The results were statistically significant and are consistent with the fact that semen contains several agents that have important roles in the prevention of preeclampsia, 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 her immune system to gradually "grow accustomed" 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.