Currently unfractionated heparin (UH) and low molecular weight heparins (LMWH) are the agents of choice for anticoagulation in pregnancy. LMWH have been used safely without monitoring in nonpregnant patients; however, because of documented changes in the pharmacokinetics of these agents in pregnancy, monitoring with anti-Xa levels is necessary in pregnancy to maintain target therapeutic ranges. Patients requiring only prophylaxis during pregnancy with either UH or LMWH might benefit from occasional assessment of anti-Xa levels to confirm that target prophylactic ranges are being achieved. Although LMWH may cause less osteoporosis than UH at therapeutic doses, the incidence of heparin-induced osteoporosis seems to be low when only prophylactic dosing is used and therefore LMWH do not seem to offer this advantage at low doses. Experience with newer agents such as pentasaccharide inhibitors and direct thrombin inhibitors are limited in pregnancy and it remains to be seen what role these agents will play in women who require anticoagulation in pregnancy.