Most fish that give birth to live young have a gestation period of about 28 days. However, some species, such as goodeids, gestate for 50 days or longer.
Only about 1 percent of all known fish species are viviparous, or livebearers. They include popular aquarium fish such as guppies, swordtails, mollies, platies and goodeids. The eggs of true livebearers are fertilized internally and retained within the female’s reproductive tract until the embryo is fully developed. At the end of the gestation period, the female delivers young ones known as "fry." Most fish species are egg laying, and their eggs are fertilized, develop and hatch externally.