Progesterone is a hormone produced by a gland in the ovaries that is functional only after eggs have been shed. It is essential to maintain pregnancy, hence PRO GESTerone, but is produced in the bitch after each season as long as ovulation has occurred, whether or not the dog becomes pregnant. When the bitch is not pregnant this phenomena is referred to as ‘pseudo’ or false pregnancy, every bitch, every season is either pregnant or pseudopregnant if ovulation has occurred, as progesterone remains at high levels during the 9 week period. The origins of this event go back to the beginnings of all canines, the wolf. In a wolf tribe only the alpha male & female mate but other females in the group cycle & produce milk to help feed the offspring.
Progesterone is therefore a very useful hormone for determining if & when ovulation is occurring. This event is vital for the workup of an ‘unproductive’ dog, as one of the main reasons for ‘apparent infertility’ is incorrect timing of mating. In canines the oestrus cycle has been named after another hormone called oestrogen. This hormone is responsible for ALL the overt behavioural & physiological signs seen when a bitch is in season. These include vulval swelling, bleeding, flagging, attraction to male, vaginal cytology changes & alterations to the pH of the vaginal fluids that can be determined by using instruments like the Draminski probe. But, in some cases, these signs due to fluctuations in oestrogen, do not correspond with actual ovulation.
Research has shown that a variation in ovulation can occur from as early as day 5 following the onset of bleeding to as late as day 28 post this sign. Sperm has remarkable robustness within the uterus, attaching to the uterine lining & obtaining nutrients from uterine secretions, and remaining fertile for around 7 days. Although live sperm have been isolated from uterine swabs for up to 14 days it is generally believed these aged sperm are not capable of fertilization. The ovulated eggs however can only be fertilized for 2-3 days.
By determining progesterone levels, matings or artificial insemination can be advised closer to the true ovulation period. This is absolutely essential for chilled & frozen inseminations. Knowing levels at the time of mating or A.I. also allows a more accurate determination of the whelping date, generally around 61 days post ovulation.
Progesterone concentrations can also be useful in determining whelping dates; levels fall dramatically around 36 hours prior to the due date. When considering an elective caeser these blood levels help to determine the best time to maximise the safe period for puppy & bitch survival. Surgery too early may result in immature pups that may not be viable & also increase the risk of life threatening bleeding from placental sites that may result in a need for an emergency spey. These tests are also useful in the rarer cases of primary inertia when a bitch may not show any signs of starting labour.