Gestation is a complex process of exchange between the mother and her fetuses. The female must offer an appropriate hormonal uterine environment, while providing optimal nourishment for the future puppies through the placenta.

Hormonal functions

Gestation is controlled mainly by just one hormone—progesterone. In the bitch, it is the yellow endocrine bodies in the ovaries (corpora lutea) that secrete this hormone, unlike other species in which it may be secreted by the placenta.

Progesterone continues to be secreted for a further two months after estrus, whether or not the bitch is gestating. This has a number of consequences. To start with, it is difficult to establish a diagnosis of pregnancy by assaying this hormone, since it is normally present even in non-pregnant bitches. Next, a sharp drop in a pregnant bitch’s progesterone level—what veterinarians call luteal failure—may bring gestation to a halt.

Fetal adnexa

The placenta is an area of close contact between the two organisms and a point of exchange between mother and puppies. In bitches, the blood supplies of mother and fetuses are not intimately mingled as in humans, but they are still very close which means that various chemical substances and pathogens can easily pass through.

The placenta forms a circular, ring-like band around the fetus (known as zonary placenta). It is pigmented dark green, almost black, and can be seen at the time of whelping when the placentas become detached.

The placenta secretes a hormone called relaxin which can be used to diagnose pregnancy.

The puppy is surrounded by several sacs. To simplify things, it lies first in the amnion filled with amniotic fluid. This is the transparent sac that surrounds a newborn puppy at birth. The fluid helps to protect against knocks, as well as having a nutritive and detoxifying role. The amnion itself is surrounded by another larger sac —the allantois. This is the well-known bag of water, which is punctured anytime from a couple of minutes to many minutes before birth.