Lack of harmony: Pregnancy is a two-person operation.

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 Dr 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, Haig argues, a mother and her unborn child engage in an unconscious struggle over the nutrients she will provide it. 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 foetuses: the copies of some genes are shut down, depending on which parent they come from. Haig has also argued that the same evolutionary conflicts can linger on after birth and even influence the adult brain. New research has offered support to this idea as well. By understanding these hidden struggles, scientists may be able to better understand psychological disorders like depression and autism.

As a biologist fresh out of graduate school in the late 1980s, Haig decided to look at pregnancy from an evolutionary point of view. As his guide, he used the work of Dr Robert Trivers, an evolutionary biologist at Rutgers University.

In the 1970s, Trivers argued that families create an evolutionary conflict. Natural selection should favour parents who can successfully raise the most offspring. For that strategy to work, they can’t put too many resources into any one child. But the child’s chances for reproductive success will increase as its care and feeding increase. Theoretically, Trivers argued natural selection could favour genes that help children get more resources from their parents than the parents want to give.

As Haig considered the case of pregnancy, it seemed like the perfect arena for this sort of conflict. A child develops in intimate contact with its mother. Its development in the womb is crucial to its long-term health. So it was plausible that nature would favour genes that allowed foetuses to draw more resources from their mothers.

A foetus does not sit passively in its mother’s womb and wait to be fed. Its placenta aggressively sprouts blood vessels that invade its mother’s tissues to extract nutrients.

Meanwhile, Haig argued, natural selection should favour mothers who could restrain these incursions, and manage to have several surviving offspring carrying on their genes. He envisioned pregnancy as a tug of war. Each side pulls hard, and yet a flag tied to the middle of the rope barely moves.

TUG OF WAR: Evolutionary conflicts linger on even after birth

“We tend to think of genes as parts of a machine working together,” Haig said. “But in the realm of genetic conflict, the cooperation breaks down.”

In a 1993 paper, Haig first predicted that many complications of pregnancy would turn out to be produced by this conflict. One of the most common complications is pre-eclampsia, in which women experience dangerously high blood pressure late in pregnancy. For decades scientists have puzzled over pre-eclampsia, which occurs in about six per cent of pregnancies.

Haig proposed that pre-eclampsia was just an extreme form of a strategy used by all foetuses. The foetuses somehow raised the blood pressure of their mothers so as to drive more blood into the relatively low-pressure placenta. Haig suggested that pre-eclampsia would be associated with some substance that foetuses injected into their mothers’ bloodstreams. Pre-eclampsia happened when foetuses injected too much of the stuff, perhaps if they were having trouble getting enough nourishment.

In the past few years, Dr Ananth Karumanchi of Harvard Medical School and his colleagues have gathered evidence that suggests Haig was right. They have found that women with pre-eclampsia had unusually high levels of a protein called soluble fms-like tyrosine kinase 1, or sFlt1 for short.