This study tested the association between multiple prenatal and postnatal early life factors and adolescent sexual orientation in a longitudinal birth cohort. Factors included birth weight, gestational age, parental age at birth, number of older brothers and sisters, breastfeeding, maternal anxiety/depression, family socioeconomic position, parent-child relationships, parental absences, pubertal body mass index, and housing issues. We used data on 5,007 youth from the Avon Longitudinal Study of Parents and Children (ALSPAC). Sexual orientation was assessed using a 5-point scale of sexual attraction at 15.5 years. Early life factors were separated into three developmental periods: prenatal (n = 9), before 7 years (n = 5), and after 7 years (n = 5). We controlled for childhood gender nonconformity, handedness, and digit ratio as markers of prenatal androgen exposure. Gender nonconformity was strongly associated with later male and female nonheterosexuality, and higher right-hand digit ratio was associated with later male nonheterosexuality. Boys with low birth weight and shorter breastfeeding duration were more likely to have a later nonheterosexual orientation. Boys with parental absence before 7 years of age were more likely to be nonheterosexual, but this effect disappeared after entering all early life history factors. Parental absence since birth, low prenatal family socioeconomic position, and poorer parent-child relationship were associated with later nonheterosexuality among girls. The results are discussed in the context of a life history framework for understanding human sexual orientation development in males and females.