Uterine atony is a loss of tone in the uterine musculature. Normally, contraction of the uterine muscle, compresses the vessels an reduces flow. This increases the likelihood of coagulation and prevents bleeds. Thus, lack of uterine muscle contraction can cause an acute hemorrhage. Clinically, 75-80% of postpartum hemorrhages are due to uterine atony. Many factors can contribute to the loss of uterine muscle tone, including: