If a female livebearer (molly, guppy, platy, swordtail) has been in a tank with a male, she will be pregnant. When livebearers breed, the males use their gonopodium (modified anal fin) to impregnate 4-6 packets of sperm into the female. The females can then use these sperm packets to fertilise eggs when conditions are good. This means females can fertilise eggs 6 months after a mating and with no other males around.