Gestation begins from fertilization of the ovum by a spermatozoon, although a breeder can really only detect this much later, around 21 days. However, even though remaining invisible from the outside, early embryonic development and implantation are crucial phases for the future puppies.

Advantages of accurate monitoring of estrus. If estrus is monitored by tests such as progesterone assays, the moment of ovulation can be determined fairly accurately. Breeders then very often find that whelping takes place exactly two months later. For example, a bitch mated on September 25—after careful monitoring of estrus and with ovulation on September 23—will most likely whelp on November 25.

Fertilization and implantation

Fertilization, i.e., the union of a spermatozoon and an ovum, takes place very high up in the uterus, in the Fallopian tubes. Toward the end of the first week that follows, the embryos migrate from the Fallopian tubes toward the uterine horns. They then float and move around in the uterine fluid and can even switch from one horn to another (they can also have been conceived on the right-hand side of the uterus before implantation in the left horn).

Embryos then become immobile, facing toward a uterine gland. They implant themselves relatively tardily, toward the 16th day after ovulation—this is nidation or implantation. It is only after this stage that they actually begin to develop, along with their adnexa.

Two distinct phases

Development of the future puppy can be split into two phases:

• Embryogenesis, during which all future organs come into place. Among other significant events, one might mention the appearance of eyelids at 32 days, fusing of the palate at33 days, and differentiation of toes at 35 days. For example, if the palate has not fused by the end of this phase—this is what is known as a cleft palate—this will not occur subsequently during gestation. At the end of the embryogenesis, future puppies are not very big, although most structures are in place and all organs have started to develop. At this stage, the embryo becomes a fetus and it is possible to recognize the species from observation.

• Weight gain: Over three-quarters of a puppy’s weight at birth is gained by the fetus after the 40th day of gestation.