This study examines the probability of twins by birth year, maternal race–ethnicity, age, and parity and the influences of these demographic factors on the probability of male in twins and singletons in a large, racially diverse population. Recent publications note steep increases in twin births while the probability of male births has been reported to vary by parental race–ethnicity and age and birth order. Probability of male stratified by plurality has not been investigated in California prior to this study. Cubic spline estimates and Poisson regression techniques were employed to describe trends in twins and males using California vital statistics birth and fetal death records over the period from 1983–2003. This study includes 127,787 twin pair and 11,025,106 singleton births. The probability of twins varied by birth year, maternal race–ethnicity, age, and parity. The probability of twins increased by 10.1% from 1983–1992 and increased by 20.1% from 1993–2003, nearly doubling the previous increase. All maternal race–ethnicity groups showed increases in probability of twins with increasing maternal age. Parous women compared to nulliparous women had larger increases in the probability of twins. The probability of males in twins decreased from 1983–1992 and increased from 1993–2003; while in singletons the probability appeared unchanged. These findings show increases in the probability of twins in California from 1983–2003 and identify maternal age, race–ethnicity, and parity groups most likely to conceive twins. The cause of the increase in twins is unknown but coincides with trends towards delayed childbearing and increased use of subfertility treatments.