Chihuahuas are the smallest breed of dog, and their tiny size may make it difficult tell a male from a female. There is a distinct difference between male and female chihuahuas, even on the day they are born' you can easily tell the difference if you know what to look for. Often, people mistake the female's vulva for a penis because it seems larger than expected, particularly when they are newly born.
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Turn the chihuahua gently onto its back. Support its body fully, from head to toe, so it will feel secure and not wiggle excessively.
Look for a penis, which will be located on the belly, not between the back legs. If you see a penis, you have a male chihuahua. You may or may not see a scrotal sac, as the dog may have been neutered or the puppy's testicles may not have descended fully. But the penis will always be there, and will be on the belly. If you are looking at a newborn puppy, the penis is just slightly lower on the belly than the umbilical cord. In adults or older puppies, you can't always see the umbilicus, or belly button.
Locate the anus, just under the tail. Look forward, toward the belly and you will see the vulva very close to the anus. The vulva is slightly swollen in appearance, and often has a few long hairs by the urethra. This often fools people into thinking it is a penis. In female dogs, the genital opening is closer to the anus than the genital opening of male dogs.