Upon becoming pregnant, up until whelping, dogs undergo a variety of hormonal changes, just as humans and other animals do. Such changes may cause subtle or quite evident effects to a dog's body. By detecting the effects of such hormones and acknowledging their levels, dog owners may recognise important stages of pregnancy in their dogs and even come up with a rough estimate for the big event: whelping day.
Whereas humans may rely on early pregnancy test kits that work by recognising pregnancy-specific hormones, in canines such tests are ineffective because of the endocrine physiology of dogs. Indeed, dogs unlike humans, do not produce chorionic gonadotrophin, the compound confirming pregnancy in test kits for humans. And levels of the hormone progesterone are not helpful in detecting pregnancy in canines since bitches secrete this hormone for two months after heat, regardless of whether they are pregnant or not.
There is, however, a hormone that indicates pregnancy in dogs--relaxin. It is primarily produced by the placenta when a fertilised egg is implanted. This hormone can be detected in a dog's plasma about 22 to 25 days after fertilisation. The levels of this hormone peak around 40 to 50 days into gestation, according to Merk Veterinary Manual. Because pseudopregnant bitches do not produce relaxin, relaxin tests are effective in distinguishing an actual pregnancy from a false pregnancy.
The levels of prolactin--a hormone produced by the pituitary gland, responsible for the stimulation of the mammary gland and lactation--tend to increase from day 30 to day 35 of a pregnancy until term is reached, according to Patrick W. Concannon, PhD. Increased levels of prolactin, though, are not reliable indicators of canine pregnancy since such levels tend to increase as well in dogs suffering from a false pregnancy.
Although progesterone, as mentioned earlier, is not effective in determining pregnancy in the first place, this hormone can be very helpful when whelping day is around the corner. In the first 60 days of pregnancy a dog's progesterone levels will be maintained around 10-80 ng/ml; however, within 48 hours of whelping the level of this hormone will dramatically decrease to the 2 ng/ml range and then to the 1 ng/ml range 24 hours prior to giving birth.
A significant decline in progesterone levels is accompanied by a significant temperature drop that can be witnessed by using a rectal thermometer. Whereas the normal temperature in dogs is around 100 to 102.5, when whelping day nears it will drop to 37.2 degrees C or lower. Generally, this drop indicates that a bitch will enter the first stage of labour within approximate 24 hours, according to the American Kennel Club.