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, 9 February, p 32). So low fertility in humans reduces 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, 3 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.
It is thought that humans are the only mammals to suffer frequent pre-eclampsia, which occurs in 3 per cent of pregnancies. We are also far less fertile: a bitch that mates just once when it is on heat usually gets pregnant, yet women typically take six months to conceive.
Research by Pierre-Yves Robillard, a neonatologist at Sud RŽunion Hospital on the Indian Ocean island of RŽunion, has shown that women who have sex with the father for over a year before getting pregnant have a 5 per cent chance of developing high blood pressure and pre-eclampsia compared with a massive 40 per cent chance for those who have only been having sex with the father for four months or less.
Robillard is now proposing that this is why we are less fertile- the extra sex gives women a better chance of surviving the placental invasion. "If we had kept the same fertility as other mammals, we would have pre-eclampsia rates of 20 per cent," he told a workshop about pre-eclampsia in Mauritius. "Humans could not have survived."
The theory has generated both interest and scepticism. "It's an interesting idea that placental invasiveness has something to do with brain expansion," says David Haig, an evolutionary biologist at Harvard University, "but other possibilities can't be eliminated." For example, pre-eclampsia may have become more common as societies became better at caring for ailing mothers and babies.
And Robert Martin, an anthropologist at The Field Museum in Chicago, questions whether invasive placentas are linked to larger brains. "Dolphins have a non-invasive placenta," he says, "yet the next biggest brain sizes after humans are found in dolphins."
Author: Douglas Fox
New Scientist issue: 23rd November 2002
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