An Introduction to Progesterone (Spoiler: It’s not about the numbers)

Measuring progesterone is the single-most helpful diagnostic in determining when your bitch is ready to be bred. If you want to maximize the chances that she will get pregnant, measuring progesterone is absolutely essential. There is a fair amount of misinformation out there when it comes to the best way to use progesterone as part of breeding management, so in this week’s blog post I will set the record straight! 

Progesterone Physiology 101 

During the majority of the year, when a bitch is not in heat, her progesterone level is very low, or at “baseline,” which can be anything under ~0.5 ng/mL (nanograms per milliliter of blood). As the cells in her ovary prepare for ovulation, small amounts of progesterone are released, causing the levels in her blood to slowly rise. When the level reaches APPROXIMATELY (but not necessarily exactly) 2.0 ng/mL, there is a surge of a hormone called LH (luteinizing hormone). The LH surge essentially tells the ovaries that it’s time to ovulate, but it still takes approximately 48 hours from the LH surge for ovulation to occur. 

When ovulation occurs, progesterone has usually reached somewhere between 5.0 ng/mL and 10 ng/mL, but the specific number is NOT as important as the TREND. In this timeframe, you will see a rapid increase in the progesterone numbers. It might go from 4.0 ng/mL to 8.0 ng/mL, from 5 ng/mL to 9 ng/mL, etc. Again, the specific numbers don’t matter, it’s all about the trend. 

The eggs are still not ready yet! The canine is different from other species in that the eggs are not ready to be fertilized as soon as they are ovulated. Instead, they need approximately another 48 hours to mature before they can be fertilized. Progesterone can do whatever it wants at this point – in some cases it continues to rise as high as 80 ng/mL, in others it might rise to 20 ng/mL and stay there. Ultimately, after I know that I’ve seen a trend that reveals ovulation (i.e. a rapid rise that ends over 10 ng/mL), I don’t particularly care what the numbers do. 

The Myth of the Magic Number

To recap, when I’m looking at progesterone numbers to determine time for breeding I’m looking for two major landmarks –

• 1) the point at which progesterone rises above 2.0 ng/mL (aka the LH surge)
• 2) the point approximately 48 hours after point #1 at which progesterone rapidly rises between 4.0 ng/mL and 10.0 ng/mL (aka ovulation).

The eggs will be ready for fertilization approximately two days after ovulation and will remain able to be fertilized for a few days after that. The timing of breeding depends on a number of factors – whether the semen is fresh, chilled, or frozen; whether this scenario is falling on a weekend; whether we’re doing only one breeding or multiple; the quality of the semen – but once I know that ovulation has occurred the timing of the breeding does NOT depend on the specific progesterone number.  It drives me bananas when I hear people declare that they “can’t” breed her until [insert their magic number here]. It just isn’t true. It’s not about the numbers, it’s about the trend. 

Machines, Machines, Everywhere and Not a One to Trust

Earlier this year, a very common brand of in-house bloodwork analyzer, the Catalyst II by Idexx, added the ability to measure progesterone. This is a double-edged sword because it gives many more veterinary clinics the ability to help responsible breeders with progesterone timing (which is great!) but many general practice veterinary clinics do not have experience managing progesterone timing, which can lead to confusion and frustration on the part of all parties involved. Additionally, there are a couple significant factors to take into consideration when discussing which machine is being used to measure progesterone, and this will be the topic of next week’s post! Until then, have a great week!

Yours in healthy, responsibly-bred puppies, 

Dr. Kristina