What is the pathophysiology of postpartum hemorrhage (PPH)?

Updated: Jun 27, 2018
  • Author: John R Smith, MD, FACOG, FRCSC; Chief Editor: Ronald M Ramus, MD  more...
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Over the course of a pregnancy, maternal blood volume increases by approximately 50% (from 4 L to 6 L). The plasma volume increases somewhat more than the total RBC volume, leading to a fall in the hemoglobin concentration and hematocrit value. The increase in blood volume serves to fulfill the perfusion demands of the low-resistance uteroplacental unit and to provide a reserve for the blood loss that occurs at delivery. [7]

At term, the estimated blood flow to the uterus is 500-800 mL/min, which constitutes 10-15% of cardiac output. Most of this flow traverses the low-resistance placental bed. The uterine blood vessels that supply the placental site traverse a weave of myometrial fibers. As these fibers contract following delivery, myometrial retraction occurs. Retraction is the unique characteristic of the uterine muscle to maintain its shortened length following each successive contraction. The blood vessels are compressed and kinked by this crisscross latticework, and, normally, blood flow is quickly occluded. This arrangement of muscle bundles has been referred to as the "living ligatures" or "physiologic sutures" of the uterus. [5]

Uterine atony is a failure of the uterine myometrial fibers to contract and retract. This is the most important cause of PPH and usually occurs immediately following delivery of the baby, up to 4 hours after the delivery. Trauma to the genital tract (ie, uterus, uterine cervix, vagina, labia, clitoris) in pregnancy results in significantly more bleeding than would occur in the nonpregnant state because of increased blood supply to these tissues. The trauma specifically related to the delivery of the baby, either vaginally in a spontaneous or assisted manner or by cesarean delivery, can also be substantial and can lead to significant disruption of soft tissue and tearing of blood vessels.

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