Summarize this article:

The average gestation period for a dog is between 58 and 68 days. Try to pinpoint when conception occurred so you can anticipate your dog's labor accordingly. Chihuahuas tend to deliver at 8 weeks gestation. If you are deliberately trying to breed your dog, take her to the veterinarian for a check-up beforehand. Re-visit the vet again about 30 days into the pregnancy. If the pregnancy wasn’t planned, however, take her to the vet as soon as you discover she’s pregnant.  The vet can give you critical information that will help you ensure your dog has a healthy pregnancy. One important thing to discuss is proper nutrition. Pregnant dogs that are overweight are more likely to have difficulties giving birth. However, pregnancy is not an ideal time to put chihuahuas on a diet, since the mother actually needs more calories in order to pass nutrients onto her litter. One recommendation is to delay putting your dog onto energy-dense, high-caloric foods (such as puppy food) until the final 2-3 weeks of pregnancy. At this point your dog will have a large womb and only want to snack anyway.  Your vet can also count the number of puppies in the litter using x-rays. This happens at 45 days of pregnancy. Three to four puppies is normal for a typical chihuahua litter. Knowing how many puppies your dog is expecting can help you better prepare for the birth. If a very large or small number of puppies is expected, your dog may experience complications during labor. Your vet can recommend whether to pursue natural labor or a planned C-section. Further, knowing the number of puppies will also help you be certain when she is finished whelping (or giving birth). For example, if your dog is carrying four puppies, gives birth to three puppies, and then stops laboring, you will be able to identify that something is wrong and contact your vet. If a puppy has trouble nursing, you'll want to have formula on hand since newborn puppies need to be fed every 2-4 hours. You can buy puppy formula and bottles at most pet supply stores. If you cannot find puppy formula, you can also use goat's milk, although this is not ideal for long-term use. Goat's milk should only be used in emergencies until you can buy some formula.

Summary:
Understand the normal progression of pregnancy in chihuahuas. Take your dog to the veterinarian. Buy puppy formula ahead of time.