The gestation period, the time interval between the event of fertilization and the delivery of the young, is highly variable. Although a broad generalization would be 4-6 weeks, at a temperature of 78-80°F. The number of young produced is also variable, it depends on the species and the size and age of the female. Livebearers with similarly shaped gonopodia can be hybridized. Thus Platies will cross easily with Swordtails, and Guppies have been crossed with Mollies.

The fact that one fertilization can suffice for the production of numerous successful broods must be kept in mind when selectively breeding livebearers. A female that is to be kept for mating must be isolated from all other males, if this is not done - even if a desired male is introduced to her later, one cannot be certain of the male parentage of the future progeny. Thus, in selective breeding the first step is to isolate breeding stock at an early age, before signs of gonopodium elongation. Then when the separated males and females are mature, the best male is mated to two or three of the best females by bringing them together in a well planted tank with surface vegetation. If fed well, nothing more needs to be done, the fish will readily mate. When the fry are born, catch the youngsters and rear them in a separate tank. The fry may be feed newly hatched brine shrimp and finely ground flake food.