Progesterone testing is the single most reliable tool for timing breeding and predicting whelping in dogs. Unlike many mammals, the dog ovulates immature eggs that need days of additional development before they can be fertilized, making a simple visual observation of “standing heat” a poor guide. Serial blood progesterone measurements track the hormonal cascade that triggers ovulation, identify when eggs actually become fertile, and signal when puppies are ready to be born. The numbers that matter at each stage are well established, but they shift depending on your testing method, and the biology has a few quirks that catch even experienced breeders off guard.
From Baseline to the LH Surge
During most of a bitch’s cycle, progesterone sits at a low baseline, typically under 1 ng/mL. The first meaningful change happens when progesterone creeps above that baseline into the 1 to 2 ng/mL range, signaling the transition from late proestrus into the fertile window. This initial rise is driven by the same hormonal shift that triggers the luteinizing hormone (LH) surge, and it’s the reason veterinarians recommend starting progesterone testing early in the heat cycle rather than waiting for obvious behavioral signs.
The LH surge itself is the anchor point for everything that follows. It lasts only about 24 hours and is difficult to catch with a single blood draw, which is why progesterone serves as a practical proxy. On the day of the LH surge, progesterone typically reads around 2.0 ng/mL. A large prospective study of over 1,400 estrous cycles found the average progesterone at estimated LH day was about 2.7 ng/mL, though individual variation is real.1PubMed Central. Accuracy and precision guidelines for optimal breeding time in bitches using in-house progesterone measurement compared with chemiluminescent microparticle immunoassay Once you see that first jump past 2 ng/mL, the clock starts ticking toward ovulation.
Pinpointing Ovulation
Ovulation in dogs typically occurs about two days after the LH surge, when progesterone has risen further into the range of roughly 4 to 10 ng/mL. A study using daily blood draws and ultrasonographic confirmation found the average progesterone on the actual day of ovulation was around 2.3 ng/mL, with most dogs showing values of 2 ng/mL or higher, while the day before ovulation averaged only about 1.1 ng/mL.2Journal of Veterinary Medical Science. Plasma LH and Progesterone Levels before and after Ovulation and Observation of Ovarian Follicles by Ultrasonographic Diagnosis System in Dogs The large 1,420-cycle study reported somewhat higher values at estimated ovulation, averaging about 4.8 ng/mL two days after the LH surge and about 7.2 ng/mL three days after.3Theriogenology. Normal progesterone profiles during estrus in the bitch: A prospective analysis of 1420 estrous cycles
This spread in reported numbers is not random error. It reflects genuine variation between individual dogs, differences in assay methods, and whether the study pinpointed ovulation by ultrasound or estimated it from the LH surge. The practical takeaway for breeders is that a progesterone reading climbing past 5 ng/mL generally means ovulation has already occurred or is happening right now. But the story doesn’t end at ovulation, and this is where dogs diverge sharply from other species.
Why Breeding Day Is Not Ovulation Day
In most mammals, eggs are released from the ovary in a mature state, ready for fertilization within hours. Dogs are different. The bitch ovulates primary oocytes that are still immature and need an additional two to three days of maturation inside the oviduct before sperm can penetrate them.4PubMed Central. Oocyte biology and challenges in developing in vitro maturation systems in the domestic dog This is strikingly unusual among domesticated animals, where the maturation window is usually measured in hours rather than days.5PubMed. Folliculogenesis, ovulation and endocrine control of oocytes and embryos in the dog
Research tracking oocyte development in detail found that eggs remained at their immature stage until about 44 hours after ovulation, with the first fully mature eggs appearing around 54 hours post-ovulation. Sperm penetration of immature eggs was exceedingly rare, and in most cases fertilization didn’t happen until roughly 90 hours after ovulation.6Reproduction. In vivo meiotic resumption, fertilization and early embryonic development in the bitch Another study confirmed that full maturation reached 100% of oocytes by about 109 hours post-ovulation.7Biology of Reproduction. Progesterone Plays a Critical Role in Canine Oocyte Maturation and Fertilization
This delayed maturation is precisely why breeders are told to wait two to three days after the estimated ovulation date to breed, especially when using fresh-chilled or frozen semen with limited viability. If you breed on the day progesterone first hits 5 ng/mL, sperm delivered at that moment may die before the eggs are ready. The classic recommendation is to breed when progesterone is in the range of roughly 15 to 25 ng/mL for natural matings or to time artificial insemination for about two to three days post-ovulation. Dogs that show a slower rise in progesterone can be trickier to time, and there is evidence that bitches with slow-climbing progesterone curves produce smaller litters when inseminated with frozen semen compared to those whose progesterone rises quickly.3Theriogenology. Normal progesterone profiles during estrus in the bitch: A prospective analysis of 1420 estrous cycles
Progesterone During Pregnancy
Once ovulation occurs, progesterone continues to climb regardless of whether the bitch is pregnant. The dog’s corpus luteum is the sole significant source of progesterone for the entire pregnancy, and unlike many species, the placenta does not take over progesterone production. This makes pregnancy entirely dependent on ovarian function from start to finish.
In the first few weeks after ovulation, progesterone peaks at high levels. A study comparing pregnant and non-pregnant dogs found that at 11 to 19 days post-ovulation, pregnant bitches averaged about 30 ng/mL while non-pregnant bitches were similar at about 28 ng/mL. By 23 to 32 days post-ovulation, both groups had declined to around 22 to 23 ng/mL. The critical difference showed up late in gestation: at 52 to 60 days post-ovulation, pregnant dogs averaged about 6.7 ng/mL, while non-pregnant dogs had already dropped to about 3.2 ng/mL.8PubMed Central. Progesterone Concentrations during Canine Pregnancy That late-pregnancy difference is significant because it shows pregnant dogs actively maintain progesterone longer than non-pregnant dogs going through a normal diestrus phase.
Through the middle weeks of gestation, progesterone follows a gradual downward slope in both pregnant and non-pregnant bitches, and that decline doesn’t appear to be driven by changes in estrogen or prostaglandin levels in any obvious way.9PubMed. Plasma concentrations of 13,14-dihydro-15-keto prostaglandin F2-alpha (PGFM), progesterone and estradiol in pregnant and nonpregnant diestrus cross-bred bitches Because progesterone is essential to maintaining the uterine environment, any condition that damages or suppresses the corpus luteum during pregnancy can cause fetal loss. Breeders monitoring a high-risk pregnancy sometimes track progesterone throughout gestation, with most reproductive veterinarians wanting to see values stay above roughly 5 ng/mL until the very end.
The Pre-Whelping Progesterone Drop
The sharp decline in progesterone before delivery is one of the most useful and most misunderstood signals in canine reproduction. In the final days before whelping, progesterone falls steeply. Measurements taken at various intervals before parturition show a clear trajectory: around 120 hours (five days) before delivery, values average about 4.5 ng/mL; by 36 hours out, about 3.1 ng/mL; at 20 hours, about 1.2 ng/mL; and by 10 hours before delivery, about 0.6 ng/mL.10Oxford Academic (Biology of Reproduction). Parturition and Lactation in the Bitch: Serum Progesterone, Cortisol and Prolactin Progesterone stays low throughout lactation.
Many breeders rely on temperature drops to predict whelping, which works because the fall in progesterone causes a transient dip in body temperature. But progesterone testing provides a more direct and quantitative picture. When progesterone falls below about 2 ng/mL, most bitches will whelp within 24 to 48 hours. This is especially useful for scheduling planned cesarean sections, where delivering too early risks premature puppies and delivering too late risks fetal distress.
There is an important caveat, though. A recent retrospective study found that the reliability of this pre-whelping progesterone drop varies with litter size. Roughly 70 to 75% of bitches carrying only one or two puppies did not show a measurable decline in progesterone by the time of surgical delivery. Brachycephalic breeds were also more likely to show the decline than other breeds. The study concluded that pre-whelping progesterone cannot reliably predict fetal readiness for birth in bitches carrying very small litters.11Theriogenology. Is the pre-partum serum progesterone decline associated with litter size in dogs? A retrospective study If you’re managing a singleton or twin pregnancy, you should not rely on progesterone alone to decide when to intervene.
Testing Methods and Why Your Numbers Depend on the Machine
One of the most practically important things about canine progesterone is that the number you get depends heavily on how it was measured. The reference ranges cited in textbooks and breeding guides were mostly established using radioimmunoassay (RIA), which was the gold standard for decades. Most veterinary clinics today use either chemiluminescent immunoassay (CLIA) on large laboratory analyzers or point-of-care (POC) analyzers designed for in-house use. These methods do not always agree with each other or with RIA.
A study comparing RIA with two different chemiluminescent systems found that one CLIA platform performed similarly to RIA, while another produced significantly lower readings, with a median difference of about 2.1 ng/mL.12PubMed. Evaluation of two chemiluminescent assays compared with radioimmunoassay for serum progesterone measurement in bitches Separate research comparing RIA, CLIA, and ELISA found that all three correlated well with each other in terms of tracking the direction of change, but the absolute numbers differed meaningfully. ELISA tended to read highest, CLIA intermediate, and RIA lowest.13PubMed. Blood periovulatory progesterone quantification using different techniques in the dog
Point-of-care analyzers, which many reproductive veterinarians use because they give results in minutes rather than hours, have their own quirks. A comparison of two POC devices against a lab-grade CLIA showed good overall correlation but found a significant positive bias at values above 2 ng/mL, meaning the POC tended to read higher than the reference method during the critical ovulation-timing window. The imprecision of POC devices was also greater, with inter-assay coefficients of variation reaching about 13% at low progesterone levels compared to about 6% for the laboratory analyzer.14PubMed Central. Comparison of a Point-of-Care Analyzer With a Chemiluminescent Immunoassay for Serum Progesterone Measurement in Breeding Management of the Bitch Another popular POC device, the Vcheck analyzer, read on average about 1.26 ng/mL lower than a laboratory chemiluminescent method, though the overall trend-tracking was considered fair.15PubMed Central. Assessment of Vcheck® analyzer for rapid progesterone concentration measurement including recommendations for achieving the optimal breeding time in bitches
What does this mean in practice? First, you should ideally use the same machine for all draws within a single breeding cycle. Switching between a POC device and an outside lab mid-cycle can create confusing jumps or dips that reflect the assay difference, not your dog’s actual hormone change. Second, the commonly cited thresholds (2 ng/mL for LH surge, 5 ng/mL for ovulation) were generated using specific assay platforms, and your machine may read consistently higher or lower. A reproductive veterinarian experienced with their particular analyzer will often adjust those thresholds based on what they’ve observed clinically. Third, serial trends matter more than any single number. If Tuesday’s reading was 2.4 and Thursday’s is 6.8, the direction of change tells you ovulation is happening regardless of whether the absolute numbers match a textbook cutoff.
Sample Handling Can Move Your Results
Beyond the analyzer itself, how the blood sample is handled between the vein and the machine can shift progesterone readings. One set of experiments found that refrigerating whole blood during the first two hours after collection significantly lowered measured progesterone in serum samples. However, if the sample sat at room temperature for at least two hours before refrigeration, the effect disappeared. Heparinized plasma samples were more forgiving and showed no significant change with different storage temperatures for at least five hours.16PubMed. The effects of storage time and temperature and anticoagulant on laboratory measurements of canine blood progesterone concentrations
A separate study tested a wider range of variables, including different collection tube types (silicone, lithium heparin, EDTA), storage durations up to two weeks, and up to ten freeze-thaw cycles. None of these factors significantly affected the progesterone result.17PubMed. Effect of blood handling conditions on progesterone assay results obtained by chemiluminescence in the bitch The overall picture is that progesterone in canine blood is reasonably stable once the sample has been properly handled in the first couple of hours. The critical point for breeders sending samples to outside labs: don’t throw a freshly drawn serum tube into a cold pack immediately. Let it clot at room temperature first.
Progesterone Antagonists and Clinical Interventions
Because the dog depends entirely on progesterone from the ovaries to maintain pregnancy, blocking progesterone’s action can terminate a pregnancy or induce labor. Aglepristone is a progesterone receptor antagonist used in veterinary medicine for exactly these purposes. It competes with progesterone for binding at the receptor, so even though blood progesterone levels may remain elevated, the hormone can no longer do its job at the tissue level.
When used to terminate pregnancy at mid-gestation, aglepristone caused pregnancy loss within four to seven days after treatment began, and blood progesterone levels stayed unchanged during that period because the drug blocks the receptor rather than reducing hormone production. The treatment also led to premature cessation of corpus luteum function and shortened the interval to the next heat cycle.18PubMed. Termination of mid-gestation pregnancy in bitches with aglepristone, a progesterone receptor antagonist
Aglepristone can also be used to induce labor in a controlled setting. In one study, the first puppy was delivered between 32 and 56 hours after the initial treatment, while plasma progesterone was still elevated, confirming that the drug works by blocking the receptor rather than waiting for natural progesterone withdrawal. The induced deliveries happened significantly earlier than spontaneous whelping in the control group (about day 59.5 versus day 62.2 of gestation).19PubMed. Induction of parturition in the bitch with the progesterone-receptor blocker aglépristone A broader review of aglepristone use confirmed it was effective and safe across multiple species for inducing parturition and scheduling cesarean sections, with no apparent adverse effects on puppy health or the dam’s future fertility, provided that the original ovulation date was accurately known.20Theriogenology. Aglepristone: A review on its clinical use in animals
That last detail deserves emphasis: accurate ovulation timing at the start of the cycle is what makes safe, well-timed interventions at the end of pregnancy possible. A breeder who skips progesterone testing during breeding and then needs to schedule a cesarean section eight weeks later is working without the single most important piece of information for determining gestational age.
The Slow-Rise Problem
Not every bitch follows the textbook progesterone curve. Some dogs show a sluggish, gradual climb rather than the brisk doubling that makes ovulation easy to pinpoint. The 1,420-cycle study identified this variation explicitly, categorizing dogs into fast and slow progesterone curves. Bitches with slow curves that were bred with frozen semen had significantly smaller litters, averaging about 3.9 puppies per litter compared to about 5.6 for fast-curve dogs.3Theriogenology. Normal progesterone profiles during estrus in the bitch: A prospective analysis of 1420 estrous cycles This effect was most pronounced with frozen semen, where sperm viability is limited and precise timing is critical. Natural matings are more forgiving because live sperm can survive longer in the reproductive tract, giving a wider window.
If your bitch consistently shows slow progesterone rises across multiple cycles, more frequent blood draws (every 24 hours rather than every 48 hours during the critical phase) can help you avoid missing the optimal breeding window. Some reproductive specialists also recommend using LH testing alongside progesterone for these dogs to better anchor the timeline, though LH kits for dogs are less widely available and less standardized than progesterone assays.
Why Dogs Are Reproductively Unusual
The canine reproductive system has several features that set it apart from nearly all other domesticated mammals and explain why progesterone monitoring is so much more central to breeding management in dogs than in, say, cattle or horses. Dogs ovulate immature oocytes that cannot be fertilized for days, as covered earlier. But they also have unusually high circulating progesterone during the period surrounding ovulation itself, a trait that appears to play a direct functional role in oocyte maturation.5PubMed. Folliculogenesis, ovulation and endocrine control of oocytes and embryos in the dog Additionally, the dog’s corpus luteum is the sole meaningful source of progesterone for the full duration of pregnancy, making the luteal phase more consequential in dogs than in species where the placenta eventually takes over.
Dogs also have a phenomenon called polyoocytic follicles, where a single follicle contains multiple eggs. This adds complexity to the relationship between follicle counts, ovulation events, and the resulting litter size, and it means that the neat one-follicle-one-egg model used for many other species doesn’t fully apply. These quirks are not just academic curiosities. They explain why progesterone testing is not an optional refinement for serious canine breeders; it is the foundation of reliable reproductive management in a species whose fertility timeline is genuinely harder to predict from external signs alone.