That’s an interesting idea. It’s known that a woman’s first pregnancy has a higher likelihood of her ending up with pre-eclampsia than in subsequent pregnancies; there is also a higher likelihood of women who are artificially inseminated (meaning, donor sperm, not her husband/boyfriend’s) ending up with pre-eclampsia than women who conceive naturally. So, researchers looked at how long the father and mother slept together, and found that women who have a short sexual relationship with the baby’s father prior to the onset of pregnancy have a higher likelihood of pre-eclampsia — possibly due to repeated exposure to sperm somehow altering or desensitizing the uterus to the “foreign” presence of the baby (with half of the genetic material being maternal, but the other half being paternal, and being contained, in one way or another, in the other sperm that didn’t make it to an egg). Here’s a link to the study’s abstract, and here’s the link to a blog talking about it [the blog links to a website that you can read the full article (not the study itself, but about the study), if you register (which is free)].
One potential confounding problem, is that women who get pregnant by a man without having been in a long-term sexual relationship with him (for the purposes of this study, it’s 6 months or less), may be at higher risk for pre-eclampsia due to other factors — not having the study, I don’t know for sure if they tried to control for all known risk factors for pre-eclampsia; and even if they did, there may be unknown risk factors for it — some underlying health issues, for instance, that may predispose to pre-eclampsia, that may also more likely to be in women who are not in a long-term, stable relationship. Married women have a lower risk for many pregnancy problems, such as preterm birth and infant mortality, so perhaps this is similar.
Also, I wonder if repeated exposure to sperm during pregnancy plays any role in pre-eclampsia. Take, for instance, a couple who gets pregnant soon after their marriage; and they were either not sexually active prior to marriage, or did not give into temptation much prior to it. They’re probably going to (and I speak from personal experience here!), “make up for lost time,” in their early months (unless she is overcome with nausea or something due to the pregnancy), and she may very well have as much, ahem, exposure to his sperm in a short time as others may have over a longer relationship. One couple may have sex only once a week (like, only after Friday-night dates) — or she could get pregnant from a one-night stand; while others may make love every day. I wonder if that was likewise taken into account — it’s one thing to say, “We started sleeping together three months ago,” and it’s another to document X number of encounters in that span of 3, 6, or 12 months. [They mention “total number of semen exposures” in the abstract, when speaking of SGA (small for gestational age) babies, but again, without the study, I don’t know if they wrote down every woman’s history, or if they took an average, or what.] Some men turn tail and run when the woman tells him she’s pregnant, so there might be a difference between a married couple getting pregnant a month or two into their sexual relationship and continuing it through pregnancy, and a similar couple getting pregnant a month or two into their sexual relationship and not continuing the relationship (for whatever reason). I wonder if there would be a large enough population of women becoming pregnant by artificial insemination to participate in a trial like this — the study arms could be women in long-term sexual relationships prior to pregnancy (in other words, they have multiple exposures to the semen of the father of the baby), women in short-term sexual relationships prior to pregnancy (perhaps separated by whether the relationship continues during pregnancy or not), women in long-term sexual relationships prior to pregnancy, but not with the father of the baby (artificial insemination by another man due to partner’s low sperm count), and women with no heterosexual relationships at the time of pregnancy (artificial insemination as a single woman or a lesbian).
However, even women in long-term relationships can and do end up with pre-eclampsia, so even if being in a stable marriage reduces your risk of pre-eclampsia, it does not totally prevent it.