Bats are mammals, not birds, so they do not lay eggs. The female bat becomes pregnant after mating with a male, just as with any other mammal.
Female bats are pregnant for 40 days up to 6 months, depending on the size of the bat. They generally give birth to one baby bat (called a pup) at a time. The pup hangs on the mother's stomach and drinks her milk until it is ready to fly on its own. This can take anywhere from 6 weeks to 4 months. Several female bats form subcolonies in which they help care for each other's young.