['To determine the role of factor V Leiden and prothrombin gene mutation in the pathogenesis of unexplained second and third trimester nonrecurrent fetal loss.', 'One hundred and fourteen women with unexplained nonrecurrent late fetal loss made up the study group, and 106 normal pregnant women with a history of delivery of at least one healthy fetus and no history of late fetal loss made up the control group. The study group was further divided into two subgroups: second (n = 36) and third (n = 78) trimester fetal loss. All women were tested for factor V Leiden and G20210A prothrombin gene mutations.', 'Twenty-one (18.4%) of the women in the study group and seven (6.6%) of the women in the control group were heterozygous carriers of factor V Leiden mutation (OR = 3.19). Eleven (9.6%) of the women in the study group and three (2.8%) of the women in the control group were heterozygous carriers of prothrombin gene mutation (OR = 3.66). In assessing with regard to trimesters, 18 (23%) factor V Leiden and 10 (12.8%) prothrombin gene mutations were present in the group of third trimester fetal loss (OR = 4.24 and OR = 5.04, respectively). Three (8.3%) factor V Leiden and one (2.7%) prothrombin gene mutation were detected in women with second trimester fetal loss (OR = 1.28 and OR = 0.40, respectively).'] Is third trimester nonrecurrent fetal loss associated with factor V Leiden and prothrombin gene mutations?