title: Risk for girls can be adversely affected post-term due to underestimation of gestational age by ultrasound in the second trimester.
abstract: BACKGROUND
  Post-term pregnancies are associated with greater risks for mother and child. Accurate determination of gestational age is necessary for safe care. Female fetuses have been shown to be smaller than males at the time of second-trimester ultrasound (US) examination, leading to underestimation of their age and, potentially, greater impacts of perinatal complications in post-term girls than in post-term boys. The purpose of this study was to investigate the sex ratio of post-term births and differences in perinatal complications (stillbirth, low Apgar score, low birthweight, meconium aspiration and low umbilical artery pH) between post-term boys and girls according to dating method [second-trimester US and last menstrual period (LMP)].
  
  
  METHODS
  Data from gestational week ≥39 to delivery of 13 338 singleton pregnancies between 13 February 2006 and 15 January 2014, were collected from the Obstetrix(®) (Siemens Healthcare) medical records system in Dalarna County, Sweden.
  
  
  RESULTS
  The neonatal male:female ratio increased with gestational age after week 40, as dated by US, reaching 1.69 in gestational week 42. This ratio remained 1 throughout gestation according to dating by the LMP. Post-term pregnancy increased the risks of meconium aspiration and low Apgar score, with no sex difference observed.
  
  
  CONCLUSIONS
  US gestational dating indicated that more boys than girls were born post-term, whereas dating according to LMP revealed no sex difference. These results support the hypothesis that female fetuses are smaller than males, leading to underestimation of their gestational age, at the time of second-trimester US examination.
authors.affiliation: Department of Obstetrics and Gynecology, Falu Hospital, Falun, Sweden.
authors.forename: Katarina
authors.identifier: http://orcid.org/0000-0002-2078-3730
authors.initials: K
authors.lastname: Björkman
authors.affiliation: Department of Obstetrics and Gynecology, Falu Hospital, Falun, Sweden.|Center for Clinical Research Dalarna, Falun, Sweden.|Department of Women's and Children's Health, Uppsala University, Uppsala, Sweden.
authors.forename: Jan
authors.identifier: http://orcid.org/0000-0003-1907-1071
authors.initials: J
authors.lastname: Wesström
mesh_terms: D000293:Adolescent; D001034:Apgar Score; D001724:Birth Weight; D005260:Female; D005865:Gestational Age; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007231:Infant, Newborn; D008297:Male; D008471:Meconium Aspiration Syndrome; D008875:Middle Aged; D011247:Pregnancy; D011256:Pregnancy Outcome; D011273:Pregnancy, Prolonged / Q000000981:diagnostic imaging*; D012189:Retrospective Studies; D012306:Risk; D050497:Stillbirth; D013548:Sweden; D016216:Ultrasonography, Prenatal; D014469:Umbilical Arteries
keywords: Post-term pregnancy; fetal size; perinatal complications; sex difference; ultrasound dating