The new U-M study is the first to show that the decline may begin even earlier, during the transition to fatherhood, said Robin Edelstein
, the study's lead author.
"We don't yet know exactly why men's hormones are changing," said Edelstein, U-M associate professor of psychology.
and colleagues examined salivary testosterone, cortisol, estradiol and progesterone in 29 first-time expectant couples between the ages of 18 and 45.
The saliva samples were obtained up to four times during the prenatal period at about 12, 20, 28 and 36 weeks of pregnancy.
Women showed large prenatal increases in all four hormones, while men saw declines in testosterone (which is associated with aggression and parental care) and estradiol (which is associated with caregiving and bonding).
No changes were found in men's cortisol (a stress hormone) or progesterone (which is associated with social closeness and maternal behavior).
So it's not just about the presence of an infant that lowers testosterone, Edelstein
One limitation of the new study-as it relates to lower testosterone-is that researchers do not have a comparison group of men who are not expecting a child.
"Thus, we can't completely rule out the possibility that the changes are simply due to age or the passage of time," Edelstein