Prior to coming in season, progesterone levels in the bitch are low, less than 2 nmol/l. The first significant change in progesterone occurs at the time of a brief but large increase in lutenising hormone (LH) when progesterone levels rise quickly to above 6 nmol/l. Roughly two days after this event, ovulation occurs. Ovulation can be as early as day 7 and as late as day 27 of the oestrous cycle, showing the importance of blood testing. Once the bitch’s progesterone level reaches 15-20 nmol/l ovulation has occurred, subsequent serum progesterone levels can vary greatly. For example, at three days post ovulation, progesterone levels may range from as low as 30mmol/l to as greater than 100mmol/l, depending on the bitch. In order to accurately identify the day of ovulation it is important to identify the rise in progesterone level that occurs at the time of the LH peak. For most bitches blood sampling for progesterone measurement should begin around 5 days after the onset of vulvular swelling and bleeding. Bitches are then retested every 2-3 days until ovulation occurs. After ovulation, progesterone levels will remain high for approximately two months, regardless of whether or not the bitch has been mated or is pregnant, at which point they return to near 0 nmol/l.

It is important to note that vaginal bleeding, swelling, “tail-turning” and receptivity to males is predominantly a response to estrogens in the bitch’s bloodstream, and not a reliable indicator of progesterone levels and consequently ovulation. If we know the day a bitch ovulates we can work out when she will be fertile. This is important with all matings, but especially so with fresh chilled extended semen and imperative with frozen semen. Once ovulation has occurred a bitch’s eggs take approximately 48 hours to become fertile. They will then remain fertile for approximately 48 hours. The idea is to maximize contact time between viable semen and fertile eggs. Next we need to look at the method we are using for breeding. Insemination can be natural (where the dog mates with the bitch) or artificial insemination (where the semen is manually introduced into the bitches vagina or womb) with fresh, chilled or frozen semen. There are major differences in the times for which the different kinds of semen will remain viable once in the bitch.

Why is measuring serum progesterone important?

Bitch progesterone measurement is a vital part of breeding management. It allows us to time ovulation and hence the optimum time for natural mating or insemination. There is a wide variety of progesterone measurements between labs so we would advise that you only do not change laboratory during a season. We usually recommend you start sampling from day 5-7 of her season (earlier if she has short heats or has been problematic to get pregnant in the past). Re-sampling intervals will be recommended by us but is usually every 2-3 days.