Mummies aren't the only ones who ride the hormone roller coaster -- first-time fathers undergo similar changes. Canadian researchers observed that from the first trimester to a few months after birth, the 23 papas-to-be who were studied had lower levels of testosterone and the stress hormone cortisol, and many had higher levels of estradiol, a hormone linked with maternal behaviour. This might help explain couvade syndrome, where 11% to 65% of men with pregnant partners experience symptoms like morning sickness.