I have many female friends in their mid-30s and early 40s, and the ones without kids are almost always sad/regretful about it themselves. Some won't admit it, but some will, especially in more intimate settings. I've had one cry on my shoulder - she is very successful professionally, but feels she missed out on the opportunity for motherhood.
Whether it was her "choice" is hard to say. Very few people get the chance to engineer their lives the way they want. Most people instead play the cards they are dealt. And now that there are many more different types of possible cards in the deck for women, they are coming to the understanding that it is possible to be dealt a bad hand, and not realize it until it is too late.
The issue is that there's no objectivity here. Are these women actually sad because they truly wanted to experience motherhood, or is it more because of pressure (even unconscious pressure) from family and friends to have kids? Or just the built-in feelings due to upbringing that having kids is "just what you're supposed to do".
I wonder, though, if these same women did have kids, and sacrificed their career for it, would they look back and have regrets in that regard, too.
So maybe it's just... given a multitude of options where we can't choose them all, perhaps humans will just naturally have some regrets around the path(s) not taken?
As counterpoint, I know women in their 40s and older who are very pleased with their decision to not have kids.
Whether it was her "choice" is hard to say. Very few people get the chance to engineer their lives the way they want. Most people instead play the cards they are dealt. And now that there are many more different types of possible cards in the deck for women, they are coming to the understanding that it is possible to be dealt a bad hand, and not realize it until it is too late.