When a female dog, also known as a bitch, becomes pregnant, her anatomy will change to support her puppies. Her reproductive organs work to nourish puppies growing in her body until birth, then she produces milk to sustain them and provide them with nutrients and antibodies until they are old enough to eat on their own.
A pregnant female dog can have anywhere between three and five pairs of breasts, which make up two rows from chest to groin. The number of breasts, and subsequently nipples, will vary from one dog to another, and can vary from one row to another as well. One row of mammary glands can have as many as two more nipples than the other row. All dogs have nipples, but upon having a first heat, they swell to a larger size which is how they remain, even if the dog does not become pregnant.
The anatomy of the female dog will change after mating. The nipples, for example, will begin to turn a deeper pink colour approximately 35 days following mating, and they will become larger during this time.
The vaginal opening of a female dog will enlarge with the first heat, and remains enlarged once the heat is over, even if the dog does not become pregnant. The initial one to two inches of the vaginal anatomy has a 45-degree upward angle, and then runs straight for three to four inches before reaching the cervix. The vaginal canal possesses a special ring of muscles that works to keep the male's penis in place until ejaculation and to ensure that the seminal fluid reaches the right place for impregnation.
The female dog's uterus is not the same as a human uterus, but is shaped as a "Y" rather than a pear. The base of the uterus is the cervix, which remains closed as much as possible to prevent foreign matter or infection from entering the uterus. The puppies develop in the two arms of the uterus, and each puppy will have a separate compartment. Dogs are known as multiple ovulators, meaning that they release several eggs over a period of time between several hours and a day.
During the heat cycle and when pregnant, female dogs produce a large amount of oestrogen and this poses a risk for tumours. This is one of the reasons why it is important to spay a female dog that you are not intentionally breeding. Unless there is some specific purpose for breeding a dog, it is healthier for her to be spayed before her first heat in order to prevent the extra production of oestrogen that leads to a greater cancer risk. Spaying involves removing the ovaries, which are responsible for producing cancer-causing oestrogen and progesterone hormones.