It is generally thought that losses of twin embryos in very early pregnancy are high, with figures that suggest that for every liveborn twin pair a further 10-12 twin pregnancies end up as a singleton birth. But, in March 14 Nature, Stephen Tong and colleagues from Monash University, Victoria, Australia, show that the presence of one embryo does not affect at all the intra uterine development of its twin.

Tong et al. used ultrasound to identify and follow-up 48 pregnant women who had double-ovulated and conceived. They found that the probability of the second egg also becoming fertilized and developing is 20-30% (Nature 2002, 416:142). These figures are comparable to the probability of conception and survival of a single conceptus.

These results show that "the presumption of huge losses of dizygotic twins in early pregnancy is unfounded," concluded the authors. In addition, they suggest "the mechanism...

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