Guppies are little fish who are easy to breed -- they basically do all the work themselves. When you're trying to determine the genders for breeding selection, the sexes of fry can be difficult to ascertain, since they're so tiny and still not fully developed. Sexing baby guppies takes some effort at first.
Gravid Spot in Females
A female guppy has a dark area at the base of the tail, near the abdomen and the anal fin, where the babies will grow when she is pregnant. With difficulty, the professional breeder can see this tiny spot in the newborn fry. For a beginner, it isn't easy to recognize. It helps to use a strong flashlight while examining the fry. The older a female guppy fry gets the more apparent the gravid spot becomes.
Gonopodium in Males
Both genders of guppy have a fin called the anal fin. In females, this fin is squared off; in the males it is pointed and called a gonopodium. This finlike appendage delivers the sperm to the female during mating. In the tiniest of fry, these fins can be identical in appearance -- so it is best to wait until the fry are at least 2 or 3 weeks old to check for this.
Within 3 weeks of age, males begin to show color formation. Since the females are less colorful, the early color is a clear sign that a guppy is male. The colors usually begin as a single dark spot on both sides of the fish. Same-age fish who display faint streaks of color only on their tails, quite often, are females. Tails are the only parts of the females that show color in adulthood.
Overfeeding the Fry
When the fry are around 3 weeks old, you can determine their genders in one rather unusual way: by overfeeding them. When the female guppy gets fat, her belly will round out while the rest of her body remains trim. In the male, the chest area will swell instead, leaving the rest of the guppy streamlined. This method is rather impractical, however, as it tends to create unhealthy fish and more fish waste to clean up.