Your dog's reproductive system consists of a vagina, cervix, uterus, oviducts and ovaries. Her ovaries produce unfertilized eggs and the hormones associated with oestrus and pregnancy. The eggs travel from the ovaries to her oviducts, where the eggs are fertilized by sperm. The fertilized eggs continue to the uterus to mature, where they develop into embryos, attached to the uterine wall by a placenta. A dog's body will change as she goes through heat cycles, however, her basic anatomy remains unchanged.
An intact female dog will experience a heat cycle approximately every six to nine months. There are four cycles to oestrus: proestrus, estrus, diestrus and anestrus. During proestrus your dog's vagina becomes swollen and leaks a bloody discharge. She's not receptive to mating at this point. She becomes interested in breeding in estrus, and her vaginal bleeding is greatly diminished or absent. During diestrus, your dog isn't receptive to breeding. If your dog is pregnant, her body will undergo significant changes during the next two months as her puppies grow and she prepares for birth. Anestrus is the longest phase, a quiet time between heat cycles.
The First Trimester
During the first three weeks of pregnancy a dog's body changes little outwardly. There's a lot happening in her uterus as her puppies began to grow, but the most significant change in appearance may be a slight enlargement to her nipples. Her appetite may increase a bit and she may become more insistent on your attention. Around the third or fourth week of pregnancy, she may experience a few days of morning sickness, prompted by her altered hormones. She may seem tired and less interested in food. If you take your dog to the vet at the end of her first trimester, your vet will be able to spot her growing puppies via ultrasound.
The Second Trimester
By the fourth week of pregnancy an experienced veterinarian will be able to palpate the puppies to confirm her pregnancy. Palpation is useful only between the 28th and 35th days of pregnancy and should be done by a trained hand.
Around weeks four or five, a pregnant dog may have a clear mucoid discharge, another sign of pregnancy. A pregnant dog secretes the hormone relaxin when the fertilized eggs implant into her uterus. A blood test can confirm the hormone after four weeks. By the end of her second trimester, your dog's nipples will be darker and larger and her belly will be bigger.
The Third Trimester
The final trimester is a period of rapid development for the puppies. Your dog's weight may increase 15 to 25 percent at this point. You'll notice it as her belly grows, perhaps even swaying a bit as she walks. As she gets closer to her delivery date, your dog's breasts will grow and she may have a bit of fluid trickling from her nipples. You may see her puppies begin to move, perhaps even feeling them, as they prepare for birth. Your vet may choose to take radiographs in the final weeks before delivery to confirm the number of puppies and make sure they won't have any trouble in the birth canal.
Dogs experience three trimesters of pregnancy, each lasting approximately 21 days.