[d4mi3n]

This is part of it. I think the rise in fertility options plays a part here as well—many people are choosing to start families later in life than was realistic or possible 20 years prior. This has been a well observed trend that has been going strong since the 1980s.

If you're female and want to pursue a career, you can now do that and wait until later in life to have a family. Wasn't long ago where that choice was binary.