Instead of releasing eggs, live-bearing fish give birth to fully developed young, much like mammals. The period from the moment of conception until the mother gives birth is relatively short for these fish, but a special survival mechanism can make the gestation period appear much longer.
When the Clock Starts
The average gestation period for live-bearing aquarium fish is four weeks, but can vary between species and even from fish to fish from three to five weeks. Visual observation is one of the best ways to tell if your live-bearing fish is about to give birth. Within a few days of the big event, she will look so fat that her body will take on a shape that is nearly square. Observation is important since the gestation period is the time between the fertilization of egg and the birth of the young, not the time since breeding. These are things for a live-bearing fish since she can store sperm from the male for up to a year before allowing the eggs to be fertilized. In fact, she can give birth several times from a single mating, which is why popular live-bearers like guppies, mollies and platys can have a relatively short gestation period but not give birth for several months after mating.