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-alpha2 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 x 109/l) at 28.6 +/- 1.0 weeks decreased over time to 91 +/- 12% and 205 +/- 59 x 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 < 0.01), and this increase correlated significantly with the decrease in antithrombin III activity in twin pregnancies (r = -0.459, p < 0.001).
CONCLUSIONS: Antithrombin III activity and platelet count show a greater decrease in twin than in singleton pregnancies, perhaps due to a greater increase in thrombin generation.