The objective was to assess fetal, antenatal, and pregnancy determinants of unexplained antepartum fetal death. This is a hospital-based cohort study of 34,394 births weighing 500 g or more from January 1995 to December 2002. Unexplained fetal deaths were defined as fetal deaths occurring before labor, without evidence of significant fetal, maternal or placental pathology. Ninety-eight unexplained antepartum fetal deaths accounted for 27.2% of 360 total fetal deaths. Two-thirds of these deaths occurred after 36 weeks' gestation. The following factors are independently associated with unexplained fetal deaths: primiparity (OR 1.74; 95% CI 1.21, 2.86); parity of five or more (OR 1.19; 95% CI 1.26, 3.26); low socioeconomic status (OR 1.22; 95% CI 1.14, 2.86); maternal age 40 years or more (OR 3.62; 95% CI 1.22, 4.52); maternal age of 18 years or less (OR 1.79; 95% CI 0.82, 2.89); maternal prepregnancy weight greater than 70 kg (OR 2.20; 95% CI 1.85, 3.68); fewer than three antenatal visits in women whose fetuses died at 31 weeks or more (OR 1.11; 95% CI 1.08, 2.48); birth weight ratio (defined as ratio of birth weight to mean birth weight for gestational age) between 0.85 and 0.94 (OR 1.77; 95% CI 1.28, 4.18) or over 1.45 (OR 2.92; 95% CI 1.75, 3.21); trimester of first antenatal visit. Previous fetal death, previous abortion, cigarette smoking, fetal sex, low maternal weight, fetal-to-placenta weight, and post date pregnancy were not significantly associated with unexplained fetal deaths. Several factors were identified that are associated with an increased risk of unexplained fetal deaths.