Obese children tend to become obese adults, especially if their parents are obese (1,2). One study showed that 80% of children who were overweight atf12 years of age were obese at f30 years (3). Among a group of elderly men and women in Helsinki, those who were obese, deﬁned by a body mass index (weight/height2) of >30 kg/m2, had had above average body mass indices at the age of 7 years (4). Such ﬁndings indicate that adult obesity may be initiated or ‘‘entrained’’ during early life (5). There may be critical periods of development when this occurs. Critical periods, during which changes tend to be irreversible, may be distinguished from high-risk periods which occur throughout life and are associated with reversible increases in body weight. Pregnancy and times of emotional stress are examples of high-risk periods. There is evidence for three critical periods for body weight-prenatal, childhood, and adolescence. This chapter focuses on the prenatal period, describing how this may inﬂuence later obesity and how it interacts with obesity in childhood and adult life to determine later disease.