More mothers are waiting until later in life to have children, but pregnancy after 50 is still quite rare.
A woman's fertility begins to decline in her early 30s. After age 35, her number of viable eggs starts to fall more quickly.
Women are born with a set amount of follicles - between one and two million - or immature eggs.
These develop at different rates. Some follicles never release an fully-fledged egg, but each month the ovaries release one egg to wait for fertilization in the uterus.
Most women only ovulate around 450 eggs over the course of their lives, so by their late 40s, most have run out and start to head toward menopause.
Menopause begins around age 51 for the average woman, at which point pregnancy becomes impossible.
However, even as fertility is declining among American women (and men) overall, one group is having a few more babies than they did in the past: older