START_REF] Yang | Mutual coupling reduction of microstrip antennas using electromagnetic band-gap structure[END_REF], so in order to improve isolation between antennas, we replace the ground plane of the microstrip antenna by the following designed AMC.

III. AMC DESIGN

To design AMC structures, it is essential to define their operational frequency band, into which the phase of the reflection coefficient varies from -90° to 90° [START_REF] Sievenpiper | High-Impedance Electromagnetic Surfaces[END_REF], in order to have constructive interference. To determine the reflection coefficient phase, a unit cell of the AMC structure is simulated with CST microwave Studio ®.

The AMC structure studied in this paper is a periodic arrangement structure of a four arms spiral shaped unit cells [START_REF] Yang | A spiral electromagnetic bandgap (EBG) structure and its application in microstrip antenna arrays[END_REF] printed on dielectric substrate with ground plane. It allows designing a compact AMC, because the use of four long arms provides a high inductance value and the gap between them a high capacitance value. The AMC structure has been realized on a Rogers TMM 100 substrate (ε r =9.82) with a total size of 115x115 mm 2 (the maximal size allowed by specifications) and a thickness of IV. PATCH ANTENNA PERFORMANCE IMPROVEMENT The dimensions of the patch antenna are optimized with an added air layer to improve the isolation between the two ports and keep directive gain higher than 6 dB (Fig. 3). The square patch width is 52 mm. It is fed by two orthogonal probes located at 9 mm from its center. The feed location is optimized to provide a good impedance matching. The AMC is composed by 18 x 18 unit cells and the air layer thickness is 0.762 mm. Because both ports are symmetrical, we only represent one radiation pattern in E and H planes.

Fig. 5 and Fig. 6 compare the simulated and measured co-polar and cross-polar radiation patterns in the E and H planes respectively. For both planes, it is found that the simulated and the measured co-polar radiation patterns are in good agreement. The broadside realized gain is about 7dB at 1.62 GHz and the HPBW is 90°. The measured cross-polar level is about 15 dB lower than the co-polar level. The simulation cross-polar level is very low and is not represented on this graph. This measured cross-polar level is probably due to some technical problems during the fabrication process of the structure. This antenna shows a rotational symmetry of the radiation pattern which is an interesting feature of AMC based antennas. The use of high dielectric substrate increases the coupling levels, due to the surface waves which propagate into the substrate from one antenna to another. In this part, we want to study the coupling between two antennas over AMC. The dimensions of the AMC are the same than previously, but the thickness of air layer is reduced to 0.3 mm, in order to decrease the width of the square patch from 52 to 43 mm [START_REF] Lee | Circular-disk microstrip antenna with an air gap[END_REF]. It is important to note that this size remains suitable to obtain directive gain close to 6 dB. The distance between the centers of the two elements is 69 mm (≃λ0/3 where λ0 is the wavelength in free space at 1.62 GHz). These two antennas have been simulated, realized, and then measured. The ports numbers are indicated in Fig. 7.

Return loss, insertion loss and mutual coupling simulations and measurements are given in Tables I and II respectively. There is a good agreement between the simulations and the measurements. The feeding ports have a return loss below -10 dB and a mutual coupling between the different ports less than -15dB.

To show the effect of AMC, we compare the performances of the structure with the AMC and without it at 1.62 GHz. Table III represents the return loss, insertion loss and mutual coupling of the two antennas without AMC. The size of the dielectric substrate is the same, only the size and location of feeding ports of antennas have been changed to operate at 1.62 GHz. AMC has been replaced by a perfect metallic ground plane.

The proposed structure with AMC has about 3 dB enhancement of decoupling in H-plane configuration (ports 2 and 4), but there is no effect at the E-plane configuration (ports 1 and 3). This is probably due to the asymmetry of the spiral unit cell, which can reduce the surface currents only in H-plane configuration. The figure 8 shows that the currents are localized around antenna and more important on passive antenna in E-plane configuration. As we can see, surface currents are present and it seems to be clear that the antenna has modified the behavior of AMC. It's necessary to remind that AMC has been characterized without antenna. To improve the efficiency of AMC, further work must be performed to take into account the antenna while characterizing the AMC.

The coupling reduction could be improved. Indeed, the two antennas are separated by only 2 unit cells in the proposed structure.

By decreasing the size of the unit cell, it will be possible to insert more unit cells between antennas, and thus to decrease the coupling.

VI. CONCLUSION

In this paper, a four arms spiral shaped AMC structure with an air layer is used to reduce the coupling between two dual polarized microstrip antennas. The parameters of the structure (spacing, width, number of the unit cells and the thickness of the air layer) were studied. First, a patch antenna over AMC has been designed. Then, two antennas over AMC and close to each other have been analyzed. It demonstrates good performances and a 3 dB mutual coupling reduction in H-plane configuration.

Fig. 1

 1 Fig. 1 AMC unit cell

  mm. Dimensions of the unit cell are lspir = 0.8 mm and g=0.3mm as shown in fig.1. At the operating frequency (f=1.62 GHz), the AMC is characterized by a null reflection phase as shown in the phase diagram (Fig.2).

Fig. 2

 2 Fig. 2 Phase of the reflection coefficient of the AMC

Fig. 3

 3 Fig. 3 Side view (a) and front view (b) of the realized structure

Fig. 4

 4 Fig. 4 Simulated and measured results (return loss and insertion loss) of the dual polarized antenna

Fig. 5 Fig. 6

 56 Fig.5 Simulated and measured radiation pattern in the E plane

Fig. 7

 7 Fig. 7 The two realized antennas

Fig. 8

 8 Fig. 8 Surface currents at 1.62 GHz for the four ports, one port is fed, the others loaded with 50 Ohms.

this paper, we present a dual polarized microstrip antenna above an Artificial Magnetic Conductor (AMC). It is printed on a high permittivity substrate (ε r =9.82). The AMC is used in order to reduce the mutual coupling between two radiating elements. Two experimental prototypes are realized in order to validate the analysis. First, the performances of a patch antenna above AMC are analyzed. The return loss is less than -15 dB at 1.62 GHz. The measured radiation pattern shows that the antenna has a broadside gain of 7 dB and a cross-polar level about -15 dB. Then, the mutual coupling between two antennas above AMC is studied. 3 dB enhancement of mutual coupling compared to the array without AMC is observed at the operating frequency 1.62 GHz in H plane configuration. A good agreement between measurements and simulations results is observed.

	1 Institut Telecom -Telecom ParisTech -LTCI -CNRS -UMR 5141 -46 Rue Barrault, 75634 Paris France
	2 Centre National d'Etudes Spatiales, 18, Avenue Edouard Belin, 31401 Toulouse Cedex 9 France
	Abstract-In

TABLE I RETURN

 I LOSS, INSERTION LOSS AND MUTUAL COUPLING SIMULATIONS FOR THE ANTENNA ARRAY WITH AMC AT 1.62GHZ

	|Sij (dB)|	i=1	i=2	i=3	i=4
	j=1	-10.1	-17.3	-14.0	-43.2
	j=2	-17.3	-18.1	-35.6	-17.2
	j=3	-14.0	-35.6	-7.7	-25.0
	j=4	-43.2	-17.2	-25.0	-18.6
			TABLE II
	RETURN LOSS, INSERTION LOSS AND MUTUAL COUPLING MEASUREMENTS FOR
		THE ANTENNA ARRAY WITH AMC AT 1.64 GHZ
	|Sij (dB)|	i=1	i=2	i=3	i=4
	j=1	-15.4	-15	-16.8	-34.7
	j=2	-14.6	-10.1	-30	-22.5
	j=3	-16.8	-30.0	-16.8	-20
	j=4	-34.7	-22.5	-20	-10
			TABLE III
	RETURN LOSS, INSERTION LOSS AND MUTUAL COUPLING SIMULATIONS FOR
		THE ANTENNA ARRAY WITHOUT AMC AT 1.62 GHZ
	|Sij (dB)|	i=1	i=2	i=3	i=4
	j=1	-11.7	-40.0	-14.0	-46.0
	j=2	-40.0	-18.3	-44.0	-14.2
	j=3	-14.0	-44.0	-11.0	-43.0
	j=4	-46.0	-14.2	-43.0	-18.3

ACKNOWLEDGMENT

The authors would like to thank the French Space Agency (Centre National d'Etudes Spatiales, CNES) for proposing and sponsoring this work.