1. If the population seriously crashes, eventually the technology level will also crash. Once birth control and abortion are no longer technologically as easy as they are now, well, people still like having sex. So there's going to be a lot of kids again, once the tech to prevent them is not as ubiquitous as it is now.

2. Once the population craters, the resources available to each person go up. In that kind of environment, populations typically grow.

But:

3. This assumes that the reason the population crashes is people choosing not to have kids. If it crashes due to environmental reasons (estrogenoid pollution, say, making males less eager to have sex and less able to reproduce), then my arguments don't work.