Antithrombin III activity and platelet count are more likely to decrease in twin pregnancies than in singleton pregnancies

Background. It is not known whether antithrombin III activity and platelet count are more likely to decrease in women with twin pregnancies than singleton pregnancies. Methods. We enrolled 56 consecutive women with twin pregnancies and 692 consecutive women with singleton pregnancies. Antithrombin III activity and platelet count were determined at 26–31 weeks and again at 32–40 weeks of gestation. Thrombin–antithrombin complex, and plasmin–α2 plasmin inhibitor complex were measured simultaneously in some women. Results. In twin pregnancies, both antithrombin III activity (111 ± 8.2%) and platelet count (244 ± 60 × 109/l) at 28.6 ± 1.0 weeks decreased over time to 91 ± 12% and 205 ± 59 × 109/l, respectively, at 35.2 ± 1.2 weeks; these decreases were significantly greater than in singleton pregnancies. However, a small number of women with singleton pregnancies had a significant decrease in antithrombin III activity in the absence of preeclampsia. The serum thrombin–antithrombin complex was significantly higher in twin pregnancies than in singleton pregnancies (9.7 ± 4.2 ng/ml vs. 6.7 ± 4.4 ng/ml, respectively, p