Placental abruption tends to recur in later pregnancies, but its time of onset is unpredictable. Women with a past history of abruption frequently are admitted, or else begin intensive ambulatory antenatal care, using thegestational age of the previous episode as a starting point. The goal of this cohort study was to determine the best time during a subsequent pregnancy to begin intensive surveillance to prevent recurrent abruption. More than 1.8 million births in Norway from 1967 through 1998, representing 928,779 women, were included in the study if the pregnancy lasted at least 16 completed weeks. A total of 492,861 women were followed through their first two deliveries. Four cohorts were assessed: 1) women with a complicated placental abruption, 2) uncomplicated abruption, 3) complicated birth without placental abruption, and 4) uncomplicated birth without abruption. A complicated birth with or without placental abruption was defined as a case with preterm delivery, fetal growth restriction, or perinatal death. Placental abruption complicated 6.4 per 1000 deliveries in birth order 1 and 6.2 per 1000 in birth order 2. The 104 recurrent abruptions represented 38 per 1000 deliveries. The recurrence rate in cohort 4 was 4.6 per 1000, and that in cohort 3 was 8.8 per 1000 deliveries. Analysis of cohort 1 disclosed recurrent placental abruption in 41 per 1000 deliveries. In this group it was estimated that 27 complicated abruptions would have occurred outside the hospital or before starting special observation at the gestational age of the initial episode, compared with only 3 if observation began 12 weeks before. Cohort 2 had recurrences at a rate of 30 per 1000. Starting surveillance 12 weeks earlier would reduce the expected number of complicated abruptions from 14 to 3. When there is a past history of placental abruption complicated by preterm delivery, a small-for-gestationalage infant, or perinatal death, observation starting 6 weeks before the gestational age of the initial event may reduce the risk of recurrence to one approaching the initial risk. An even longer surveillance time may be appropriate after an uncomplicated term placental abruption.
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