If you are concerned that your Chihuahua may have contracted an "accidental" pregnancy that you may wish to terminate, the fastest way to tell is to have the vet do an ultra sound.
If you are sure that you don't want to breed you Chihuahua, spay her before her first heat, which usually occurs at around the eighth month. She will almost certainly become pregnant if she mates during her heat cycle.
If a Chihuahua is suffering from a hormone imbalance, she may have a false pregnancy. This can cause the stomach and teats to swell, and she may start to produce milk. In such cases, the symptoms begin to fade in a few weeks.
Vets and breeders advise against trying to handle a Chihuahua's labor and birth without experienced help. Chihuahua can suffer serious problems during delivery, which may result in death.
Chihuahuas are the smallest breed of dog, and they often experience complications during pregnancy and delivery. Sometimes puppies are born prematurely or may become lodged in the birth canal because of their relatively large head size. In fact, a higher than average number of Chihuahuas give birth via cesarean section. Because of the higher risk involved in Chihuahua pregnancy, owners and breeders often will not breed their animals until their second or third heat. The risk factors involved mean that owners of these little dogs need to identify a pregnancy immediately–whether the pregnancy was planned or not.
Check to see if your dog's vulva has returned to normal size after she was in heat. The vulva, located just beneath the tail, is slightly swollen during heat, which lasts about 25 days. Once heat is over, the swelling disappears, unless your dog has become pregnant.
Note whether your Chihuahua seems to be less energetic than usual. Like a human, she will feel lethargic in early pregnancy.
Pay attention to her eating habits. Some animals suffer from severe morning sickness early in pregnancy, but, unlike in humans, this lasts for only a few days. You may miss it if you are not watching her closely.
Watch her nipples to see whether they are enlarged. This happens early in the pregnancy and the teats closest to the hind legs generally show signs of swelling the first.
Palpate her stomach gently. Unlike other breeds Chihuahua stomachs enlarge very quickly and become quite hard during pregnancy.
Watch to see if she grooms herself more than usual and appears to be nesting.