In the U.S., birth rates fell pretty much continuously for at least 150 years until WWII and the Baby Boom. Now they are slightly declining again. Many industrialized nations have a similar graph.

So that pretty much kills any explanation that depends on our recent experience, like pressure to get a degree, good job, etc. Seems like there is probably something far more fundamental at work to create a 200+ year trend with a one brief interruption for a couple decades.