One of the most extensive studies on prenatal hormones in first-time expectant parents has revealed that both men and women experience hormonal changes during pregnancy.

The study, published in the American Journal of Human Biology, found significant prenatal declines in testosterone and estradiol in men, which researchers say suggests that impending fatherhood may cause men’s hormone levels to change.

“Other studies have shown that men’s hormones change once they become fathers, but our findings suggest that these changes may begin even earlier, during the transition to fatherhood,” Dr. Robin Edelstein, lead author of the study said in a news release.

“We don’t know exactly why men’s hormones are changing; these changes could be a function of psychological changes that men experience as they prepare to become fathers, changes in their romantic relationships, or even physical changes that men experience along with their pregnant partners,” Edelstein said.

The results also showed large prenatal increase in salivary testosterone, cortisol, estradiol and progesterone in women, which researchers expected. Researchers say additional studies are needed to understand whether partners’ prenatal hormone changes are linked to postpartum behavior and adjustment.