Definition of fourth trimester
: the three month period immediately following giving birth in which the mother typically recovers from childbirth and adjusts to caring for her infant The "fourth trimester" is a catchphrase used to describe the postpartum period, defined as the first 12 weeks after delivery.— Meera Varma Simões Women and their families experience substantial physiological, social, and emotional changes. … Most women would benefit from much closer follow-up during the fourth trimester.— Ilona T. Goldfarb especially : the first three months of an infant's life The concept of the fourth trimester—the three-month period of adjustment for babies to life outside of the womb—was introduced in the late 1970s and then popularized by pediatrician Harvey Karp … in 2002. — Maggie Overfelt
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