The population-based epidemiologic study was designed to evaluate potential adverse reproductive outcomes associated with prenatal exposure to Iodine-131 as a result of radioactive releases from the April 1986 Chernobyl nuclear accident. The study population is a well-defined cohort of 2,582 mother-child pairs from contaminated areas of northern Ukraine, originally assembled to investigate the risk of benign and malignant thyroid disease through in-depth clinical screening examinations of the offspring and structured interviews with the mothers carried out in 2003-2006. Data for the current study of reproductive outcomes were ascertained by retrieving and reviewing cohort members' prenatal, delivery and newborn records. Data were recorded on an Abstract Form designed with the help of study gynecologists and registered in analytic data base. Individual estimates of fetal thyroid Iodine-131 dose are available for all subjects. Statistical analyses to examine the relationship of fetal I-131 dose with neonatal anthropometrics and length of gestation have been carried out on a subset of cohort members with complete data (N=1167). Preliminary results show dose-dependent relationships with head circumference and gestational length. We are currently carrying out more extensive analyses to pursue these initial findings. We are also extending the work to explore radiation effects on reproductive function/fertility among the in-utero exposed offspring.