The Boy Who Gave Birth to His Twin

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Fraught business

The first two months are the most hectic time for a developing embryo, as it changes from a fertilised egg to a recognisable human form. Not surprisingly, it is during this period that the developmental schedule is most likely to go awry. Most human embryos die in their first eight weeks of life, often before the mother is even aware that she is pregnant. It has been estimated that as many as three quarters of all human embryos perish during this critical phase. It's an astonishingly high rate compared to other mammals, and the reason is unclear. But with development relying on the interaction of so many factors, it's no wonder things do go wrong.

And if the process is complicated with one embryo, imagine what it's like with two. Twins, especially identical ones, can throw an extra degree of unpredictability into the mix. Identical twins arise when a fertilised egg splits in two before it implants into the uterus. But again, there are a multitude of ways in which this can go askew. One twin, for example, may end up completely enclosed inside the other, resulting in the rare condition known as 'foetus in foetu'. Or the embryos may fail to separate completely, resulting in conjoined twins.

Given the many pitfalls that litter the developmental journey of an embryo, it may seem a miracle that any of us are here at all. But with six billion people living on Earth today, all who completed that journey, something must be working right.