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Frequent sex reduces pregnancy complications

19:00 20 November 02

Exclusive from New Scientist Print Edition

Low fertility and frequent pregnancy complications may be the price that humans have paid for evolving a large brain.

For the fetus to get enough nutrients to grow a hefty brain the placenta has to aggressively invade a mother's uterus, says a new theory. But that can also provoke her immune system, causing dangerous complications.

However, recent research suggests that exposure to a man's semen helps a women's immune system prepare for pregnancy (New Scientist print edition, 9 February, p 32). So low rates of conception in humans reduce complications during pregnancy by giving a woman's immune system more time to adapt.

Human fetuses spend 60 per cent of their energy on their brain, three times as much as other mammals. Twenty weeks into pregnancy, the placenta attacks the uterine wall for a second time, burrowing in more deeply than in any other mammal.

But burrowing deeper is risky. It can provoke the mother's immune system to attack the placenta, which is loaded with foreign genes from the father. This can trigger pre-eclampsia, where the placenta leaks toxins into the mother's circulation, causing blood pressure to spike dangerously. Within hours it can escalate into kidney failure, brain haemorrhaging and death....

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