Maternal age and risk of early neonatal mortality: a national cohort study.

Advanced maternal age (AMA) is a growing trend world-wide and is traditionally defined as childbearing in women over 35 years of age. The purpose of our study was to determine the maternal age group within the Korean population, in which the risk of early neonatal mortality is increased. Korean birth and mortality data from 2011 to 2015 were used to estimate the influence of maternal age on the risk of early neonatal mortality. A Poisson regression was used for the analysis of multiple clinical variables such as year of delivery, maternal age, gestational age, infant gender, birth weight, multiple birth, parity, and socioeconomic variables. Furthermore, a generalized additive model was used to determine the maternal age at which the risk for neonatal mortality increases. We included 2,161,908 participants and found that 49.4% of mothers were 30-34 years of age at delivery. The proportion of mothers aged 35 and above increased over the 5-year analysis period. A maternal age lower than 29 years or higher than 40 years was associated with a relatively higher risk of early neonatal mortality. The trend and magnitude of the age-related risk on early neonatal mortality were independent of maternal socioeconomic factors such as living in an obstetrically underserved area, education level, and employment status. Furthermore, we showed that the risk for early neonatal mortality was higher until the maternal age of 28. However, there were no significant changes in the risk between the age of 35 and 40 years. According to recent national-wide data, age-related risk for early neonatal mortality is only apparent for mothers ≥ 40 years old whereas, age between 35 and 39 are not at increased risk for early neonatal mortality, despite being classified as AMA.