That is NOT a proper interview question!

I was asked that once in the 70's - about a PIANO job in a restaurant. The fear is that women will leave the job if and when they do have babies, and they will have to train someone else.

I think if they subsidized daycare, provided it, or even covered maternity leave, they would have less to worry about.

Check here:
http://www.collegegrad.com/ezine/23illega.shtml

From that page:
It should be noted that just because an illegal question has been asked does not necessarily mean a crime has been committed. Just because the question has [been] asked does not establish intent. It is up to a court of law to determine whether the question or any resulting information was used in a discriminatory manner.