The Fetal Formation of Puppies

It's just over two months, approximately 63 days, from canine conception to birth. Within 12 hours after mating, cell division takes place for the first time, repeating every 12 hours. Once the division reaches 16 cells, this proto-puppy mass moves to the mother's uterine horn. That clump of cells has become an embryo.

Two weeks after entering the uterine horn, the embryo attaches itself to tissues in the womb and starts forming a placenta. The placenta provides oxygen and nourishes the embryo via the mother's blood, and also removes waste from the embryo back to the mother for excretion. The entire embryonic stage lasts for about a month, before it becomes a fetus. During this period, the cells differentiate, ready to form the various parts of the fetus. These cells form systems beginning with the head and ending with the tail.

By the third week after conception, the heartbeat is visible via ultrasound. That's the point at which your vet can manually palpate the mother dog to confirm the pregnancy, feeling the gestational sacs with her fingers. Fetuses all start out as female, without a Y chromosome, but those fetuses destined to be male develop this chromosome, with sex differentiation occurring about day 36.

After the 30-day mark, development occurs that makes the fetus resemble a puppy. The head forms in two sections, as cranium and face. Between days 33 and 35, limb buds sprout, the future front and hind legs. A day or so later, fetal movement starts. The skeleton develops between days 33 and 39, with the bladder and stomach developing right after. Lung development occurs between days 38 and 42, with the liver, kidneys and eyes appearing between days 39 and 47. The umbilical stalk sprouts between days 40 and 46, while the intestines don't develop until day 57 to 63, right before birth.

The fetal skeleton mineralizes at approximately day 42. After that time, your vet can radiograph your pregnant dog's abdomen to determine how many puppies she's carrying. For best results, wait until day 50 or later. Manual palpation and ultrasound doesn't accurately reveal the number of fetuses. The puppies' baby teeth develop late in the pregnancy. They can be spotted on an X-ray about four days before parturition.