In this study by Kho and colleagues at the University of Auckland, published in the Journal of Reproductive Immunology, 2507 first-time pregnant women were interviewed about the length of their relationship with the baby's biological father.

When the pregnancies came to term, pre-eclampsia (pregnancy-induced hypertension) was found to be less common in women who had long-term sexual relations exclusively with the biological father, than in those who had been with their partner only for a short time (i.e. less than six months).

The study also revealed that women who had undersized babies (SGA, or 'small for gestational age') were also more likely to have been in shorter relationships, but only when 20 week ultrasounds demonstrated reduced blood flow to the fetus.

F1000 reviewer Dr Larry Chamley explains that "in normal pregnancies … prolonged exposure of the female immune system to paternal antigens following intercourse (without barrier contraception) [could induce] tolerance of the maternal immune system to the paternal antigens.

But the exaggerated maternal inflammatory response in pre-eclampsia is due to a failure of the maternal immune system to down-regulate or tolerate its response to paternal antigens."

"Although the issue of whether prolonged semen exposure does protect against developing pre-eclampsia is not yet resolved," continued Dr Chamley, "this paper seems to tip the weight of evidence back in favour of suggesting that prolonged semen exposure is protective."

More information:

• The full text of the evaluation of is available free for 90 days at: http://www.f1000medicine.com/article/b1q9v25c8dj7d5b/id/1164869
• An abstract of the original paper by Kho et al. (Duration of sexual relationship and its effect on preeclampsia and small for gestational age perinatal outcome.) is at: http://www.ncbi.nlm.nih.gov/sites/entrez/19679359?dopt=Abstract&holding=f1000,f1000m,isrctn

Source: Faculty of 1000: Biology and Medicine