Semen protects against preeclampsia.

A dangerous form of high blood pressure that strikes during pregnancy appears to be linked to the duration of the mother's sexual relationship with the father, a new study shows. The researchers believe that prolonged exposure to the father's semen helps trigger a protective response in the mother that later wards off this illness.

Doctors understand very little about what causes pregnancy-induced hypertension, including preeclampsia. Researchers know that the disorder most often affects women pregnant for the first tiem. But the new findings suggest that women who have just begun a sexual relationship with the father face the highest risk of this condition.

From February to July 1993, Pierre-Yves Robillard of the University Hospital of Pointe a Pitre, Guadeloupe, French West Indies, and his colleagues collected information from 957 women who had just delivered a baby in the hospital. The researchers gathered data about paternity by interviewing the mothers without their partners present.

The investigators noted that 102 of the women had developed pregnancy-induced hypertension. The majority had the mildest form of the disorder. However, 19 had progressed to preeclampsia, characterized by high blood pressure and protein in the urine. And two women had full-blown eclampsia, which can cause death.

The threat of preeclampsia increases significantly when pregnancy occurs within the first year of a sexual relationship, the researchers discovered. If a woman becomes pregnant within the first 4 months of such a partnership, her risk of developing preeclampsia is 12 times higher than if she had been with her partner for at least a year.

Although the incidence of preeclampsia declines in subsequent pregnancies, women who change their sexual partner reinstitute the risk of this disorder. In fact, such women are five times as likely to develop the condition as pregnant women who have remained with the father of their previous children, the team discovered. Robillard and his colleagues report their findings in the Oct. 8 LANCET.

Something in male ejaculate may help protect a woman from preeclampsia--if she's been repeatedly exposed to it, says David A. Clark of McMaster University in Hamilton, Ontario. Researchers don't know whether the sperm itself, the accompanying white cells, or the nourishing liquid called seminal plasma is responsible for the shielding effect.

Such a concept is not as far-fetched as it may sound. For example, scientists already know that substances from the father lead to a beneficial immune response in the mother that helps sustain a healthy placenta. In preeclampsia, blood flow through the placenta is inadequate.