The Effect of Maternal Age at the First Childbirth on Gestational Age and Birth Weight: The Japan Environment and Children's Study (JECS)

Background: In Japan, mean maternal age at first childbirth is increasing. The aim of this study was to investigate whether maternal age at the first childbirth is a risk factor for preterm birth (PTB), low birth weight (LBW), and small for gestational age (SGA). Methods: We used the results of Japan Environment and Children's Study (JECS) who gave birth in 2011-2014. Cases of primiparous singleton pregnancies where the subject was >= 20 years and delivered after 22 weeks were included. All subjects were categorized into five groups according to maternal age: 20-24, 25-29, 30-34, 35-39, and >= 40 years. Adjusted odds ratios (aORs) for PTB (before 37 and 34 weeks), LBW (<2,500 g and <1,500 g), and SGA were calculated using a logistic regression model, with the 20-24-year age group as reference. Results: We analyzed 38,412 singleton primiparous pregnancies. The aORs of all outcomes increased in parallel with each maternal age group >30 years. The aORs of PTB before 37 and 34 weeks, LBW <2,500 g, LBW <1,500 g, and SGA in the 30-34-year age group were 1.39 (95% confidence interval [CI], 1.16-1.67), 2.23 (95% CI, 1.45-3.41), 1.34 (95% CI, 1.18-1.53), 2.30 (95% CI, 1.35-3.94), and 1.24 (95% CI, 1.05-1.46), respectively. Conclusion: The present study showed that higher maternal age (>30 years) at the first childbirth was an independent risk factor for PTB, LBW, and SGA.