Subinvolution of the uterus refers to the failure of the uterus to return to its normal pre-pregnancy size within the expected postpartum period. Normally, the uterus undergoes a process called involution, where it contracts and shrinks after delivery. When this process is delayed or incomplete, it leads to subinvolution, which can cause prolonged vaginal bleeding and increase the risk of infection. The condition is often due to retained products of conception, infection, or inadequate uterine contractions but is distinct from excessive contractions or uterine rupture. Recognizing subinvolution is crucial for timely management to prevent complications.

Reference: Williams Obstetrics, Cunningham, 25th Edition.