For both cohorts, we obtained parental age at delivery from their children’s birth records. We classified maternal age into five categories: 1) younger than 25.0 years; 2) 25.0–29.9 years; 3) 30.0–34.9 years; 4) 35.0–39.9 years, and 5) 40.0 years and older. The Japan Society of Obstetrics and Gynecology considers 35 years or older as advanced maternal age for first-time births .