Ask experienced breeders what causes most failed breedings and you will rarely hear “infertility” as the first answer. Far more often, the problem is timing. The clinical consensus is blunt: mistimed breeding is by far the most common cause of failure to conceive in the bitch, ahead of any real fertility problem.
Progesterone testing for dog breeding is the tool that closes that gap. But a number on a report is only useful when you understand what it says about where the bitch is in her cycle. This guide covers the milestones that matter, what each concentration represents physiologically, and how to turn a result into a breeding decision.
Before you use any number on this page
Progesterone thresholds are specific to the assay that produced them. Different analyzers report systematically different values on the same blood sample, so a threshold from one instrument is not valid on another. Read your results against the reference range published for your analyzer, and use the day counts below as the part that transfers. See why your analyzer’s chart doesn’t match the textbook.
- In dogs, progesterone rises before ovulation. It forecasts the fertile window rather than confirming it has passed.
- A reading of 2 to 3 ng/mL marks the LH surge, conventionally called Day 0. Everything else is counted from there.
- Ovulation follows around Day 2, but the eggs need another 2 days to mature. The fertile window opens around Day 4.
- Fresh semen: breed Days 4 and 6. Chilled or frozen: Days 6 and 7.
If you run a Finecare Vet or Pro-DX
The laboratory-scale figures in this article come from reference-method literature. This analyzer reads on its own scale and sits systematically higher: independent testing puts the regression at Finecare = 5.87 + 1.28 × reference, so on the Finecare chart estimated ovulation falls at 8–15 ng/mL and the fertile range at 15–40 ng/mL. Time from the chart supplied with your kit lot, not from the numbers in this article. See how the common analyzers compare, and why thresholds belong to the assay that produced them →
Why dogs are different from almost every other species
In most mammals, progesterone rises after ovulation. The follicle releases the egg, then collapses and reorganizes into a corpus luteum, and only then does progesterone climb. That makes progesterone a confirmation hormone: it tells you ovulation already happened.
Dogs do not work this way. The canine follicle undergoes unusually strong luteinization before it ruptures — a phenomenon Concannon, Hansel and McEntee documented in Biology of Reproduction in 1977 — so progesterone starts rising in the days leading up to ovulation. That single quirk of physiology is what makes serial progesterone testing so powerful in breeding management. In the bitch, progesterone is not a rear-view mirror. It is a forecast.
There is a second quirk that matters just as much. The bitch ovulates immature primary oocytes. As De Cramer summarises it: canine eggs cannot be fertilized immediately after ovulation because they need another 2 days to mature, and by that time progesterone usually averages 10 ng/mL or above. So the day of ovulation is not the day to breed.
A third complication is that the cycle itself is elastic. Drawing on Johnston, Root Kustritz and Olson, proestrus averages 9 days but can range from 1 to 27 days in extreme cases; Beijerink puts estrus at an average of 9 days with a range of 3 to 21. That variability is exactly why counting days from the first sign of heat does not work, and why a hormonal anchor is needed.
The milestone concentrations in progesterone testing for dog breeding
The chart below shows the shape of a typical rise. Treat the values as landmarks on a map rather than exact coordinates, because individual dogs vary and assay platforms vary too.
The interpretation table below follows De Cramer’s reference table for bitches in heat.
| Progesterone (ng/mL, reference lab scale) | Likely event and action | Reference day |
|---|---|---|
| Under 1.0 | Anestrus or proestrus. Retest in 3 to 4 days. | Before Day 0 |
| 1.0 to 2.0 | Proestrus and early estrus, pre-LH surge. Retest in 2 days. | Approaching Day 0 |
| 2.0 to 3.0 | LH surge. Retest in 2 days to confirm a continued rise, then aim to breed 4 to 7 days after the rise above 2. | Day 0 |
| 3.1 to 4.9 | Post-LH surge, pre-ovulation. Retest in 2 days; breed 3 to 5 days from this date. | Day 1 |
| 5 to 8 | At or near ovulation. Retest in 2 days; breed 2 to 4 days from this date. | Day 2 |
| 8 to 10 and above | Fertilizable period. Breed on this day and for another 2 to 3 days. | Days 4 to 6 |
| Established luteal phase | No longer carries timing information. Values vary widely by analyzer. | Days 15 to 30 post-ovulation |
Notice how compressed the useful part of that scale is. Almost every decision you will make happens between about 1 and 10 ng/mL, across four or five days. Once the rise is established the number stops carrying timing information, which is why a single late test often arrives too late to act on.
Same blood, different rulers: the milestones by analyzer
The figures above come from a laboratory analyzer. Point-of-care systems are calibrated to their own scale — the published reference range for the Wondfo cProg test on the PETlife Pro-DX, for instance, is 15 to 35 ng/mL over a 1 to 50 ng/mL linear range. Same event, different ruler. The chart below makes the size of that difference visible: the same schematic rise as Figure 1, drawn where each analyzer actually reports it.
The table adds the sources and the practical reading of each row. If you have been handed the two-column miniVIDAS-versus-IDEXX card that many reproduction clinics use, the same four events are laid out against the Pro-DX chart in why your analyzer’s chart doesn’t match the textbook.
| Analyzer (method) | LH surge, Day 0 | Ovulation, Day 2 | Fertile window, Days 4–6 | How it sits vs the reference scale |
|---|---|---|---|---|
| Reference lab, CLIA (Immulite-type) interpretation-table bands | 2–3 | 5–8 | 8–10 and above | The scale used throughout this article. |
| bioMérieux Mini Vidas (ELFA) interpretation-table bands | ≈ 2–3 | ≈ 5–8 | ≈ 8–10 and above | Tracks the reference scale closely: validated against RIA with r = 0.989 and deviations within about 15% in both directions (Brugger 2011). Reference-scale thresholds are a reasonable starting point; confirm with your lab. |
| Bionote Vcheck V200 (FIA) measured stage means (95% CI) | ≈ 1.4–2.3 | ≈ 4.5–6.3 | ≈ 10–15 | Reads systematically lower — on average 1.26 ng/mL below CMIA across the cycle, with the gap widening after ovulation (Wutthiwitthayaphon 2024, stage means and 95% CIs). |
| Wondfo Finecare Vet / PETlife Pro-DX (FIA) ★ THE ANALYZER WE CARRY vendor interpretation chart | 2–4 | 8–15 | 15–40 | Reads systematically higher — independent testing puts the regression at Finecare ≈ 5.87 + 1.28 × reference. Values here follow the chart supplied with the Pro-DX kit. |
| IDEXX Catalyst One (immunoassay) | — | — | — | Reads higher above 2 ng/mL, rising roughly 1.4–1.5× faster than the Immulite (Zuercher 2021). IDEXX publishes no stage table on this scale; use the interpretation chart supplied with the instrument. |
How to read this table
These are approximate anchors from single studies and vendor charts, not conversion factors — agreement between instruments also drifts with kit lot and calibration. Do not convert a reading from one machine to another machine’s chart. Pick one analyzer, stay on it for the whole cycle, and interpret against the chart supplied with that kit lot.
One row looks contradictory, and is not. The Vcheck reads lower than the reference lab, yet its Days 4–6 figure (≈ 10–15) is higher than the reference figure (8–10 and above). The two are different kinds of number, which is why the rows are labelled: 8–10 and above is a threshold — the value at which the interpretation table says the fertilizable period has begun. 10–15 is a measured range — what the instrument actually read across that stage. Progesterone keeps climbing through Days 4–6 on every scale, so a measured range for the window will always sit above the threshold that opens it. Compare like with like: read down a column only within rows carrying the same label.
- The shape of the curve: baseline, rise, sustained climb
- The day counts: ovulation on Day 2, breed Days 4–6 (fresh) or 6–7 (frozen)
- The confirmation rule: one crossing is not Day 0
- Every absolute ng/mL threshold, including the ones in this article
- Readings taken mid-cycle on a different machine
- Another breeder’s numbers from another analyzer
Day count beats the raw number
This is the concept that separates breeders who get consistent results from breeders who chase a magic number. Once you have identified Day 0, the calendar takes over — and the calendar is the same on every instrument.
The value of progesterone testing is not the value itself. It is establishing a reliable Day 0, after which the rest of the cycle unfolds on a predictable schedule.
Common mistake
Testing once, getting a mid-range result, and breeding the same afternoon. A value at ovulation says the eggs are being released. It does not say they are ready. Breeding on the day of ovulation is, in dogs, breeding early. A second mistake is treating a single crossing of 2 ng/mL as Day 0 — see the false rise.
Breeding windows by semen type
How much slack you have depends entirely on how long the sperm survives once it is in the reproductive tract. This is where timing precision either becomes optional or becomes everything. All of the days below are counted from Day 0, so they apply whatever analyzer you use.
| Semen type | Survival in the tract | Suggested breeding days | Interval |
|---|---|---|---|
| Natural breeding or fresh AI | 4 to 6 days, but as little as 1 to 2 days for many studs | Days 4 and 6 | 48 hours |
| Fresh chilled or compromised semen | Reduced | Days 6 and 7 | 1 day |
| Frozen | Probably not exceeding 12 hours | Days 6 and 7 | 1 day |
Those survival figures come from specific work. Doak, Hall and Dale, publishing in the Journal of Reproduction and Fertility in 1967, established that spermatozoa can remain alive in the reproductive tract of the bitch for 4 to 6 days and longer — though in diminishing numbers. But England and Concannon add the caveat that matters: for many stud dogs, sperm may survive no longer than 1 or 2 days in the female tract.
That is why breeding should start at the beginning of the fertile period rather than late in it, with 48 hours between breedings. You cannot know in advance which kind of stud you have.
Frozen semen has no cushion at all. Thawed sperm survive in the female genital tract for a period probably not exceeding 12 hours, so insemination is timed to the start of the fertile period with a 1-day interval. The Days 6 and 7 timing for frozen and chilled semen comes from Steckler, Nöthling and Harper’s multi-sire frozen-thawed insemination trial.
The rate of rise carries information too
Two bitches can both cross 2.5 ng/mL on the same day and then behave very differently. One climbs steeply within 48 hours. The other drifts. Those are not equivalent cycles, and a slow, shallow rise is worth flagging to your veterinarian, particularly with frozen semen where the window is unforgiving. This is another argument for serial testing over single-point testing: a curve tells you something a dot never can.
What progesterone cannot tell you
Progesterone is a timing tool, not a general-purpose diagnostic. As De Cramer states, the progesterone profiles of pregnant and non-pregnant bitches do not differ significantly, and therefore progesterone cannot be used for pregnancy diagnosis. It will not assess semen quality, uterine health, or whether a breeding was physically successful. And it cannot compensate for a bitch who is cycling abnormally, which is a conversation for your veterinarian rather than a test strip.
Vaginal smears are worth mentioning here too, since they are often proposed as a substitute. De Cramer’s assessment is that they are useful for confirming a bitch is in heat and for spotting pathology, but they cannot accurately reflect where she is in the cycle, when a mating should take place, or how many days of heat remain. They are, however, genuinely useful for determining when she is out of heat, which helps with whelping dates.
Keep the whole curve, not just the last result
A progesterone program produces data across every cycle, every bitch, and every season. Kept as loose paper slips, that data is nearly worthless by the following year. Recorded consistently, it becomes one of the more valuable assets in a breeding program: you learn which of your bitches rise quickly, which run slow, and which reliably ovulate later in proestrus than the textbook suggests.
De Cramer warns against the opposite habit — assuming each cycle replicates the last. A bitch bred successfully on day 10 one season does not necessarily ovulate on day 10 the next, and breeding to that assumption works in some cases but certainly not all. Records are useful as a starting point for when to begin testing, not as a substitute for testing.
The free Pup Planner app was built with that in mind, letting you plot test results over time and keep breeding history attached to the dog rather than to a notebook. Whatever system you use, the principle holds: the second cycle is easier to time than the first, but only if you kept the first one.
Frequently asked questions
What progesterone level means a dog is ready to breed?
There is no universal number, because the answer depends on your analyzer’s scale. What is universal is the day count: breed 4 to 7 days after progesterone first rose above 2 ng/mL, which works out to Days 4 and 6 for fresh semen and Days 6 and 7 for chilled or frozen.
How many progesterone tests does one cycle need?
For critical breedings — frozen or chilled semen, compromised semen, or an atypical cycle — De Cramer recommends three, four or more assays to cover the LH peak, ovulation, and the onset of the fertile period. A single reading is generally not advised.
Can progesterone confirm that a dog is pregnant?
No. Dogs produce progesterone on essentially the same schedule whether or not conception occurred, so the profiles overlap. Pregnancy diagnosis uses relaxin testing, ultrasound, or radiography instead.
Why did two labs give different numbers for the same dog?
Different assay platforms genuinely produce different values on the same sample, sometimes by several ng/mL — a Vcheck reads lower than a reference lab, a Finecare reads higher, and a Mini Vidas sits close to it (see the comparison table). Check the unit as well, since ng/mL and nmol/L differ by a factor of about 3.18. Stay on one platform for the whole cycle.
Sources
Merck Veterinary Manual. Breeding Management of Bitches. In: Management of Reproduction: Dogs and Cats. Merck & Co., accessed September 2026.
De Cramer K. Simplifying the Canine Reproductive Cycle Using the Concentration of Progesterone. IDEXX Laboratories, CLD-12211-00.
Concannon PW, Hansel W, McEntee K. Changes in LH, progesterone and sexual behavior associated with preovulatory luteinization in the bitch. Biol Reprod. 1977;17(4):604–613.
Doak RL, Hall A, Dale HE. Longevity of spermatozoa in the reproductive tract of the bitch. J Reprod Fertil. 1967;13:51–58.
England GWC, Concannon PW. Determination of the optimal breeding time in the bitch: basic considerations. In: Recent Advances in Small Animal Reproduction. IVIS; 2002.
Steckler D, Nöthling JO, Harper C. Prediction of the optimal time for insemination using frozen-thawed semen in a multi-sire insemination trial in bitches. Anim Reprod Sci. 2013;142(3–4):191–197.
Johnston SD, Root Kustritz MV, Olson PNS. Canine and Feline Theriogenology. WB Saunders; 2001.
Beijerink N. Endocrinology of physiological and progestin-induced canine anoestrus [dissertation]. Utrecht University; 2007.
Brugger N, Otzdorff C, Walter B, Hoffmann B, Braun J. Quantitative determination of progesterone (P4) in canine blood serum using an enzyme-linked fluorescence assay. Reprod Domest Anim. 2011;46(5):870–873.
Wutthiwitthayaphon S, Suwannachote T, Arayatham S, Prasitsuwan W, Ruenphet S. Assessment of Vcheck® analyzer for rapid progesterone concentration measurement including recommendations for achieving the optimal breeding time in bitches. Vet World. 2024;17(2):427–433.
Zuercher J, Boes KM, Balogh O, Helms AB, Cecere JT. Comparison of a point-of-care analyzer with a chemiluminescent immunoassay for serum progesterone measurement in breeding management of the bitch. Front Vet Sci. 2021;8:660923.
Keep reading
Threshold values in this article are drawn from published veterinary literature and are assay-dependent: absolute numbers are not interchangeable between analyzers. Always interpret results against the reference range for the instrument you are using. This article is educational and is not a substitute for veterinary advice. Breeding decisions, test interpretation, and any medical intervention should be made in consultation with your veterinarian or a board-certified theriogenologist.