While many dogs end up being euthanized because of overpopulation, legitimate breeders and even pet owners whose dogs become unexpectedly pregnant may experience excitement at the thought of a litter of tiny puppies running around. However, pregnancy is a major event in the life of a dog, so it's important for dog owners to understand the endocrinology-related changes involved.
Endocrinology is the branch of medicine that deals with the endocrine glands, their secretions, and the functions of those secretions in the body. One main type of secretion of the endocrine glands is hormones. Endocrinology plays an important role in the pregnancy not just of humans, but also of canines, because the endocrine glands and their hormonal secretions are responsible for many changes that occur during pregnancy.
The term "estrus" refers to the time period when female dogs are sexually receptive, or in heat. This happens to unspayed females every 17 to 21 days, according to the University of Missouri's College of Veterinary Medicine. The first sign of pregnancy in female canines is the failure to return to estrus following a heat cycle during which the pooch was exposed to unfixed males. Palpation is often also used to determine pregnancy. While the uterus of dog that is not pregnant is hard to identify by palpation, the uterus of a pregnant dog is enlarged by the secretion of certain hormones, so it can be more easily felt.
This refers to the phase of a canine pregnancy that involves a peak in the progesterone levels. This happens early on, between the 15th and 25th day. From that point on, progesterone levels slowly decline, according to Dr. Patrick W. Concannon of the Cornell University Department of Biomedical Sciences. After this initial peak, a second peak in progesterone levels occurs between the 25th and 35th day of pregnancy in canines.
The levels of estradiol and relaxin, two powerful female hormones, also increase in the body as canines progress through pregnancy. Prolactin levels peak late in the gestation period, surging during birth. Prolactin is the hormone responsible for milk production after pregnancy.
When they give birth, female canines also experience a brief increase in cortisol levels and increased levels of circulating level of prostaglandin and prolactin, while levels of progesterone decline. These hormonal changes are responsible for kick-starting the birth process and expelling the newborn puppies and placenta from the uterus.