Objective: To determine whether there is a summer peak in conception of births to adolescents (up to 17 years) compared with older teenagers (18-19 years) and adults (20-29 years), and to assess the influence of season of conception on late initiation of prenatal care. Methods: We analyzed 1,178,607 singleton births to women aged 29 years and younger in Texas between 1994 and 1998. Dates of conception were estimated using last menstrual period and clinical estimates of gestation. Proportions of births conceived per month were assessed for seasonal patterns. Proportions of births with late initiation of prenatal care were also compared by month of conception. The outcomes were further stratified by sociodemographic variables. Results: There was a consistent summer trough (7.5% in August) and year-end peak (9.1% in December) in conception of births to adolescents (P < .001), a pattern similar to that of older teenagers and adults. Among the adolescents, students and non-Hispanic women giving birth presented a secondary early summer peak (8.8% each in May and June) in their conceptions. There was a modest bimodal effect of season of conception on initiation of prenatal care in all age groups. Adolescent conceptions in April-May and September-October were 14-18% and 6% significantly more likely to have late prenatal care compared with other months, respectively. Conclusion: Adolescents giving birth in Texas were not more likely to conceive in the summer. They did present seasonal patterns of conception and late initiation of prenatal care similar to older women. © 2001 by The American College of Obstetricians and Gynecologists.