Objective: To study the associations between low maternal age at first birth and the risks of very and moderately preterm birth, in first birth and in second successive birth in adulthood.
Design: Population-based cohort study.
Setting: Sweden. POPULATION Women aged less than 25 years at first birth (n = 275,933), having two successive live single births from 1973 through 1993.
Methods: Odds ratios with 95 percent confidence intervals were calculated to estimate the effect of low maternal age at first birth. Analyses of first births were adjusted for year of first birth and maternal education and in second births also for previous pregnancy outcomes and interpregnancy interval.
Main outcome measures: Very preterm birth (less than 33 completed weeks) and moderately preterm birth (33-36 completed weeks).
Results: Compared with women aged 20 to 24 years at first birth, mothers aged 13 to 15 years were at increased risk of very preterm birth (odds ratio = 4.8). The corresponding risks among women aged 16 to 17 years at first birth were doubled (odds ratio = 2.3). The influence of maternal age on risks of moderately preterm birth was similar, although the age-related risks were lower. At second birth, risks of preterm birth were reduced in all age groups. However, mothers who were 17 years or less at first birth, faced, compared with mothers aged 20-24 years, significantly larger reduction in risks of very and moderately preterm birth.
Conclusions: Our results suggest that there may be a biological effect of very young maternal age, affecting foremost very preterm birth.