By Dr. Becker
Determining the sex of a cat, even if it's a tiny kitten, is actually easier than many people think. In fact, probably the trickiest thing about this undertaking is convincing kitty that you mean no harm as you move his/her tail out of the way to get a closer look at those tiny private parts. For purposes of accuracy, it's best to wait until a kitten is at least a month old to try to determine its gender.
First Things First: Keeping Kitty Calm
It's very important to minimize stress on your cat during this process. I recommend you engage in some calm petting and maybe a little playtime before you begin your gender identification project. If you're a stranger to the cat, or it's very stressed or standoffish, you may want to wait for a better time. You may also want to ask someone to assist you by either gently holding and cradling the kitty, or helping to move the tail out of the way.
Another approach that might work is to scratch the cat's back right at the base of the tail. Many cats absolutely love to have that spot scratched, and automatically lift their tail in response. If you're dealing with a palm-sized kitten, you can try gently holding it on its back in your hand. The tail may drop out of the way, or you may have to move it.
It's a Boy! It's a Girl!
The goal so far, if you haven't guessed, is to get a clear look at your cat's backend, which means the tail must be out of the way. Once you have an unobstructed view of kitty's backside, the first opening you'll see right below the tail is the anus. This is true for both males and females, and tells you zip about which yours is, but it's a good point of reference!
It's the genital opening below the anus that tells the tale. These openings are in slightly different spots, and are slightly differently shapes in male and female cats. If your cat is female, you'll find her genital opening (vulva) immediately below her anus, and it will be in the shape of a vertical slit.
If your cat is male, his genital opening (penis) will be a noticeable distance from his anus – about ½ inch in kittens and 1 inch in adults – and will be a rounded shape. The testicles of unneutered mature males will be visible in the area between the anal and genital openings.
Cheat Sheet for Sexing a Cat
Did you know the color of your kitten may give you a clue to its gender? Almost all (but not ALL) calico (black, white, and orange) and tortoise shell (black and orange) kittens are females. Also, more orange kittens are male than female.