Anticoagulation
Direct oral anticoagulants, so far, are prohibited in pregnancy as there is little information on maternal and fetal safety. The choice between therapeutic anticoagulation with low molecular weight heparin (LMWH) or vitamin K antagonist (VKA) is made according to the stage of pregnancy. Warfarin, a VKA, crosses the placenta and can cause fetal bleeding, including intracranial hemorrhage, and increase fetal anomalies, with the latter occurring mainly during the first trimester. Neither unfractionated heparin nor LMWH crosses the placenta, so these agents do not cause fetal bleeding or teratogenicity, although bleeding at the uteroplacental junction and fetal wastage are possible. Therefore, heparins are recommended for anticoagulation in the first trimester, being replaced by VKA after that. Women receiving VKA should be changed to either LMWH or unfractionated heparin after 36 weeks of gestation to reduce the risk of fetal hemorrhage at the time of vaginal delivery as well as delivery-associated maternal bleeding.