Expectant moms aren't the only ones whose hormones fluctuate.
Expectant moms aren't the only ones whose hormones fluctuate. In a small study at Queen's University in Ontario, Canada, researchers found that first-time dads-to-be experienced a significant decrease in the male hormone testosterone as well as cortisol, a stress hormone. They were also more likely to have detectable levels of estradiol, a naturally occurring estrogen known to influence maternal behavior. The change in body chemistry may help men respond differently to the new situations they'll face as dads—and possibly encourage the father-child bond.