Late childbearing and changing risks of adverse birth outcomes in Korea.

This study aimed to examine whether the relative importance of maternal age as a correlate of adverse birth outcomes has changed and to investigate if social inequalities in birth outcomes have widened during the past decade when the marriage and fertility related social environment has undergone tremendous change in Korea. Probabilities of adverse birth outcomes (prematurity and intrauterine growth retardation [IUGR]) were estimated with multinomial logistic regression models, utilizing the Korean birth registration data of 1995 and 2005. The main effects of maternal age and parental socioeconomic characteristics were compared between two study years, net of infant sex, birth order, and plurality. The association between maternal age and adverse birth outcomes, relative to the maternal and parental social characteristics, has clearly diminished between 1995 and 2005. During this period, differences in prematurity and IUGR by maternal age have also diminished, while those by parental social characteristics, particularly maternal education, have substantially widened. The intensified overall socioeconomic polarization since the economic crisis of the late 1990s is most likely responsible for the increased social inequality in adverse birth outcomes in Korea. A massive structural change in macro-economic conditions and culture during the study period may have modified the relationship between maternal age and birth outcomes.