According to Practical Fishkeeping, the first sign of pregnancy is that the fish's belly enlarges and appears rounder. Pregnant fish also exhibit a black spot on the belly called a gravid mark. As the pregnancy progresses, the fish's belly becomes very large and distended, and the gravid mark becomes larger and more obvious.
The gravid mark is more visible on olive-colored fish but may be visible on red or orange fish. It is not visible on black fish, such as pregnant black mollies. Livebearer fish do not get pregnant in the complex way that humans do; instead, the females retain their eggs inside of themselves until the eggs hatch. The newly hatched babies are called fry. Livebearers do not care for their newly born fry at all. In fact, most livebearers eat their own fry.
Common livebearer fish kept in aquariums include guppies, mollies, swordtails and platies. The males of these species have a modified fin called a gonopodium that is used to fertilize the eggs inside of the female. Most species of fish lay eggs instead of getting pregnant. The females lay eggs, and then the males release sperm into the water over the batch of newly laid eggs.