A pregnant woman with prosthetic valve switches to heparin at :

Anticoagulants are necessary in cases of congenital hea disease who have pulmonary hypeension, aificial valve replacements or atrial fibrillation. The patient taking warfarin should discontinue it as soon as pregnancy is diagnosed and replace by heparin 5000 units twice daily subcutaneously up to 12th week. This is then replaced by warfarin tablet 3 mg. daily to be taken at the same time each day and continued up to 36 weeks. Thereafter it is replaced by heparin upto 7 days postpaum. Warfarin is then to be continued. Reference: Dutta's Textbook of Obstetrics 7th edition page 277