A female dog is usually pregnant for about 63 days but this can vary. How the pups develop and the physical and behavioral changes their mother experiences during pregnancy fall roughly into three stages, each lasting about 21 days.
The mother dog will look and generally behave as she always does during the first three weeks of her pregnancy but things are happening inside her. The female dog's uterus or womb is divided into two thin tubes known as horns. During the first couple of weeks, her fertilized eggs, called zygotes, attach to the wall of these horns where each egg develops into two parts. One part becomes the fetus and the other part is the placenta, which feeds the fetus with nutrients from the mother's bloodstream. At two weeks, the tiny fetuses have developed heads, spines, limb buds and tails and by the end of the third week, the development of tissue and organs, means the pups are roughly at the same stage of development as a human fetus at three months.
Where pups are positioned in their mother's womb can affect their development and behavior later in life. While the fetuses are evenly spaced along their mother's uterine horns helping to make sure each pup has an equal advantage as they develop, placentas in the middle sections of the horns tend to be larger and healthier, and this usually results in larger, healthier pups.
Pups' neighbors in the womb can have an interesting effect later on in life. While in the womb, boy pups secrete the male hormone testosterone as their testes develop. This helps the boy pups develop male characteristics such as dominance. If their sisters are positioned between two boys in their mother's womb, it's believed, the female pups can be exposed to small amounts of testosterone and they are likely to develop more dominant personalities as adult dogs.
A veterinarian usually can diagnose pregnancy at about three weeks by feeling the fetuses inside their mother through her abdominal wall, especially if she is a lean-bodied breed. For the first five weeks of being pregnant the mother will do fine eating well balanced, normal-sized meals. At week five, the demands of eating not just for two, but maybe for four, five or many more developing pups, will mean it's important her calories increase by about 10 percent each week from week five onward. By the end of this middle stage, at six weeks, the fetuses are like miniature dogs, and have eyelids, claws and hair.
In the last three weeks, the mother dog usually will look pregnant as her abdomen becomes noticeably enlarged and her nipples swell and get darker. A couple of days before the pups are due she will start producing milk. These final weeks are when the mother's behavior is most likely to change. She may shred up paper and bedding to build nests, become restless and, understandably, at times be irritable and want to be left alone. As for her pups, they are now fully developed and just keep growing. About 12 to 14 hours before labor starts the mother dog gets anxious, her temperature drops and she won't want to eat. Once contractions start the first pup is usually born within two hours. Most canine births go smoothly but always have the veterinarian's phone number to hand and seek their help and advice throughout pregnancy.