Trend in parental age at childbirth and comparison with the general population. (A, B) Box plots for paternal (A) and maternal (B) ages at childbirth between 1993 and 2010. The thick band inside the box is the median, and the bottom and top of the box are the first quantile (Q1) and the third quantile (Q3). The ends of the whiskers represents data within 1.5 *interquartile range (IQR) from the lower quantile (Q1) or the upper quantile (Q3), where IQR is equal to Q3 minus Q1. The fitted simple linear regression lines for parental age at childbirth are highlighted in red (paternal line: slope =1.6e-01 (95% confidence interval (CI): 1.2e-01 to 1.9e-01), P value =1.5e-15, maternal line: slope = 1.6e-01 (95% CI: 1.3e-01 to 1.9e-01), P value <2e-16). (C) Comparison of maternal age at childbirth of mothers of the Beijing Stars and Rain Education Institute for Autism (SR) children with a reported diagnosis of autism spectrum disorder (ASD) or ‘autistic tendencies’ and mothers in the general population. Fitted simple linear regression lines based on the SR data for maternal age at childbirth between 2000 and 2010 are represented by red thick solid lines and grouped by maternal education level, with 95% CI for the mean shown as red thin solid curves. The blue thick solid lines, based on 2000 and 2010 Chinese census data, connect the mean maternal ages at childbirth and are grouped by maternal education level. The blue thin solid segments show the 95% CI for the mean of each subset calculated using the mean and standard error.