title: Monozygotic twinning associated with day 5 embryo transfer in pregnancies conceived after IVF.
abstract: BACKGROUND
  This study examines the association between day of embryo transfer and monozygotic (MZ) twinning.
  
  
  METHODS
  We used a population-based sample of 108,36 IVF/embryo transfer procedures in which the patients oocytes' were freshly fertilized (non-frozen; non-donor) and 39,98 resultant pregnancies from US clinics in 1999 and 2000. Cases were pregnancies for which the number of fetal hearts observed on ultrasound exceeded the number of embryos transferred. These pregnancies were considered to contain at least one set of MZ twins. A total of 226 MZ pregnancies were compared with two control groups: 23,880 singleton pregnancies (one fetal heart) and 15,092 other multiple-gestation pregnancies (> or = 2 fetal hearts but the number of fetal hearts on ultrasound was less than or equal to the number of embryos transferred).
  
  
  RESULTS
  Cases of presumed MZ multiple-gestation pregnancies were more likely to have had a day 5 embryo transfer compared with day 3 embryo transfers than singleton pregnancies [adjusted odds ratio (AOR) = 3.92, 95% confidence interval (CI) = 2.97-5.17] or other multiple-gestation pregnancies (AOR = 3.91, 95% CI = 2.96-5.17) conceived with IVF/embryo transfer.
  
  
  CONCLUSIONS
  Day 5 embryo transfer may be associated with increased MZ twinning.
authors.affiliation: Division of Reproductive Health, National Center for Chronic Disease Prevention and Health Promotion, Centers for Disease Control and Prevention, Atlanta, GA, USA. vwright@cdc.gov
authors.forename: Victoria
authors.initials: V
authors.lastname: Wright
authors.forename: Laura A
authors.initials: LA
authors.lastname: Schieve
authors.forename: Anjel
authors.initials: A
authors.lastname: Vahratian
authors.forename: Meredith A
authors.initials: MA
authors.lastname: Reynolds
mesh_terms: D000328:Adult; D017677:Age Distribution; D016022:Case-Control Studies; D004624:Embryo Transfer / Q000401:mortality / Q000706:statistics & numerical data*; D005260:Female; D005307:Fertilization in Vitro / Q000401:mortality / Q000706:statistics & numerical data*; D006801:Humans; D011247:Pregnancy; D011256:Pregnancy Outcome; D012307:Risk Factors; D013997:Time Factors; D014430:Twins, Monozygotic