New moms are used to experiencing hormonal fluctuations, thanks in part to the estrogen surge that happens at the very start of pregnancy and persists through the next nine months.
But while most men don’t notice it, fatherhood does a number on their hormone levels as well, namely testosterone. Recent studies have detected a testosterone dip in new dads — and the decrease appears to happen the moment he picks up his newborn, Dr. Helen Fisher, a biological anthropologist and professor at Rutgers University, tells Yahoo Parenting.
Evidence even suggests that the testosterone drop might precede the baby’s birth. Expectant fathers showed decreased testosterone levels during their partner’s pregnancy, according to a 2014 study from the University of Michigan.
So what’s behind the dive? Experts don’t know for sure, but they think it might be part of Mother Nature’s plan to transform aggressive guys into nurturing and involved husbands and fathers.
“Testosterone is the hormone that drives men to be more aggressive and seek out sex, so reduced testosterone levels make guys more stable, less likely to spread their seed elsewhere,” New York-based sex therapist Dr. Ian Kerner, author of She Comes First, tells Yahoo Parenting. “From an evolutionary standpoint, it makes sense for guys who have a pregnant partner or are already a father to be less incentivized to stray and more invested in the home front and their family,” he says.
The transition from dude to dad isn’t the only thing that can make a dent in testosterone levels. Now that he’s on diaper duty and dealing with the challenges of a baby, the following factors can also dial it down — to the point where his sex drive and sperm production may be affected:
Packing on pounds: With less free time to spend at the gym, a new dad may find himself gaining weight. But if his BMI qualifies as overweight or obese, testosterone can nosedive. “Testosterone decreases as it is replaced by estrogen, which is made by fat cells,” Dr. Joseph Alukal, assistant professor of urology at NYU Langone Medical Center, tells Yahoo Parenting. “The more fat cells he has, the more his testosterone is affected.”
Not getting enough sleep: Babies don’t always like to cooperate with adult sleep schedules, and that takes a toll. “If your sleep is disrupted or you don’t get enough on a regular basis, it causes changes in sleep hormones that can lower testosterone,” says Alukal.
Excessive stress: Infants are adorable, cuddly stress bombs. Taking care of one can elevate levels of the stress hormone cortisol, which makes a dent in a dad’s testosterone. Lower testosterone means a lower sex drive. It’s almost as if his body is saying that with all this stress going on, right now is not the best time to have a lot of sex…and create another baby, says Alukal.