Without fibrinogen replacement, pregnancies in patients with congenital afibrinogenemia end in miscarriage between 6 and 8 weeks of gestation. Studies in animal models suggest that disturbance of the embryo's decidual attachment contributes to miscarriage in afibrinogenemia. Ectopic pregnancy in human congenital afibrinogenemia has not been previously documented. We present a case of right interstitial pregnancy in a woman with congenital afibrinogenemia, who had not received the recommended fibrinogen replacement for the first 6 weeks of pregnancy. The pregnancy followed a right salpingo-oophorectomy for ovarian hemorrhage. Therefore the embryo most likely entered the uterine cavity via the left tube only to move upstream into the lumen of the interstitial portion of the right tube. Our experience suggests that disturbance of the embryo's decidual attachment may have contributed to the development of an ectopic pregnancy. Therefore, fibrinogen replacement early in the first trimester is of utmost importance.