Almost any owner of a live-bearing female fish has been surprised to find a tiny fishlike worm or two swimming around the aquarium, hiding from the other inhabitants. Livebearers are the only fish that experience pregnancy in any scientifically recognised term. Female fish are inseminated internally by a male; the baby fish, called fry, develop within the mother until birth. Care should be taken when handling any pregnant fish because stress may cause her to "drop her fry" too soon.
Most female guppies are pregnant from the time they reach sexual maturity until death. If a male guppy has been present with the female guppy in the past six months, it is best to assume that she is probably pregnant. However, guppies are well equipped to live in a state of constant pregnancy. The "gravid spot," which is the dark spot along the abdomen of the female guppy, will become even darker as the pregnancy wears on. Just before she delivers, the spot will likely be pitch black. If you are intent on keeping fry from the waiting mouths of the mother and other tank mates, it is best to place the mother in a breeding box before the gravid spot turns black.
There are two common varieties of mollies that experience pregnancy in the home aquarium, balloon mollies and sailfin mollies. Pregnancy signs for the two are quite different. Balloon mollies will start off by eating slightly more than normal, though this may not be noticeable to the beginner fish keeper. These fish will swell noticeably along the entire bulbous portion of their bodies. If the fish is female, swelling and still swimming correctly, she is likely pregnant. Sailfin mollies will exhibit the same eating habits as balloon mollies, but only the vent and area near it will swell, not the entire abdominal area. Sailfin mollies may also show they are pregnant by chasing male mollies and harassing them or tearing at their fins in an effort to be left alone.
Pregnancy in swordtails can be difficult to diagnose because of their body structure. In addition, these vividly coloured fish may not show many signs except a slight bulging around the abdomen near the end of the pregnancy. Small signs to look out for with a female swordtail include constant waiting for food near the top of the water, chasing males that she is usually friendly toward or becoming territorial with other tank mates. A fish that is hiding in plants may be about to start or already in labour.