CONTEXT:The associations between timing of first live birth and previous sexual behavior and pregnancies are not well understood.
METHODS: Members of a 1972–1973 New Zealand birth cohort were surveyed at ages 21, 26 and 32 about their sexual and reproductive histories; 506 men and 479 women participated in at least one assessment. Relative risks and 95% confidence intervals were calculated using Poisson regression to examine associations between the likelihood of first live birth at specific ages (prior to age 21, at age 21–25, at age 26–31) and selected characteristics.
RESULTS: Birth prior to age 21 was more likely for men and women who initiated intercourse before age 15 (relative risks, 3.1 and 2.0, respectively), and less likely for those who initiated at age 18 or later (0.3 and 0.1, respectively), than for those aged 15–17 at first coitus. Prior miscarriage was associated (although sometimes marginally) with an elevated likelihood of first birth across genders and ages (1.7–1.8). Prior abortion was associated with an elevated likelihood of first birth at age 21–25 for women (1.6) and a reduced likelihood at age 26–31 for men (0.5). Having multiple sexual partners at age 21–25 was negatively associated with the likelihood of a first birth at age 26–31 for men. Marriage and cohabitation were positively associated with birth timing.
CONCLUSIONS: Early sexual initiation and relationship instability may promote parenthood at younger ages, whereas greater relationship stability may do so at older ages.