title: Trends and correlates of monozygotic twinning after single embryo transfer.
abstract: OBJECTIVE
  To evaluate trends of monozygotic twinning after single embryo transfer and its association with patient and treatment factors.
  
  
  METHODS
  Our retrospective cohort study included 28,596 pregnancies after fresh, nondonor single embryo transfer during 2003-2012 reported to the National ART Surveillance System. We examined trends of monozygotic twin pregnancies (number of fetal heart tones on first-trimester ultrasonography more than one or number of neonates born more than one) and assessed patient and treatment factors for monozygotic twin compared with singleton pregnancies. Modified Poisson regression models were used to estimate adjusted risk ratios (RRs) and 95% confidence intervals (CIs) for association between monozygotic twinning and selected factors stratified by day 2-3 and day 5-6 transfer.
  
  
  RESULTS
  During 2003-2012, the incidence of monozygotic twinning after single embryo transfer was lower for day 2-3 transfers than for day 5-6 transfers (1.71%, 95% CI 1.45-1.98, n=162 compared with 2.50%, 95% CI 2.28-2.73, n=472); the incidence did not change significantly over the study period. Among day 2-3 transfers, assisted hatching increased the risk for monozygotic twinning compared with singletons (adjusted RR 2.16, 95% CI 1.53-3.06); use of intracytoplasmic sperm injection decreased the risk (adjusted RR 0.60, 95% CI 0.42-0.85). Having one or more prior pregnancies increased the risk for monozygotic twinning among day 5-6 transfers (adjusted RR 1.26, 95% CI 1.03-1.53).
  
  
  CONCLUSIONS
  Monozygotic twinning after single embryo transfers was more common among day 5-6 embryo transfers than day 2-3 transfers. Use of assisted hatching was associated with increased risk for monozygotic twinning for day 2-3 transfers.
  
  
  METHODS
  II.
authors.affiliation: Department of Gynecology and Obstetrics, Emory University School of Medicine, and the Division of Reproductive Health, National Center for Chronic Disease Prevention and Health Promotion, Centers for Disease Control and Prevention, Atlanta, Georgia.
authors.forename: Jessica R
authors.initials: JR
authors.lastname: Kanter
authors.forename: Sheree L
authors.initials: SL
authors.lastname: Boulet
authors.forename: Jennifer F
authors.initials: JF
authors.lastname: Kawwass
authors.forename: Denise J
authors.initials: DJ
authors.lastname: Jamieson
authors.forename: Dmitry M
authors.initials: DM
authors.lastname: Kissin
mesh_terms: D000328:Adult; D005260:Female; D020073:Gravidity; D006801:Humans; D015994:Incidence; D011247:Pregnancy; D059285:Pregnancy, Twin / Q000706:statistics & numerical data*; D012189:Retrospective Studies; D056826:Single Embryo Transfer / Q000379:methods / Q000706:statistics & numerical data*; D020554:Sperm Injections, Intracytoplasmic; D013997:Time Factors; D038522:Twinning, Monozygotic; D014481:United States