What is the average age that women start having children in the U.S.?
Advanced maternal age
Kalberer et al.[16] have shown that despite the older maternal age at birth of the first child, the time span between the birth of the first and the second child (= interpregnancy interval) decreased over the last decades. If purely biological factors were at work, it could be argued that interpregnancy interval should have increased, as fertility declines with age, which would make it harder for the woman to get a second child after postponed birth of the first one. This not being the case shows that sociologic factors (see above) prime over biological factors in determining interpregnancy interval.