title: Effect of in vitro culture period on birthweight of singleton newborns.
abstract: OBJECTIVE
  Does prolonged in vitro culture influence newborn birthweight?
  
  
  CONCLUSIONS
  The absolute mean birthweight and gestational age- and gender-adjusted birthweight (Z scores) of singletons born from blastocyst transfer are higher than singletons born from Day 3 transfer.
  
  
  BACKGROUND
  An increased proportion of large-for-gestational age (LGA) newborns occurs after blastocyst transfer compared with Day 2 transfer, and Z scores for newborns after blastocyst transfer are higher than newborns after transfer on Day 2 or Day 3.
  
  
  METHODS
  This study was a retrospective analysis of newborn birthweight, including 2929 singletons at the Reproductive Medical Center of Peking University Third Hospital between January 2009 and June 2012. The number of singletons after Day 3 transfer was 2833 and the number of singletons after blastocyst transfer (Day 5-6) was 96.
  
  
  METHODS
  Only cycles with fresh embryo transfer were included. Patients ≤40 years of age with a BMI < 30 kg/m(2) were analyzed. Only data from singleton newborns born alive after the 20th week of gestation were included in the data analysis. Patients with more than one fetal sac diagnosed by ultrasound but who delivered singletons were excluded. Patients who received PGD and cycles with donor oocytes were excluded. Multiple linear regression analysis was performed to determine the significance of individual factors on absolute birthweight of singleton newborns. The absolute birthweight and Z scores of singletons were compared.
  
  
  RESULTS
  Multiple linear regression analysis indicated that maternal age, maternal BMI, paternal BMI, type of infertility, gestational age, infant gender and culture period were significantly associated with birthweight. The absolute birthweight for singletons resulting from blastocyst transfer was significantly greater than singletons resulting from Day 3 transfer (3465.31 ± 51.36 versus 3319.82 ± 10.04 g respectively, P = 0.009). The Z scores for singletons after blastocyst transfer were significantly higher than singletons after Day 3 transfer (0.347 versus 0.029 respectively, P = 0.016).
  
  
  CONCLUSIONS
  In our clinic, blastocyst culture is mainly offered to patients with unsuccessful IVF cycles but also to patients with uterine malformations, and therefore this protocol introduced a potential selection bias in our study. Moreover, as certain culture media are associated with fetal overgrowth, the media used may be also a confounding factor, even though the absolute birthweights of singletons were comparable.
  
  
  CONCLUSIONS
  Our study suggests that a prolonged (5-6 days) in vitro culture period has a significant effect on the mean absolute birthweight and Z scores of singleton newborns. The effect of prolonged in vitro culture on epigenetic changes in the embryo needs further study.
  
  
  BACKGROUND
  National Natural Science Foundation of China for Young Scholars (81300483). The authors have no conflicts of interest to declare.
  
  
  BACKGROUND
  Not applicable.
authors.affiliation: Department of Obstetrics and Gynecology, Reproductive Medical Center, Peking University Third Hospital, No. 49 North Huayuan Road, Haidian District, Beijing 100191, China.
authors.forename: Jinliang
authors.initials: J
authors.lastname: Zhu
authors.forename: Shengli
authors.initials: S
authors.lastname: Lin
authors.forename: Ming
authors.initials: M
authors.lastname: Li
authors.forename: Lixue
authors.initials: L
authors.lastname: Chen
authors.forename: Ying
authors.initials: Y
authors.lastname: Lian
authors.forename: Ping
authors.initials: P
authors.lastname: Liu
authors.forename: Jie
authors.initials: J
authors.lastname: Qiao
mesh_terms: D000328:Adult; D001724:Birth Weight; D001755:Blastocyst; D002681:China; D046149:Embryo Culture Techniques / Q000379:methods*; D004624:Embryo Transfer / Q000379:methods*; D005260:Female; D005320:Fetal Macrosomia / Q000209:etiology*; D005865:Gestational Age; D006801:Humans; D007231:Infant, Newborn; D008423:Maternal Age; D011247:Pregnancy; D012189:Retrospective Studies
keywords: IVF; birthweight; blastocyst transfer; singleton