Progesterone, a steroid hormone, is the primary regulator of the female canine reproductive cycle. It is produced by structures called corpora lutea on the ovaries following ovulation. Monitoring its concentration in the bloodstream provides breeders and veterinarians with insight into a dog’s fertility status and the progression of pregnancy.
The Core Function of Progesterone in Dogs
Progesterone (P4) is often called the “pregnancy hormone” because its main function is to prepare the uterus for receiving a fertilized egg. During the initial stages of the heat cycle (anestrus and proestrus), P4 levels remain at a low baseline, typically below 1.0 ng/mL. The hormone’s level begins its rise just before the fertile period, signaling a shift in the dog’s reproductive physiology.
The hormone thickens the lining of the uterus and promotes the necessary secretions to sustain a developing embryo. P4 also suppresses uterine contractions, which helps maintain the integrity of the pregnancy once established. A unique aspect of canine reproduction is that P4 levels rise and remain elevated after ovulation, regardless of whether the dog is successfully bred.
This sustained elevation of P4 lasts for approximately 60 days, characterizing the luteal phase in both pregnant and non-pregnant dogs. This hormonal similarity explains why non-pregnant dogs often experience a “pseudopregnancy,” or false pregnancy, complete with physical and behavioral changes. The high P4 level indicates that the ovaries are prepared to support a pregnancy.
Using P4 Levels to Time Optimal Breeding
Tracking the initial rise of P4 is the most accurate method for pinpointing the fertile window and maximizing conception. Testing typically begins a few days into the heat cycle to establish a baseline, usually less than 1.0 ng/mL. The first event is the luteinizing hormone (LH) surge, which occurs concurrently with a slight P4 increase to between 1.5 ng/mL and 3.0 ng/mL.
The LH surge triggers the release of eggs (ovulation), which occurs about two days later, when P4 levels are generally between 4 ng/mL and 10 ng/mL. However, the eggs released are not immediately fertilizable, requiring another two days to mature. Therefore, the optimal breeding window begins when the P4 concentration reaches this 4-10 ng/mL range, as the eggs will be ready for fertilization in two to three days.
For fresh or natural breeding, the optimal time is typically two to three days after the P4 level hits 5 ng/mL. This timing allows fresh sperm, which survives for several days, to be present when the eggs are mature. When using frozen or chilled semen, which has a shorter lifespan, precise timing based on P4 levels is necessary to ensure the semen is introduced when the eggs are at peak maturity.
P4 Monitoring During Pregnancy and Whelping
Once pregnancy is established, P4 monitoring focuses on maintaining gestation and predicting the time of birth. Progesterone levels must remain above a minimum threshold, typically 2.0 ng/mL, throughout the pregnancy to prevent fetal loss. If a dog has a history of pregnancy failure, P4 levels may be monitored regularly; a drop below 4 ng/mL to 5 ng/mL before day 55 may indicate a need for veterinary intervention.
The corpora lutea continue to produce P4 until the end of the pregnancy, with levels often peaking between 10 ng/mL and 80 ng/mL. The most significant P4 event occurs in the last 24 hours before whelping begins. The P4 level drops abruptly, signaling the activation of the birthing process.
This sharp decrease in P4 to below 2.0 ng/mL is the most reliable hormonal indicator that labor will commence within 24 hours. When the P4 concentration falls further, often dropping below 1.0 ng/mL, whelping is imminent. Monitoring this drop is helpful for planning elective C-sections or ensuring immediate veterinary support is available for delivery.
Understanding P4 Testing and Result Interpretation
Progesterone testing uses a blood sample, typically serum or plasma, to obtain a quantitative measurement of the hormone concentration. The most reliable method is a quantitative blood test that provides a specific numerical result, usually reported in nanograms per milliliter (ng/mL). Results can be obtained from external laboratories or from in-house veterinary analyzers.
The type of testing equipment used can influence the absolute numerical results, making consistency important when tracking a dog’s cycle. It is recommended to use the same laboratory or machine for serial testing to accurately track the rate of P4 rise, which is more important than a single reading. Some serum separator tubes can also affect the results, a factor veterinarians consider when processing samples.
Because interpretation hinges on a dynamic rate of change, serial testing is necessary, usually every one to three days, until ovulation is confirmed. While numerical thresholds provide a roadmap, they should always be interpreted by a veterinarian. The veterinarian combines the hormonal data with the dog’s clinical signs and the specifics of the testing method. P4 testing is a powerful tool, but its successful application requires professional guidance.