A more appropriate name for a bat nursery is a maternity roost.
Most bat species give live birth to only one pup per year. However, some species can give birth to twins or multiple pups. For 3 to 4 weeks, the baby is incapable of flying and is latched onto its mother’s nipple. It is not until they are about five weeks old that the baby bat can begin flying lessons and leaves its roost.
Ok, I found a video of a bat giving birth. Edited my original comment to correct that they hang upside down. The video is awesome.