When Molly fish are pregnant, they develop large, swollen abdomens with large dark spots near the pectoral fin. Mollies start to develop this appearance up to a week before giving birth.
Molly fish are livebearers of the Poeciliidae family. They are colorful freshwater fish that are quite hardy. Molly fish get pregnant fairly often and can give birth up to once a month. After fertilization by a male, molly females can release live young each month for up to 6 months. The first birth is the largest with between 12 and 30 babies. The next month she will give birth to 50 percent fewer babies and so on.
When pregnant, molly fish look larger around the abdomen. They grow bigger as they get further along in the pregnancy. As they get close to giving birth, their bellies may take on a wide, almost squarish shape. Fish owners may also notice a darkening of the fish's belly that coalesces into a large, very dark spot near the pectoral fin. This dark spot is actually the small fry moving into position to be born.
Before giving birth the female molly may also start rubbing her abdomen against leaves of aquarium plants. She may also start hiding out in any plant growth at the surface of the aquarium.