Dogs will usually go into their first heat at 6-12 months of age, and if unspayed will have an estrus cycle twice a year at which time they may become pregnant. Pregnancy in a dog last only two months, somewhere between 58-68 days. Smaller breeds of dogs tend to have a slightly shorter pregnancy than large breed dogs. About halfway through the pregnancy the expectant mother dog will increase her appetite to eat about twice as much food as normal, and during nursing will eat three times as much. She's not just eating for two as the cliche goes, she may be eating for up to twelve puppies! Pregnant dogs require a high quality puppy chow through pregnancy, and for the entire time while nursing the puppies.
Some signs of pregnancy in dogs besides a voracious appetite, are enlarged nipples and a clear vaginal discharge, after about three weeks into the pregnancy. Veterinarians are able to confirm a pregnancy diagnosis at 25 days with an ultrasound, with a blood test at 30 days, or an x-ray at 45 days. Towards the end of the pregnancy term, it is recommended to keep the new mom away from other dogs, with a comfortable place to give birth to her puppies. Within 24 hours of giving birth, a dogs temperature will fall below 100, as it is normally 101.5 fahrenheit. In the initial stage of labor she will begin to feel contractions, and will show signs of being uncomfortable, this lasts for about up to twelve hours until the first puppy is born. The puppies are usually born about 30-60 minutes apart.
Here is a video of a Jack Russell Terrier giving birth:
