The postpartum period, also known as the postnatal period, puerperium and the “fourth trimester,” refers to the time after birth when the physiologic changes related to pregnancy return to the nonpregnant state.

Most medical professionals agree that the postpartum period begins once the baby is born. However, the end of this period is not concrete. It’s generally considered to be the 6 to 8 weeks after birth because most of the pregnancy effects on the mother’s body have returned to normal by this time. UpToDate

Having said this, because not all of the organ systems have returned to normal, the American College of Obstetricians and Gynecologists considers the postpartum period to last up to 12 weeks, even up to 12 months for some individuals. NIH: National Library of Medicine

Note: The terms “postpartum period” and “postnatal period” are often used interchangeably. However, if they are used separately, “postpartum” would refer to the mother and “postnatal” would refer to the baby. NIH: National Library of Medicine