Objective : In pregnant women after mechanical heart valve replacement is risky for mother and fetus because required anticoagulant as prophylactic thromboembolism. Effective anticoagulants that are safe for babies and mothers are still a dilemma. Warfarin crosses the placental barrier and has a teratogenic effect. Low Molecular Weight Heparin (LMWH) is safer than warfarin but less effective for the prevention of thromboembolic events. 
Case : We reported the case of a 27 years old pregnant woman who LMWH (Enoxaparin) was given subcutaneously during the first trimester of pregnancy and switched over to warfarin on the second trimester. Warfarin was substituted to LMWH at 36 weeks until the baby born. The patient has just given birth to her second child. Her second child a male born by cesarean section at 36 to 37 weeks of pregnancy with a birth weight of 2300 g and body length of 47 cm. His APGAR (Appearance, Pulse, Grimace, Activity, and Respiration) scores were 7 at 1 min and 8 at 5 min and with respiratory rate 40 breaths per minute, cyanosis did not appear when the baby cried strongly. Both children were born with normal conditions without any abnormalities and no signs of bleeding. The first child 3 years old has grown and developed well.
Conclusion : The incidence of teratogenicity in the fetus is low when warfarin given in the first semester with low doses less than 5 mg per day and use of LMWH could be used as an alternative anticoagulant therapy during pregnancy, but giving anticoagulants to pregnant women with heart valves replacement as prevention of clot actually can cause intrauterine growth restriction (low birth weight) in the newborn.