Abnormal Uterine Involution May Lead to Atony and Postpartum Hemorrhage: A Hypothesis, With Review of the Evidence.

Uterine involution has 2 major components-(1) involution of vessels; and (2) involution of myometrium. Involution of vessels was addressed by Rutherford and Hertig in 1945; however, involution of myometrium has received little attention in the modern literature. We suggest that the pathophysiology of myometrial involution may lead to uterine atony and postpartum hemorrhage. The myometrium dramatically enlarges due to gestational hyperplasia and hypertrophy of myocytes, caused by hormonal influences of the fetal adrenal cortex and the placenta. After delivery, uterine weight drops rapidly, with physiologic involution of myometrium associated with massive destruction of myometrial tissue. The resulting histopathology, supported by scientific evidence, may be termed "postpartum metropathy," and may explain the delay of postpartum menstrual periods until the completion of involution. When uterine atony causes uncontrolled hemorrhage, postpartum hysterectomy examination may be the responsibility of the perinatal pathologist.Postpartum metropathy may be initiated when delivery of the baby terminates exposure to the hormonal influence of the fetal adrenal cortex, and may be accelerated when placental delivery terminates exposure to human chorionic gonadotrophin (HCG). This hypothesis may explain why a prolonged third stage of labor, and delays in management, are risk factors for severe hemorrhage due to uterine atony.