Background: Pregnancy is associated with a 10-fold increased risk of venous thrombosis (VT), with different risk profiles for the antenatal and postnatal periods. The purpose of this study was to assess the risk of pregnancy-related VT associated with the factor (F)V Leiden and prothrombin gene G20210A polymorphisms.
Materials and Methods: The study comprised 377 155 women with 613 232 pregnancies at 18 Norwegian hospitals from 1 January 1990 to 31 December 2003. Of a total 559 cases with a validated first lifetime diagnosis of VT in pregnancy or within 14 weeks postpartum, and 1229 controls naive for VT, 313 cases and 353 controls donated biological material.
Results: The odds ratios for VT during pregnancy or puerperium were 5.0 [95% confidence interval (CI) 3.1–8.3] and 9.4 (95% CI 2.1–42.4) for heterozygous carriers of the FV Leiden and the prothrombin gene polymorphisms, respectively. All homozygous carriers of the FV Leiden polymorphism (n = 8) and the prothrombin polymorphism (n = 1) developed VT, indicating a very high risk of VT. We estimated that pregnancy-related VT occurred in 1.1/1000 non-carriers, in 5.4/1000 heterozygous carriers of the FV Leiden polymorphism, and in 9.4/1000 heterozygous carriers of the prothrombin polymorphism. To avoid one VT, the number of pregnant women needed to be screened for these two polymorphisms and the number needed to be given thromboprophylaxis were 2015 and 157, respectively.
Conclusions: Although the relative risk for VT during pregnancy and after delivery was increased among carriers of the FV Leiden and the prothrombin polymorphisms, the overall probability for pregnancy-related VT was low.