The male guppy extends a tube from his gonopodium and inserts it into the female's genital pore during guppy reproduction. Once the male inserts the tube, he ejaculates sperm into the female. The sperm mixes with one of the female's egg cells to form a new cell that eventually becomes a baby guppy.
Female guppies can hold on to male sperm for up to three spawning cycles. This means that if the guppies mate and the male is removed, the female is still able to give birth up to three times without mating again. Each time the female gives birth, she is able to produce approximately 20 to 200 babies. The exact number usually depends on her size.
As the babies grow, they are attached to a yolk that nourishes them. The average gestation time for a guppy is 28 days, but females can have gestation periods that range from 20 to 40 days. It is not possible to tell if a female guppy is pregnant merely by looking for a growth in her belly area, but it is possible to tell by looking for a gravid spot. A gravid spot is a dark patch on the female guppy's belly.