Pregnancy can be the most wonderful experience life has to offer. But it can also be dangerous. Around the world, an estimated 529,000 women a year die during pregnancy or childbirth. Ten million suffer injuries, infection or disability. To David Haig, an evolutionary biologist at Harvard, these grim statistics raise a profound puzzle about pregnancy. “Pregnancy is absolutely central to reproduction, and yet pregnancy doesn’t seem to work very well,” he said. “If you think about the heart or the kidney, they’re wonderful bits of engineering that work day in and day out for years and years. But pregnancy is associated with all sorts of medical problems. What’s the difference?” The difference is that the heart and the kidney belong to a single individual, while pregnancy is a two-person operation. And this operation does not run in perfect harmony. Instead, Dr. Haig argues, a mother and her unborn child engage in an unconscious struggle over the nutrients she will provide it. Dr. Haig’s theory has been gaining support in recent years, as scientists examine the various ways pregnancy can go wrong. His theory also explains a baffling feature of developing fetuses: the copies of some […]