INTRODUCTION: Monozygotic twinning, associated with increased infant morbidity and mortality, is more common after assisted reproductive technology (ART) than in the general population. Although multiple factors have been proposed as contributors, studies seeking to define causality have been underpowered or inconclusive. We evaluated trends of monozygotic twinning in ART and its association with patient and treatment factors.
METHODS: We analyzed 392,136 pregnancies resulting from fresh, nondonor embryo transfers conducted between 2000 and 2011 and reported to the National ART Surveillance System. We examined trends of monozygotic twin pregnancies (defined as those in which number of fetal heart tones on first trimester ultrasound exceeded number of embryos transferred) using the Cochrane-Armitage test and subsequently compared monozygotic twins with (1) singleton pregnancies (one fetal heart) and (2) other multiple-gestation pregnancies (more than one fetal heart but not more than number of embryos transferred). Modified Poisson regression models were used to estimate adjusted risk ratios (aRRs) for monozygotic twinning association with assisted hatching for both comparison groups, stratified by day-3 and day-5 transfer.
RESULTS: Incidence of monozygotic twinning after ART increased from 2000 to 2011 (P<.001 for trend), with greater incidence in day-5 (1.72%) than day-3 (0.48%) embryo transfers. Monozygotic twinning was associated with assisted hatching among day-3 embryo transfers when compared with both singleton (aRR 2.19, confidence interval [CI] 1.93-2.48) and other multiple-gestation (aRR 2.27, CI 2.00-2.57) pregnancies and among day-5 embryo transfers when compared with other multiple-gestation pregnancies (aRR 1.18, CI 1.05-1.32).
CONCLUSIONS: Monozygotic twin pregnancy incidence after ART has increased over the past decade. Day-5 transfer and assisted hatching are associated with increased monozygotic twinning risk.
(C) 2014 by The American College of Obstetricians and Gynecologists.