Uterine atony

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Uterine atony is a loss of tone in the uterine musculature. Normally, contraction of the uterine muscle, compresses the vessels and 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:

- overdistention of the uterus
- multiple gestations
- polyhydramnios
- fetal macrosomia
- prolonged labor
- oxytocin augmentation of labor
- grand multiparity (having given birth 5 or more times)
- precipitous labor (labor lasting less than 3 hours)
- magnesium sulfate treatment of preeclampsia
- chorioamnionitis
- halogenated anesthetics
- uterine leiomyomata

(Hacker, Neville, J. G. Moore, and Joseph Gambone. Essentials of Obstetrics and Gynecology. 4th ed. Vol. 1. Philadelphia: Elsevier Inc., 2004. 151.)