Comment by cyberax

Yes, but it happened in stages. The fertility fell each time people moved into higher density areas.

You can even see mild recovery when de-densification happens. It's very interesting to compare the fertility rate in Denmark and Netherlands:

You can see the dip and a recovery in Denmark and essentially no recovery in Netherlands (until post 2000, but that was due to immigration). Why?

Here's the answer:

Denmark de-densified its cities during the late 70-s (that's why Copenhagen is the world's most liveable city, btw).