It has been suggested that fellatio may, through "immune modulation", have a beneficial role in preventing dangerous complications during pregnancy, reducing specifically the risk of miscarriage and pre-eclampsia. Specifically, several research groups have reported that pre-eclampsia, a life threatening complication that sometimes arises in pregnancy, is much less frequent in couples who have practiced oral sex, and even more rare in couples where fellatio regularly ended with a woman's swallowing of her partner's semen.
The results were statistically significant and are consistent with the fact that semen contains several agents that have important roles in the prevention of pre-eclampsia, which may arise out of an immunological condition. According to that view, preeclampsia is caused by a failure of the mother to accept the fetus and placenta, which both contain "foreign" proteins from the father's genes.
Regular exposure to the father's semen helps cause immunological tolerance to their proteins. Other studies also found that, while any exposure to the partner's sperm during sex appears to decrease the chances of various disorders, women in couples who have practiced "sex acts other than intercourse" are less than half as likely to suffer preeclampsia.
The studies noted that it would be impossible to assume conclusively the likely protective effect of the "other sex acts" including oral sex, or that the correlation between these sexual practices was due to the presence of collinearity induced by some other protective factor not noted in the studies: for example, greater overall frequency of sex. The standard way to resolve such confounding questions in medical science would be through a randomized trial, but there are unique challenges to research in sexual health.