Mating takes place in the autumn, just before hibernation, but the sperm does not fertilize the female's egg cells until the spring - this is called delayed implantation. Once the embryo starts to grow, its development depends on the weather and the food supply. It will stop developing if conditions are poor.
The female pipistrelles form their own nursery colonies in early summer, often in a roof space behind tiles. The males roost in separate small colonies during the summer, only joining the females during the autumn and winter months.
Gestation is normally 44 - 50 days and usually only one baby is born in June. The baby bats are very tiny, hairless and blind for about a week. Warmth is very important and the warmer they are the faster they grow. The young are weaned and ready to fly independently at 3 weeks old. The females reach sexual maturity in 1 year and the males in 2 years.
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