But whenever there is a baby in a scene (say a birth) Star Wars Episode 3 comes to mind. I've been watching Fiddler on th roof (it was a class history film that I had to watch) towards the end there was a baby and it was obviously making baby sounds. I was wondering are those sound effects or recorded on set?

I would be blow away if wasn't added in post production.

I agree with ydoolz. 95% of the time - it's in post.

But, watch the mouth. If you don't see it - they probably did it in post. If you see the mouth moving and the baby's expression matches the sounds - probably managed to catch it on set.

It is, of course, unethical to MAKE a baby cry on cue.