The Internal Anatomy of a Pregnant Dog

Updated November 21, 2016

A female dog is normally referred to as a “bitch." She can go into heat starting at five months of age, at which time, if she is not spayed, she can become pregnant. Most female dogs go into heat twice yearly, lasting about two weeks. The gestation period until the birth of her puppies is about nine weeks. The anatomy of a pregnant female dog functions similarly to the female anatomies of other mammals.

Heat Cycle

There are four stages of the heat cycle. Small dogs can go into heat as young as five months old whereas giant breeds may not go into heat for over a year. The heat is brought on by the production and release of the ovaries. Stage one is the proestrus, in which the vagina swells with a bloody discharge. Males start showing interest at this point although she is as yet unreceptive. Step two is the oestrus, when the eggs are released and travel through the oviducts. The female at this point will let a male mount her to mate. Stage three is the diestrus, during which, if fertilised, embryos will attach to the wall of the uterus with the placenta. The last stage of heat is anestrus, which is the period between cycles.

Mammary Glands/Nipples states that a female dog, depending on her size, can have between three and five sets of mammary glands or nipples, which are tiny at birth. They get larger when a female goes into heat and engorge with milk about halfway through her pregnancy, ready to feed her pups when they are whelped. Female dogs with great maternal instincts have been known to produce milk for baby animals, not their own, which are orphaned.


According to and, the ovaries are where the eggs are produced along with reproductive hormones. These eggs develop within fluid-filled sacs called follicles. The ovulated eggs then pass into little tubes called the oviducts, where they will be fertilised by sperm.


The uterus is shaped like two long horns on either side of a short body, like a “Y” shape. Within two days of ovulation and fertilisation, an egg implants itself inside the walls of the uterus. This is where the embryos will grow, surrounded by the placenta, which nourishes them.


This is the “path” from the uterus to the vagina. During pregnancy, the cervix is closed to the birth canal. This protects any microorganisms from entering the uterus. During labour, the cervix will dilate, getting ready for the birthing process from the uterus to the vagina for delivery. Most dogs do not need help in the birthing process.


The vagina is the organ that extends from the cervix to the vulva. The vulva is the outer organ, which consists of the external genitalia, a clitoris, and two vertical lips.

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