Determine if a platy fish is pregnant by examining its size and shape. A pregnant platy fish starts to look very large, becoming almost square in appearance, and very swollen in the middle by the time it is ready to give birth.
A female platy fish differs from the male in its fan-shaped anal fin. The female platy fish gives birth to live young rather than to eggs every three to four weeks when it is paired with a male. When a pregnant platy fish looks ready to give birth, move her into her own tank to protect her young from the male fish. Moving the female too early may cause her to miscarry due to stress, however, so make sure to wait until the appropriate time to move her.
The use of a net breeder also greatly increases the survival rate of the fry. Place it in the corner of the tank after the female has spawned and ensure the fry are the only fish inside the net. A net breeder made of mesh is preferable to one made of plastic. Feed the fry little and often, and crush the fish food between the fingers before sprinkling it in the tank.