Although several factors have been identified to predispose to an increased incidence of monozygotic twinning in assisted reproductive technologies (ART), the relative risks associated with each have yet to be fully established. Moreover, the focus has been predominantly on monozygosity, which, in the absence of monochorionicity, does not increase perinatal risk. The present objective was to undertake an analysis of the relative risks of factors associated with monochorionic pairs resulting from ART.METHODS
Study cycles included the last cycle, of each patient undergoing ART at Brigham and Women's Hospital from January 1998 to December 2004, that resulted either in a pregnancy with a monochorionic pair (n=41) or a pregnancy without a monochorionic pair at 12 weeks (n=2460). We used multivariable logistic regression to estimate odds ratios (OR) and 95% confidence intervals (CI) to identify factors significantly associated with a monochorionic pair.RESULTS
Independent predictors of a monochorionic pair were assisted hatching (OR 2.23, 95% CI 1.06–4.67), ICSI (OR 2.42, 95% CI 1.22–4.83) and Day 5 embryo transfer (OR 2.48, 95% CI 1.62–3.80). The effects of ICSI and Day 5 transfer were amplified when cycles involved both interventions.CONCLUSIONS
ICSI and Day 5 embryo transfer synergistically increase the risk of monochorionic placentation. Patients undergoing these procedures should be counselled regarding these increased risks.