The 5e Guide to Sex has condoms and certain herbs as reasonably effective, but not perfect, birth control methods. The herbs give disadvantage to fertility checks, while the condoms aren't up to our modern standards, but give what is essentially super disadvantage to fertility checks.<br/><br/>Any exposure to necrotic energy also works as an instant prophylactic/termination, so there could easily have been enough access to Chill Touch cantrips available to avoid too much in the way of pregnancies at the College of Magic. Whether or not Muula is pregnant, on the other hand, is probably up to you.