Official websites use .gov
A
.gov website belongs to an official
government organization in the United States.
Secure .gov websites use HTTPS
A lock (
) or https:// means you've safely
connected to the .gov website. Share sensitive
information only on official, secure websites.
Seasonal variation in births by mothers’ age. Data are for period (a) 1940–1959 (total number of births N = 1 909 300), (b) 1960–1979 (N = 2 009 475), (c) 1980–1999 (N = 1 802 088) and (d) 2000–2012 (N = 1 047 947). –––– <25, ······ 25–29, - - - - 30–34, 
35+ (error bars are 95% CI).
