Etiology of primary postpartum hemorrhage

Physiological control of postpartum bleeding occurs by contraction and retraction of the interlacing myometrial fibers surrounding maternal spiral arteries of the placental bed. Myometrial contraction compresses the spiral arteries and veins, thereby obliterating their lumens; hemostasis following placental separation is initially a mechanical process that is not primarily dependent upon an intact coagulation system. Primary postpartum hemorrhage due to uterine atony occurs when the relaxed myometrium fails to constrict these blood vessels, thereby allowing hemorrhage. Since up to a fifth of maternal cardiac output, or 1 l/min, enters the uteroplacental circulation at term, primary postpartum hemorrhage is capable of exsanguinating the mother within 5 min.

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