The "4th trimester" refers to the transition period after childbirth when infants are adjusting to life outside the womb and mothers are adjusting to new parenthood. This critical period is marked by significant biological, psychological, and social changes, which are currently insufficiently supported. The 4th trimester perspective draws attention and calls for resources to address overlapping maternal health domains. Multiple issues intersect during this critical time: maternal mood and emotional well-being; infant care and feeding; sexuality, contraception, and birth spacing; sleep and fatigue; physical recovery from childbirth; and medications, substances, and exposures. By comprehensively addressing these interconnecting needs, service providers can improve health and well-being across two generations.