This study examined changes in coparenting after the birth of a second child. Mothers and fathers from 241 two-parent families reported on their spouse’s coparenting cooperation and conflict with their firstborn child before (prenatal) and four months after the birth of a second child. Parents completed questionnaires (prenatal) on gender role attitudes, marital satisfaction, and firstborn children’s temperamental characteristics. Parents also reported on the secondborn infant’s temperament at 1 month following the birth of the second child. Coparenting conflict increased across the transition, whereas cooperation decreased. Couples in which fathers reported greater marital satisfaction were more cooperative 4 months after the birth. Firstborns’ difficult temperament contributed to less cooperative coparenting by both parents. When mothers had more traditional gender role beliefs, fathers engaged in more conflictual coparenting behavior, and when fathers had more traditional gender role beliefs, mothers engaged in more conflictual coparenting behavior. Mothers, but not fathers, engaged in more coparenting conflict regarding the firstborn when both the firstborn and infant sibling had difficult temperaments.