The fetal spot of guppy is generally brown or black, but it is difficult to find at ordinary times. Only after pregnancy, a triangular plaque will appear between their anal fin and ventral fin, which is the fetal plaque. When guppies are first pregnant, the color of fetal spots is not obvious, which is difficult to find if breeders do not observe carefully. However, guppies that have been pregnant many times or are close to production will have obvious fetal spots and become darker and darker. As guppy mothers become pregnant, their fetal spots will become bigger and bigger.

Guppies grow fetal spots because their eggs have a black membrane, and their bodies are transparent. When they are pregnant, fetal spots will appear. Fetal spots are only found in female guppies, but not in male guppies. The fetal spots of guppies produced for the first time are not particularly obvious, and the color of fetal spots will become darker and darker after they reproduce many times.

For guppies with high production frequency, fetal spots will become more obvious when they are just pregnant. If breeders can't tell when guppies are pregnant, they can tell when they are born by the color of their fetal spots.