Placental abruption (also called abruptio placentae) refers to bleeding at the decidual-placental interface that causes partial or total placental detachment prior to delivery of the fetus. The diagnosis is typically reserved for pregnancies over 20 weeks of gestation. The major clinical findings are vaginal bleeding and abdominal pain, often accompanied by hypertonic uterine contractions, uterine tenderness, and a nonreassuring fetal heart rate (FHR) pattern.
Abruption is a significant cause of maternal and perinatal morbidity, and perinatal mortality. The perinatal death rate is approximately 12 percent (versus 0.6 percent in non-abruption births) . The majority of perinatal deaths (up to 77 percent) occur in utero; deaths in the postnatal period are primarily related to preterm delivery [1-4]. However, perinatal mortality associated with abruption appears to be decreasing .
Placental abruption complicates 0.4 to 1 percent of pregnancies [5-7]. The incidence appears to be increasing, possibly due to increases in the prevalence of risk factors for the disorder and/or to changes in ascertainment of cases [7,8]. In one review, 40 to 60 percent of abruptions occurred before 37 weeks of gestation and 14 percent occurred before 32 weeks . However, gestational age-specific incidence rates vary considerably depending on the etiology [9,10].
PATHOGENESIS AND PATHOPHYSIOLOGY
The immediate cause of the premature placental separation is rupture of maternal vessels in the decidua basalis, where it interfaces with the anchoring villi of the placenta. Rarely, the bleeding originates from the fetal-placental vessels. The accumulating blood splits the decidua, separating a thin layer of decidua with its placental attachment from the uterus. The bleed may be small and self-limited, or may continue to dissect through the placental-decidual interface, leading to complete or near complete placental separation. The detached portion of the placenta is unable to exchange gases and nutrients; when the remaining fetoplacental unit is unable to compensate for this loss of function, the fetus becomes compromised.
The etiology of bleeding at the decidua basalis remains speculative in most cases, despite extensive clinical and epidemiologic research. A small proportion of all abruptions are related to sudden mechanical events, such as blunt abdominal trauma  or rapid uterine decompression, which cause shearing of the inelastic placenta due to sudden stretching or contraction of the underlying uterine wall [9,10]. In motor vehicle crashes, an additional factor is rapid acceleration-deceleration of the uterus, which causes uterine stretch without concomitant placental stretch, leading to a shearing force between the placenta and the uterine wall. Although even minor trauma may be associated with an increased risk of preterm birth, severe maternal trauma is associated with a six-fold increased risk of abruption .