The fourth trimester refers to the crucial three to six-months after birth when many of the physical, psychological, emotional, and social effects of pregnancy continue.
Resources
https://www.eurekalert.org/pub_releases/2019-05/uouh-olc050619.php
https://www.acog.org/Clinical-Guidance-and-Publications/Committee-Opinions/Committee-on-Obstetric-Practice/Optimizing-Postpartum-Care