"Things You Wouldn't Know If We Didn't Blog Intermittently."
One in five women aged 40 to 44 reported that they’ve never had children. Meanwhile, just 41 percent of Americans say having children is necessary to a good marriage, compared to 65 percent in 1990.
In my interpretation (and I'm not by any means a sociologist), the reason for the reduction in slope is that formerly, a woman would have had to work extremely hard (harder than a man in the same position) and sacrifice other aspects of her life to achieve a PhD or professional degree. More recently, it has become "easier" for a woman to obtain one, and as a result more women are able to balance this with procreation.
I obtained my PhD at 27; there would have been time enough to procreate. When asked I say it takes a smart woman to make wise decisions.