Is It A Boy or A Girl?
Sexing kittens is not easy - sometimes even vets and experienced breeders get it wrong with very young kittens. The differences are subtle, but they are there.
All newborn kittens have two openings under their tails. In the females these are obviously the anus (closest to the tail) and the vagina. Some people are surpised to learn that in male kittens the lower opening is in fact the penis.
Therefore at first glance, both male and female kittens look the same under their tails. However, on closer examination there are two differences.
In males, the distance between the two openings is greater than in females. As mentioned, even the professionals get it wrong, especially when faced with a single kitten. The best way to notice the difference in this distance is to look at a number of kittens and compare what you’re seeing.
The reason for the larger gap in males is to make room for the testicles, which do not appear outside the body for a couple of months.
The second difference is the shape of the lower opening. In females it is a narrow vertical opening, whereas with males it is rounder. Some people compare the area under a female kitten’s tail to a lower case letter i, and the male’s to a colon :
That’s a huge simplification of reality, but nonetheless goes somewhat towards describing the situation.
One more tip - if you’re feeling up to it, you can give the lower opening a small squeeze. If the tip of the penis appears, it’s a boy.
There is also a health issue related to sexing. You should check kittens under their tail as a matter of course. This is because on rare occasions there is only one opening. This requires immediate veterinary attention.