Many people explain low fertility rates in the West as a fruit of women finally having control over how many babies they have.

But there's evidence that the birth dearth is explained by a large portion of women not having as many babies as they want.

Pew looks at surveys of European women near the end of their childbearing years and finds that while most have as many children as they want, about 30 percent have fewer than they would want. Only ten percent have more.

In the U.S., 40 percent of women near the end of their childbearing years have fewer children than they would like.

Now, it's one study, and maybe moms are unwilling to say they regret one of their kids, but it strikes me as plausible.

Possible causes: Many families think they can't afford as many kids as they would like. Also, women's and men's bodies don't always comply with their wishes. Pew explains that timing is an issue:

So what’s driving this gap between ideal and actual family size? Among others things, delays in childbearing, which may be caused by increases in educational attainment, or by the lack of a suitable partner, may play a role. Starting childbearing at a later age means that there are fewer years for a woman to meet her fertility ideals, plus it increases the risk of age-related infertility.