Stages of Fetal Puppy Development

Pregnancy begins after the sperm from a male dog fertilizes, or unites with, the egg in a female during mating. Fertilization occurs in the oviducts and only 12 short hours later, the cell begins to divide, initiating the development process. According to Julio E. Correa at Alabama A&M University, the canine gestation period is between 56 and 66 days, the average being 63 days, or nine weeks.

During this first stage of development the fertilized egg divides as undifferentiated, or unspecialized, cells until it reaches an 8-cell mass. At this point, the cells begin to specialize into specific cell types to develop the necessary parts of the embryo. Division continues, and at the 16-cell stage, it now is considered an active embryo, which enters the uterus. After about two weeks, the cells become a 64-cell mass, which then will implant into the uterine wall between days 17 and 21. The placenta also begins to develop, which will provide oxygen and nutrients to the implanted embryo, allowing it to grow and develop further.

After implantation, the specialized cells of the embryo differentiate into distinct layers, the endoderm, mesoderm and ectoderm. Each layer of cells will develop into different organ systems. The endoderm layer of cells is destined to become the respiratory and digestive systems, and mucosal membranes, like the lining of the mouth and nostrils. The mesoderm layer becomes the muscles, bone, circulatory system, reproductive system, urinary system and connective tissue. The ectoderm layer will result in the sensory organs for sensation, hearing, sight, balance, and smell, as well as nervous tissue, the brain, spinal cord, skin, glands and hair. At this stage, the embryo is now considered a fetus. During these next few weeks, approximately days 20 to 34, the organ systems are developing. In week four, the fetus is most susceptible to birth defects. The eyes, face and spinal cord take shape and the fetus grows to about 14 to 15 millimeters. Toes, whiskers and claws develop in week five, and the fetus begins to resemble a dog. By the end of week six, the fetus is about 45 millimeters long, and has developed skin pigment.

Now that most of the major organ systems have developed, these last few weeks are for further growth and maturation. The eyes are an exception and continue to develop through these last few weeks and even after birth. The fetal puppy gains weight, and the puppy’s movements can be seen when the mother is at rest. The puppy can be born safely at week eight.