Guppies are usually able to first breed by the age of three months. One mating can fertilize several batches of live-born fry. After breeding, the babies develop over one to two months depending on water temperature and the health of the mother. She can give birth early or delay birth depending on her health and stress level. It is common for newly bought female guppies to give birth months after being obtained even when kept in a tank devoid of males.
Females have a gravid spot which is a dark spot near their vent.
As her young grow, her abdomen will become larger and larger. When you can see a large dark mass near her vent and she appears ready to pop, she is nearing delivering. A female ready to release fry may appear afraid and hide in the plants or dart about the tank. Place her in a breeding box or a small tank by herself with lots of floating and other live plants. Stores sell a number of plastic breeding boxes for guppies.