title: Impact of fetal gender on the risk of preterm birth, a national cohort study.
abstract: BACKGROUND
  Fetal gender is associated with preterm birth; however, a proper subdivision by onset of labor and corresponding neonatal outcome by week of gestation is lacking.
  
  
  METHODS
  Data from the Netherlands Perinatal Registry (1999-2010) were used to calculate relative risk ratios for gender by week of gestation and gender-related risk on adverse neonatal outcomes using a moving average technique. White European women with an alive fetus at onset of labor were included. Adverse neonatal outcomes were defined as neonatal mortality and a composite of neonatal morbidity. Onset of labor was categorized as spontaneous onset with intact membranes, premature rupture of membranes, and induction or elective cesarean section.
  
  
  RESULTS
  The study population comprised 1 736 615 singleton deliveries (25(+0) -42(+6) weeks). Male fetuses were at increased risk of spontaneous preterm birth with intact membranes compared with a female fetus with a peak between 27 and 31 weeks [relative risk (RR) 1.5; 95% CI 1.4-1.6]. Male fetuses were also at increased risk of preterm premature rupture of membranes between 27 and 37 weeks (RR 1.2; 95% CI 1.16-1.23). No gender effect was seen for medically indicated preterm birth. No significant differences were seen for neonatal mortality. Males were at significantly increased risk of composite neonatal morbidity from 29 weeks onwards (RR 1.3; 95% CI 1.3-1.4).
  
  
  CONCLUSIONS
  Male fetal gender is a relevant risk factor for spontaneous preterm birth, both for intact membranes and for preterm premature rupture of membranes in white European women. In addition, male infants are at increased risk of neonatal morbidity.
authors.affiliation: Department of Obstetrics and Gynecology, Academic Medical Center, Amsterdam, the Netherlands.
authors.forename: Myrthe J C S
authors.identifier: http://orcid.org/0000-0002-1281-0911
authors.initials: MJ
authors.lastname: Peelen
authors.affiliation: Department of Obstetrics and Gynecology, Academic Medical Center, Amsterdam, the Netherlands.
authors.forename: Brenda M
authors.identifier: http://orcid.org/0000-0002-6656-2482
authors.initials: BM
authors.lastname: Kazemier
authors.affiliation: Department of Medical Informatics, Academic Medical Center, Amsterdam, the Netherlands.
authors.forename: Anita C J
authors.identifier: http://orcid.org/0000-0002-3447-8286
authors.initials: AC
authors.lastname: Ravelli
authors.affiliation: Department of Obstetrics and Gynecology, VU Medical Center, Amsterdam, the Netherlands.
authors.forename: Christianne J M
authors.initials: CJ
authors.lastname: De Groot
authors.affiliation: Department of Obstetrics and Gynecology, Academic Medical Center, Amsterdam, the Netherlands.
authors.forename: Joris A M
authors.initials: JA
authors.lastname: Van Der Post
authors.affiliation: The Robinson Research Institute, School of Medicine, University of Adelaide, The South Australian Health and Medical Research Institute, Adelaide, Australia.
authors.forename: Ben W J
authors.identifier: http://orcid.org/0000-0001-8337-550X
authors.initials: BW
authors.lastname: Mol
authors.affiliation: Department of Obstetrics and Gynecology, Academic Medical Center, Amsterdam, the Netherlands.
authors.forename: Petra J
authors.identifier: http://orcid.org/0000-0003-1092-4322
authors.initials: PJ
authors.lastname: Hajenius
authors.affiliation: Department of Obstetrics and Gynecology, Academic Medical Center, Amsterdam, the Netherlands.
authors.forename: Marjolein
authors.initials: M
authors.lastname: Kok
mesh_terms: D015331:Cohort Studies; D005260:Female; D005322:Premature Rupture of Fetal Membranes / Q000453:epidemiology; D005865:Gestational Age; D006801:Humans; D007231:Infant, Newborn; D008297:Male; D009426:Netherlands / Q000453:epidemiology; D011247:Pregnancy; D047928:Premature Birth / Q000453:epidemiology*; D012042:Registries; D012307:Risk Factors; D017678:Sex Distribution; D012737:Sex Factors
keywords: Fetal gender; morbidity; mortality; neonatal outcome; preterm birth; sex