By Carl Zimmer　

　　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 unconsciousness 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 genes are shut down, depending on which parent they come from. Dr. 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.

　　A fetus 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.

　　Dr. Haig argued that natural selection should favor mothers who could restrain these incursion, and manage to have several surviving offspring carrying on their genes. He envisioned pregnancy as a tug of war.

　　In a 1993 paper, Dr. Haig first predicted that many complications of pregnancy would turn out to be produced by this conflict. One common complication 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 6 percent of pregnancies.

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

　　In the past few years, Ananth Karumanchi of Harvard Medical School and his colleagues have gathered evidence that suggests Dr. 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.

　　Other labs have replicated their results. Dr. Karumanchi’s group has done additional work that indicates that this protein interferes with the mother’s ability to repair minor damage builds up, so does her blood pressure.

　　And as Dr. Haig predicted, the protein is produced by the fetus, not the mother. “When I first came across David Haig’s hypothesis, it was absolutely cool,” Dr. Karumanchi said. “And it made me feel like I might be on the right track.”

　　Dr. Haig also made some predictions about the sorts of maternal defenses that have evolved. One of the most intriguing strategies he proposed was for mothers to shut down some of the genes in their own children.

　　This strategy takes advantage of the fact that most of the genes we carry come in pairs. We inherit one copy from our mother and one from our father. In most cases, these pairs of genes behave identically. But in the past 15 years, scientists have identified more than 70 pairs of genes in which the copy from one parent never makes a protein. In some cases, a parent’s gene is silenced only in one organ.

　　Scientists do not fully understand this process, known as genomic imprinting. They suspect that it is made possible by chemical handles called methyl groups that are attached to units of DNA. Some handles may turn off genes in sperm and egg cells. The genes then remain shut off after a sperm fertilizes an egg.

　　Only a few of these genes have been carefully studied to understand how they work. But the evidence so far is consistent with Dr. Haig’s theory.

　　One of the most striking examples is a gene called insulin growth factor 2 (Igf2). Produced only in fetal cells, it stimulates rapid growth. Normally, only the father’s copy is active. To understand the gene’s function, scientists disabled the father’s copy in the placenta of fetal mice. The mice were born weighing 40 percent below average. Perhaps the mother’s copy of Igf2 is silent because turning it off helps slow the growth of a fetus.

　　Dr. Haig has enjoyed watching his theory mature and inspire other scientists. But he has also had to cope with a fair amount of hate mail. It comes from across the political spectrum, from abortion opponents to feminists who accuse him of trying to force patriarchy into biology.

　　“People seem to think, ‘ He must have a political agenda,’” Dr. Haig said. “But I’m not talking at all about conscious behaviors. I’m just interested in these mechanisms and why they evolved.”