694 Stillbirths in which no cause of death can be found have contributed substantially to perinatal mortality rates in developed countries in recent decades. 1 These deaths constitute a major challenge for obstetricians, both in terms of fetal care and in terms of the need for information and counseling of the affected couples. Nev- ertheless, little is known of the epidemiologic character- istics of unexplained antepartum stillbirths. A few au- thors have approached the epidemiologic details of unexplained stillbirths specifically, 2-6 and it has been proposed that this could be a continuum of the sudden infant death syndrome. 2 In sudden infant death syn- drome a definition that included the obvious need for a thorough postmortem examination was established in 1969, 7 but there is no such consensus on the criteria for the use of the term unexplained stillbirth. The earlier re- ports on unexplained stillbirth have unfortunately in- cluded both unexplained and unexplored cases without postmortem examinations, and the causes of death were in some cases collected from unreliable death certifi- cates. 3, 4 The classification of a fetal death as a sponta- neous abortion, an antepartum stillbirth, an intrapartum stillbirth, or an early neonatal death not only is difficult but also may vary with the personal view of the reporting physician. Finally, there has been no attempt to differen- tiate between the unexplained deaths of singleton fe- tuses and the more complex situation of unexplained deaths of multiple fetuses. We propose a definition of sudden intrauterine unex- plained death. The aim of this study was to identify risk factors for this type of fetal death and to compare them with risk factors for other types of stillbirth. We per- formed a double case-control study comparing cases of validated singleton sudden intrauterine unexplained death with controls consisting of both live births and ex- plained singleton stillbirths occurring in the city of Oslo, Norway, and also the total population of singleton births in Norway. From the Department of Obstetrics and Gynecology, the Department of Pe- diatric Research, the Institute of Forensic Medicine, and the Section for Clinical Epidemiology, The National Hospital, University of Oslo. Supported by the Norwegian Women’s Public Health Association. Received for publication December 2, 1999; revised July 12, 2000; accepted August 8, 2000. Reprint requests: J. Frederik Frøen, MD, Department of Pediatric Research, The National Hospital, University of Oslo, Pilestredet 32, N-0027 Oslo, Norway. Copyright © 2001 by Mosby, Inc. 0002-9378/2001 $35.00 + 0 6/1/110697 doi:10.1067/mob.2001.110697 Risk factors for sudden intrauterine unexplained death: Epidemiologic characteristics of singleton cases in Oslo, Norway, 1986-1995 J. Frederik Frøen, MD, Marianne Arnestad, MD, Kathrine Frey, MSc, Åshild Vege, MD, PhD, Ola Didrik Saugstad, MD, PhD, and Babill Stray-Pedersen, MD, PhD Oslo, Norway OBJECTIVE: The epidemiologic characteristics of unexplained stillbirths are largely unknown or unreliable. We define sudden intrauterine unexplained death as a death that occurs antepartum and results in a stillbirth for which there is no explanation despite postmortem examinations, and we present risk factors for this type of stillbirth in singleton gestations. STUDY DESIGN: Singleton antepartum stillbirths (n = 291) and live births (n = 582) in Oslo were included and compared with national data (n = 2025 and n = 575,572, respectively). Explained stillbirths (n = 165) and live births in Oslo served as controls for the cases of sudden intrauterine unexplained death (n = 76) in multi- ple logistic regression analyses. RESULTS: One fourth of stillbirths remain unexplained. The risk of sudden intrauterine unexplained death (1/1000) increased with gestational age, high maternal age, high cigarette use, low education, and over- weight or obesity. Primiparity and previous stillbirths or spontaneous abortions were not associated with sud- den intrauterine unexplained death. CONCLUSIONS: Risk factors for sudden intrauterine unexplained death are identifiable by basic antenatal care. Adding unexplored stillbirths to the unexplained ones conceals several risk factors and underlines the necessity of a definition that includes thorough postmortem examinations. (Am J Obstet Gynecol 2001;184:694-702.) Key words: Unexplained stillbirth, antepartum, epidemiology, smoking, obesity