A cat may have a sex that is ambiguous. It can be difficult to determine the sex of a small kitten since the organs are not immediately visible like that of a puppy. A male puppy's penis is perfectly visible from the moment of birth, while the penis of a male kitten will not begin to show until he is approximately 6 to 10 weeks old. With there being little, if any, physical definition in the genital region of a small kitten, it can be hard to distinguish males from females.
What to Look for
Determining the sex requires close observation of the subtleties of the animal's physical anatomy, since it is determined by the distance between the kitten's anus and sexual organ; you will need to lift your cat's tail gently to view the area.
The distance between a male kitten's anus and penis is approximately 1/2". Adult neutered males are often times confused for adult females (due to the alterations made during the neutering procedure) and therefore, the same relative equation is applied to distinguish adult neutered males from adult females. Again the sex will be determined by the distance between the cat's anus and penis, which in an adult male is approximately 1" wide.
As for females there should be slight distance between the anus and the vaginal opening of a female kitten, and a distance of approximately 1/2" between between the anus and vaginal opening of an adult female.
If you are having difficulty determining the sex of your cat or kitten, consult your veterinarian, and always remember to have your cat spayed or neutered.