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pre-eclampsia isn’t spotted and prevented, sudden convulsions can develop during labour. When full eclampsia sets in, the mother’s mouth twitches and her body contracts, then becomes completely rigid; violent muscular spasms break out and the woman foams at the mouth. So alarming is this complication that it probably prompted the first Caesarean sections to be conducted about two thousand years ago. Unfortunately, eclampsia remains a serious, potentially fatal condition, and can harm a woman’s kidneys, liver, and blood vessels.

These terrible complications, however, do not affect all animals with placentas. In fact, they are only known to happen in three species alive today: patas monkeys, lowland gorillas, and us. What distinguishes these three primates from other mammals is the extent to which the placenta penetrates the mother’s blood supply. If the placenta does not invade deep enough, the mother’s heart has to work harder, increasing the pressure of the blood in order to keep the foetus alive.

The link between pre-eclampsia and high blood pressure has been acknowledged since 1896, when the inflatable arm-band for measuring blood pressure was invented, but doctors still do not know exactly why, in some women, the placenta stops receiving blood as it should. The only risk factor that is universally accepted is being pregnant for the first time. Why should that be?

Our immune systems evolved to protect us from a staggering variety of parasites – anything that is in our bodies that shouldn’t be. Once an outsider is recognized, the body’s aim is to get rid of it. But we have seen that evolution has worked around this line of defence in many ways, for the simple reason that if a species is reproducing via sex, it benefits the mother’s genes to become pregnant. Yet, it isn’t easy for foreign sperm to get to an egg. Out of the three hundred million sperm that might be released into a woman’s vagina, only one, if any, will normally succeed in fertilizing an egg. All the barriers are in place to prevent it: to prevent infection, the woman’s vagina has an acidic pH that is also a killer for sperm; to stop microbes from invading, the cervix is filled with compact mucus, which also makes it incredibly difficult for sperm to make it to the womb; and then, the womb is armed with the soldiers of the immune system, white blood cells, which will physically engulf and destroy unwanted invaders, including most sperm. In pre-eclampsia, it may be that the woman’s immune system has put up yet another line of defence and refuses to accept the incursion of the placenta’s foreign DNA.

Pre-eclampsia occurs mostly in first-time pregnancies, but not all first-time pregnancies are the same. If a couple has had unprotected sex for less than four months before conception, the rate of pre-eclampsia is approximately four out of five. This decreases to one in four among those couples who have been having unprotected sex for five to eight months, and further to one in twenty among those who have been doing so for more than twelve months. Even a woman who has already conceived several children runs a heightened first-time-pregnancy risk when she takes a new male partner who isn’t the father of her earlier children. Put simply, it may be that being exposed to a particular partner’s sperm ‘acclimatizes’ a woman’s immune system to his genes, breaking down the defences against foreign intruders and improving the negotiations that take place between the womb and the child. Becoming tolerant of a partner’s sperm appears to protect the mother and the embryo, once it implants in the woman’s womb.

So while humans may not ourselves be strictly monogamous, evolution has built women to have more successful pregnancies with long-term sexual partners.

During pregnancy, the mother, too, has an active role in protecting the foetus from her immune system’s attackers: substantial numbers of a woman’s immune cells cross through the placenta and settle in the developing lymph nodes of the foetus, disguising the baby’s immune system from her own. In a way, the body is tricked into seeing the foetus as a ‘temporary self’. These cells also serve to suppress the foetus’s immune system, which could be set against the mother’s blood.

These maternal immune cells have an incredibly long-lasting influence on the foetus, even long after the baby is born. As they cross from the mother to the child, the cells ‘teach’ the foetus how to balance the need for self-defence against the need for tolerance to the surrounding environment. This is a tricky balancing act. If the foetus is taught to be too tolerant, the newborn baby may be left unprotected from a common but potentially lethal infection. If, on the other hand, the foetus’s self-defence mechanisms become too keen, a child may be overly sensitive to certain foods and environments; worse, the body might start attacking itself – a condition called autoimmunity. Indeed, until at least early adulthood, a mother’s immune cells influence how her child’s body regulates its own defences and how tolerant, or susceptible, it will be to allergies and infection.

It used to be thought that a baby in the womb somehow made itself completely invisible to the mother’s immune system, but this isn’t strictly true. What happens, instead, is that the immune systems become interlocked. This means that diseases which are not normally transmissible between two adults can pass from mother to child. Diseases such as cancer.

In 2007, a twenty-eight-year-old Japanese woman gave birth to a girl. The pregnancy was uneventful, and the baby seemed perfectly healthy on delivery. When the baby was about one month old, however, the mother had to be rushed back into hospital: she was bleeding uncontrollably from her vagina. She died not long after being admitted. Although she had not known it, the mother had leukaemia, a cancer of