Among new parents, fathers' testosterone declined over partners' pregnancy and correlated w/ partners' changes in testosterone. Men whose testosterone decreased more (vs less) sharply were more invested in and committed to their relationships postpartum.
This study addresses the concern you raise. Levels of testosterone are measured frequently in pregnancy & after birth. Greater within-person reductions in fathers T were associated with higher post-partum commitment, satisfaction, etc.
Dads' decline in #testosterone across partner's pregnancy --> higher postpartum rltnshp satisfaction in mom & dad!