At a cellular level, involution is characterized by the process of proteolysis of the basement membrane (basal lamina), leading to epithelial regression and apoptosis, with accompanying stromal fibrosis.
The thymus continues to grow between birth and sexual maturity and then begins to atrophy, a process directed by the high levels of circulating sex hormones.
It is a physiological process occurring after parturition; the hypertrophy of the uterus has to be undone since it does not need to house the fetus anymore.
The completion of this period is defined as when the diameter of the uterus returns to the size it is normally during a woman's menstrual cycle.
[citation needed] During pregnancy until after birth, mammary glands grow steadily to a size required for optimal milk production.