Have you taken a tour? If not, scheduling a tour is a good time to ask. I did not start actively calling until my baby was born, at which point I said something to the effect of, "I'm calling to let you know that our little girl has arrived. Her name is X, and she was born on Y date. Can you tell me more about the movement of the waitlist and the likelihood of a space opening up on Z date?"
Then I called once a month after that until I got an answer. They get these calls all the time. After a while, I stopped even pretending to have a legitimate reason to call and started saying "Well, it's an alternate Wednesday! Do you have any good news for me this week?" They didn't, until close to the end of my maternity leave, but we did have a good laugh.