While they may come into the world seeming clumsy and helpless, puppies go through quite a bit of development before they are born. There are three main stages of development, the ovum period, the embryonic period, and the fetal period, each with its own important contributions to the development of a litter of puppies. Gestation takes place over a period of 64 days.
The Ovum Period
The first period is the shortest phase of a puppy's development; it takes place immediately after the ovum is fertilized. The cell forms an outer coating that prevents other sperm from entering the egg; after twelve hours, a cell division occurs. Cells continue to divide every twelve hours as the ovum makes its way toward the uterus, forming a bundle of cells known as a blastocyst. Around day 14, the cells will implant in the uterus and begin the embryonic phase.
The Embryonic Period
In the approximately 30-day embryonic phase, the cells continue to divide into three main layers that in time will become everything from hair and teeth to internal organs: the ectoderm, mesoderm and endoderm. The ectoderm will form the hair, skin, nervous system and sensory organs. The mesoderm will become the muscles, bone, circulatory system and reproductive system. The endoderm will develop into the respiratory and gastrointestinal systems.
After implantation, the placenta develops, providing all the nutrients a puppy will need throughout the pregnancy and maintaining separation between the mother's and puppy's circulatory systems. The nervous system will begin to form around day 16. Around day 22, the puppy's heart will begin beating; by day 24 the limb buds will start to form. As development progresses, the fetus will start looking more like a puppy, forming proportional limbs and tail by day 34.
The Fetal Period
The fetal period will last from around day 34 until delivery. During this stage, the organs and nervous system will finish developing. The joints, paws and separate toes will form, and the bones will begin to harden. This is also the stage during which puppies' reproductive systems form. Around day 55, fetal puppies will begin growing fur.
Identifying the Pregnancy
A dog's pregnancy can be difficult to identify before the fetal stage. It is sometimes possible for a veterinarian to identify a pregnancy by feeling the abdomen as early as 20 days, but embryonic puppies are more likely to be palpable around day 30. A pregnancy test may be accurate between 30 and 35 days. An ultrasound will show the puppies between days 25 and 35, and will allow the veterinarian to see if they are mobile and have heartbeats. Around day 45 it is possible to see puppies in an X-ray, the most reliable way to determine the number of puppies being carried.
Approaching Labor and Birth
As the time for labor nears, the mother will typically begin nesting, looking for a safe and warm place to give birth. A few days before birth, the mother may become restless and dig at her bedding. Approximately 12 to 24 hours before whelping begins, the mother's temperature will drop slightly. Puppies are generally born one or two hours apart. Four hours between deliveries or labor lasting more than 24 hours spells trouble; your vet should be advised.