Instructions
Pick up the baby rat gently in your hands after he or she has nursed for a while. Place the pup on its back in the palm of your hand and hold it down gently with your thumb, so the pup doesn't fall or squirm away.
Hold the base of the pup's tail away from its body gently with your other hand. Look near the base of the rat's tail, and you are able to see the anus. The opening of the rat's urethra is the small hole further up towards the stomach. The male's urethra is farther away from his anus and the urethra will stick a little out more. A female's urethral opening is right near her anus. It's easier to see the difference when a male and female are compared together.
Pick up the baby rat when it is about 1 week old if you are still not sure about the sex. Hold it on its back in your hand so that its belly is in the air, and you will be able to see the marks of the developing nipples if it is a female. Nipples aren't visible on a female until she is 5 days to 1 week old.
How to Tell If a Baby Rat Is Female or Male
Finding out the sex of your baby rats is the first thing you want to do when adopting them. When they are 2 weeks old, you can adopt a rat but it may still be hard to determine whether they are male or female. The male testes don't drop until they are about 3 weeks old. But you should be able to tell the difference when baby rats are only a few days old, if you can see a male and female pup together.