From The Aquarium Wiki
A gravid spot is a darkly colored spot on female livebearers, situated just behind the anal fin. It grows larger during pregnancy, getting larger the closer a fish is to giving birth. It is commonly used as a useful indicator of pregnancy in livebearers such as platies and guppies, with some aquarists claiming that close to birthing it is also possible to see dark spots corresponding to the eyes of the fry in the same area.
- See also: gravid