Scientific Approach The Seychelles Child Development Study (SCDS) started in the mid-1980s and now includes several large cohorts of children whose mothers consumed fish frequently during their pregnancy. The study objective has always been to determine if prenatal MeHg exposure from fish consumption has adverse effects on the children’s neurodevelopment. Another objective has been to document child development in the Seychelles. The Seychelles was selected because fish consumption in the islands was high and indeed the mothers who were enrolled consumed more than 8 meals containing fish per week. The Seychelles also presented several advantages for an epidemiology study looking for subtle differences in developmental outcomes. These included being a Westernized developing nation; the initiation in about 1978 of a universal, readily accessible and free system of health care and education; limited industrial development with no local sources of pollution; and low alcohol consumption among women. Mercury exposure has been measured in maternal hair growing during pregnancy and in child hair at various points after birth. The Pilot cohort is comprised of 789 children who joined the study in 1986 and who have been intermittently followed for child development ever since. The Main cohort was recruited in 1989 and is comprised of 779 children. This cohort has been tested for developmental outcomes at 6, 19, and 29 months and again at 5.5, 9, 10.5, 17, 19, 22 and 24 years of age. A subsequent Nutrition cohort (NC 1) was developed to test the hypothesis that nutrient and dietary status of the mother during pregnancy modulates the neurotoxicity of methylmercury. In 2001, a cohort of 300 mothers was enrolled during their first trimester of pregnancy. At enrollment and delivery we obtained hair and blood from the mothers for Hg analysis and cord blood from the infants. A variety of nutritional factors that might influence child development were measured in the mother’s blood. Child development of the offspring was tested at 5, 9, 25, and 30 months, and at 5, 8 and 10 years of age. Most recently, a second Nutrition cohort (NC 2) was enrolled to follow up on findings from the NC 1 cohort. The larger cohort was needed to test the hypothesis that certain micronutrients in fish may be beneficial for child development and protective against the neurotoxic effects of methylmercury. Between 2008 and 2011, a cohort of over 1,500 mothers was enrolled during pregnancy. Similar to the NC 1 cohort, hair and blood were obtained from mothers and cord blood from the infants for the analysis of Hg and various nutritional and genetic factors that might influence child development and / or methylmercury toxicity. The cohort has been tested for developmental outcomes at 20 months of age, and will be tested again at 7 years of age.