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We performed a meta-analysis of all published studies of deep vein thrombosis during pregnancy and the puerperium using MEDLINE between 1966 and May 1998. Data were pooled using a random effects model. Fourteen studies included relevant information on deep vein thrombosis in pregnancy or the puerperium, and used objective testing to diagnose deep vein thrombosis. The pooled event rate for left sided or bilateral deep vein thrombosis was 82.2 percent (95 percent CI 75.1 to 87.5). There was no statistical evidence of heterogeneity for this figure (P = .08). Twelve studies reported on the trimester in which deep vein thrombosis was diagnosed. The deep vein thrombosis event rate during the first trimester was 21.9 percent (95 percent CI 17.4 to 27.3), 33.7 percent (95 percent CI 28.1 to 39.8) during the second trimester, and 47.6 percent (95 percent CI 39.2 to 56.2) for the third trimester. Heterogeneity testing was not significant. Nine studies compared deep vein thrombosis events between the antepartum and puerperium periods, with 65.5 percent (95 percent CI 58.1 to 72.1) arising during pregnancy, and 34.5 percent (95 percent CI 27.9 to 41.9) postpartum (P = .08, not heterogeneous). Using these figures, the estimated relative distribution of 100 deep vein thrombosis events during pregnancy and the puerperium would be 0.23 per day during pregnancy, and 0.82 per day in the postpartum period. During pregnancy and the puerperium, deep vein thrombosis is more likely to arise in the left leg. More than half of all deep vein thromboses during pregnancy occur during the first and second trimesters. Furthermore, during the puerperium, the risk of developing deep vein thrombosis is significantly higher than the antepartum.Obstetricians & Gynecologists, Family PhysiciansAfter completion of this article, the reader will be able to understand the incidence and distribution of DVTs during pregnancy and to be able to appreciate the differing risk of DVT during the various time periods in a pregnancy.