I called Air France, Emirates, and Northwest (the most likely carriers for us), and they all said that an infant ticket can only be purchased after the birth. We can book the other family members' tickets before then, and add the infant later, since she won't need a seat.
An infant fare is 10% of the adult fare at the time of purchase, so it might be higher than 10% of the adult fares we ticket now.
Thanks for your responses!