Increased live birth rate in twin pregnancies resulting from embryo assistance.

Objective: To estimate why the live birth rate per implanted embryo is higher in twin than in singleton pregnancies.

Methods: Data from 1,159 singleton and 523 dichorionic twin pregnancies, obtained after assisted conception, were included. To account for the higher live birth rate observed in twin pregnancies, two possibilities were considered: embryo collaboration and assistance. To test these hypotheses, we compared the observed values for each possible outcome in twin pregnancies (double birth, single birth, double loss) with the expected values regarding as the null hypothesis that the survival of either embryo is independent from the presence of the other. The effect of maternal age was also examined.

Results: Live birth rate, per implanted embryo, was higher in twin than in singleton pregnancies: 83% compared with 76% (odds ratio 1.53, 95% confidence interval 1.24-1.88; P<.001). There were significant differences between the observed outcomes in twin pregnancies and those expected assuming no interaction between embryos (P<.001). The number of double births was higher than expected (71.9% observed compared with 57.7% expected), whereas single births were lower to a similar extent (22% observed compared with 36.5% expected). In contrast, observed and expected double losses were similar (6.1% observed compared with 5.8% expected). Although the overall rate of miscarriage was higher for women aged 34 years and older, the difference in live birth rate between twin and singleton pregnancies was 2.4-fold higher than in younger women.

Conclusion: The higher live birth rate occurring in twin pregnancies can be accounted for by assistance, whereby some embryos that would fail as singletons survive in a twin pregnancy when implanted along with a fit sibling. This effect is more pronounced in older mothers.