I see too many users here always asking the same old question, which is essentially: "Where did all these fry come from?" A lot of new fish-keepers are often stumped by the mysterious pregnancies and fry they find when they keep livebearers. This is a little guide to understanding the reproduction of livebearers.
There are four main livebearing species commonly kept by aquarists: Platties, guppies, mollies, and swordtails. These are very special fish, because unlike most fish, they give birth to live young. What's even more astounding, is that not only do the young develop inside the mother, they also develop without an encasing egg. They are nourished by means of an umbilical, much like humans.
Mystery Pregnancies - It's not uncommon for an owner to discover, maybe months after purchasing their fish, that it is pregnant. In the cases in which no males were with the females, you can imagine the confusion. Fear not, this is normal. In most cases, when you purchase one of the "Big Four" breeds, the odds are any female will be pregnant already, if not, carrying sperm.
Carrying sperm? Sound odd, yes, but mollies, platies, guppies and swordtails have a unique pouch inside their bodies that allows them to store sperm after mating for up to about 6-9 months. The males are also unique in the way they deliver their sperm in 'packets.' These packets are designed to be stored by the female and used whenever conditions are optimal for releasing fry. Again, not all livebearing species have this little feature, but the four mentioned above do.
The gestation period for the average livebearer is approximately a month. Towards the end of the pregnancy, you'll notice the mother 'squaring off,' which is when she will literally look square in shape from the back. The gravid spot by her anal fin will also get bigger. The gravid spot is essentially the eyes of the developing fry that you can see through the mother's scales.
Finally, it's time for the mother to drop the fry. The fry can be dropped over the course of a couple days or several hours. For the 12 hours during and after dropping the fry, the mother's body will produce a hormone that will keep her from eating, and eating the fry, but after those 12 hours, the fry will need someplace to hide, or else be eaten by other fish, including their parents.
This is all for now. More may be added later.
For more info, feel free to visit Breeding Livebearers