A total of 2,828,068 State of Texas singleton conceptions, estimated from the date of the first day of the last menstrual period, were assessed for annual periodicity by multiple-component (1-yr fundamental plus the 6- and 3-month harmonics) Cosinor analysis. An annual pattern (p < 0.001) of conception was detected with a December peak, end of July-beginning of August trough and modest amplitude (total peak-to-trough variation) equal to 13.3% of the yearly mean. Annual patterns (p < 0.005) of generally comparable peak and trough times were also documented for conceptions categorized by maternal marital status, age, ethnicity, years of education, birthplace, and county (Texas-Mexico border vs. non-border) of residence. The amplitude, a measure of the prominence of the annual pattern, varied by race (two-fold greater for non-Hispanic Blacks and Hispanics than non-Hispanic Whites), age (50-60% greater for the 19-year-old and younger group than the 20-29 and 30-44-year-old groups) and maternal education (two-fold greater for those with 0-8 than 13 or more years of schooling). Annual patterns (p < 0.001) in conceptions, generally with December peak and end of July-beginning of August trough, were also detected for neonatal variables categorized by birth weight, birth order, gestational age, and gender. The amplitude of the annual pattern in conceptions varied directly with gestational age and inversely with birth weight class.