After the guppy is pregnant, a triangular color block will appear between the anal fin and the pelvic fin, which is the guppy fetal spot. When guppy is just pregnant, the fetal spot is not obvious, and it is generally difficult to find it if the breeder does not observe it carefully. When guppy is pregnant to the middle stage, the fetal spot will become more obvious, and the fetal spot color at this time is brown. When they are near production, the color of tire spots will get darker and darker, and eventually turn black. If breeders can't determine when guppies mate, they can judge the approximate pregnancy time according to the color of their fetal spots.

Although breeders can judge the time of pregnancy by fetal spots, it is difficult to find the first-time female fish when they are pregnant, and it will become more obvious only when they are about to give birth. Repeated births of guppies will be more obvious after pregnancy, and will deepen the color with the time of pregnancy.

Guppies grow fetal spots because of their eggs, because the eggs show a black membrane, and the bodies of female guppies are very transparent, so fetal spots can only be observed when they are pregnant.

However, the fetal spot of guppies is not born due to pregnancy, but will appear after maturity, but it is not obvious. It is only because the abdomen of the female fish becomes bigger after pregnancy, and because of the color of the eggs, the fetal spots will be clearer.