By Shereen Lehman

Men waiting to become fathers for the first time experienced hormonal changes before their babies were born, and levels of some hormones appeared linked to those of the men’s wives, according to a new study.

The expectant fathers showed drops in testosterone and estradiol – a form of estrogen – but no changes in cortisol or progesterone, two hormones that are implicated in stress, say the authors.

Past research has suggested that new fathers have lower levels of testosterone, but it wasn’t known when the decline begins.

“The previous studies have shown that men with children have lower testosterone than men who don't have children,” said Robin Edelstein, who led the new study.

Her team’s results are the first to show those hormonal changes might actually begin early in the pregnancy, said Edelstein, a psychologist and director of the Personality, Relationships, and Hormones lab at the University of Michigan in Ann Arbor.

“That's what I think is interesting - that it's not about having the baby there (physically) but that there may be some process happening even when just thinking about becoming a father,” she said.

For their study, published in the American Journal of Human Biology, Edelstein and her colleagues enrolled 29 couples who were expecting their first babies. They chose first-time parents because their experiences are often different from those of couples who’ve been through pregnancies before.

The men and women each provided saliva samples at least two times and up to four times during their pregnancies, at roughly eight-week intervals. The researchers used the samples to measure levels of testosterone, estradiol, cortisol and progesterone.

As expected, the mothers-to-be showed large increases in all four hormones during their pregnancies while the testosterone and estradiol levels declined in the fathers-to-be.

Although there were no changes in the men’s progesterone and cortisol levels during the study, the researchers found that within couples, levels of those hormones were generally high or low in both members of the couple.

Edelstein said the study team isn’t sure why these hormone levels were closely correlated in couples, but it might be due to pregnancy being both an exciting and stressful experience that’s shared by both parents.

“That's something we would like to look at a little more,” she said. “It might suggest there is some kind of interdependence or something going on between the partners that is reflected in the correlated hormone levels.”

Edelstein said the study team has also looked at data taken from some of the fathers after their babies were born. She said it appears that men who had larger declines in testosterone have reported being more engaged with their infants and more supportive of their spouses.

“This suggests that there's some kind of benefit to these changes, that there may be something that’s helpful about having declines in testosterone,” she said.

Lee Gettler, director of the Hormones, Health, and Human Behavior Lab at the University of Notre Dame, said despite being somewhat small, the study had many strengths.

“First, the scientists included both mom and dad in the same study - most studies on parents’ biology study either mom or dad,” Gettler told Reuters Health by email.

“Second, they track the couples through time and at many time points during the pregnancy,” he said.

Gettler, who was not involved in the study, said that being able to collect data from both parents at four time points during the pregnancy is exceptional.

“Trust me, it is not especially easy to get expectant couples or new parents to participate in this type of research,” he said.

The testosterone changes might relate to factors between the partners - how their relationship dynamics and interactions change during the pregnancy, Gettler said.

“All of the subjects in this study were living together, so there are many day-to-day dynamics related to social support, intimacy, psychosocial stress and relationship quality that could impact both partners and, likely, men’s biology,” he said.

Gettler has previously studied changes over time in the testosterone levels of single men who were not fathers. He and his colleagues found that men who got married or became new fathers experienced much larger declines in testosterone than men who remained single and childless.

“In that (2011) study we found that new fathers’ testosterone dropped about 40 percent in the first month after they became dads,” he said.

Gettler said that lab experiments indicate fathers with lower testosterone are more sensitive to infant cues and more in tune with their babies, and they behave more affectionately with them.

SOURCE: American Journal of Human Biology, online December 15, 2014.