Interpreting Progesterone Test Results

A progesterone number on its own does not tell you much unless you know what it means. This guide covers exactly that: given a ng/mL reading, what stage of the cycle your dog is in and what to do next.

For guidance on when to start testing, how often to retest, and how the blood draw itself works, see our progesterone testing protocol guide. This article picks up once you already have a result in hand.

It is educational in nature and does not replace veterinary guidance. Breeding decisions should be made in partnership with a veterinarian who can see the trend across your dog's full testing history.

Quick reference: what your progesterone level means

These are commonly cited reference ranges for a rising progesterone curve. Treat them as a general guide to the shape of the cycle, not an exact cutoff — see the note on lab variability below.

ng/mL vs. nmol/L: check your lab's units first

Before you compare a single number to any chart, confirm which unit your report uses. Labs in the United States usually report progesterone in ng/mL, but many labs elsewhere — and some reference labs anywhere — report in nmol/L. The same blood sample produces a number that looks about three times larger in nmol/L, which is a very easy way to badly misread a result.

The conversion is simple: multiply ng/mL by 3.18 to get nmol/L, or divide nmol/L by 3.18 to get ng/mL. Here is how the key breeding thresholds translate:

LH surge vs. ovulation vs. egg maturation — three different events

Breeders sometimes treat "progesterone is rising" as a single event, but it actually describes three distinct biological steps that happen a few days apart. Confusing them is the most common reason a breeding happens a day or two too early.

In plain terms: the LH surge starts the clock, ovulation follows about two days later, and the eggs need roughly two more days after that before they can actually be fertilized. A progesterone value tells you where you sit on this curve — the days-since-surge matter as much as the raw number.

A worked example: one dog's cycle, reading by reading

This is why a single number is nearly meaningless on its own — the story is in the rise. Here is an illustrative series for one dog, showing how a vet reads the trend rather than any one value. Your dog's exact numbers and dates will differ.

Read in isolation, the Day 9 value of 5.4 could be mistaken for the breeding day. In context — a clean climb from baseline through the surge two days earlier — it is clearly ovulation, and breeding waits until the eggs mature. That is the difference the trend makes.

Natural mating vs. AI: how timing shifts by method

The same progesterone curve produces a different ideal breeding day depending on how sperm is being delivered, because sperm longevity varies enormously by method.

• Natural mating or fresh semen: the widest window. Sperm can survive over a week in the female reproductive tract, so breeding a day or two before ovulation still commonly works.
• Fresh-chilled AI: a tighter window. Chilled sperm survives roughly 48–72 hours (about two to three days), so insemination is usually timed to land close to ovulation itself.
• Frozen semen AI: the tightest window of all. Thawed sperm survives only about 12–24 hours, so timing needs to be accurate to within a day — this is where progesterone testing matters most.

For the full breakdown of optimal timing windows by method, see the progesterone testing protocol guide.

How your progesterone result affects due-date accuracy

This is the part most breeders don't realize until it matters: your due date is only as accurate as the reference point you count from, and mating date is the least reliable one.

Gestation length is remarkably consistent when measured from the right starting point:

• ~65 days from the LH surge — the most repeatable predictor, since the surge itself is a sharp hormonal event.
• ~63 days from ovulation — ovulation follows the LH surge by about two days, so this shifts the count accordingly.
• 57–72 days from a single mating date — a much wider range, because sperm can survive over a week waiting for eggs that themselves take days to mature. Two dogs bred on the same calendar day can conceive several days apart.

Once you know your ovulation date from testing, enter it into the Whelping Date Calculator for the most accurate due-date estimate — using the ovulation date rather than the mating date is what makes the calculation reliable.

Progesterone also predicts whelping day

Progesterone isn't only a breeding tool — the same hormone signals the end of pregnancy. Throughout gestation, progesterone stays high to hold the pregnancy in place. Then, in the final day or two before labor, it falls off a cliff.

When progesterone drops below about 2 ng/mL, labor typically begins within 24 to 48 hours — this is the single most reliable hormonal predictor of imminent whelping. Below roughly 1 ng/mL, delivery is essentially certain within a day. This same progesterone collapse is what triggers the well-known pre-whelping temperature drop to around 97–98°F, seen in about 80% of dams roughly 12–24 hours before labor.

Vets sometimes call this a "reverse progesterone" test: running the assay near the due date to decide whether whelping is truly imminent, or whether a planned cesarean should go ahead. One caveat — in very small litters (one or two puppies) the drop can be muted, so it is used alongside temperature tracking, not instead of it. To turn your ovulation date into a full whelping timeline, use the Whelping Date Calculator.

What testing costs and how often you'll test

Progesterone timing is a series, not a single test, and it helps to budget for that from the start. A single test commonly runs $50–$100, with in-house analyzers usually faster and cheaper than send-out reference labs.

• How many: most cycles need three to five tests to pin down ovulation cleanly — occasionally more if the surge is slow to appear.
• How often: testing usually starts every 2–3 days as the surge approaches, then tightens to every 1–2 days once progesterone begins climbing, so ovulation isn't missed.
• Why it's worth it: an extra test is inexpensive next to a missed breeding — especially with frozen semen, where a single mistimed day can cost the whole cycle.

For exactly when to begin testing in a given heat cycle, see the progesterone testing protocol guide.

When results are ambiguous or borderline

Not every reading falls cleanly into one bucket. A few situations are worth knowing how to handle:

• A plateau reading — the same value on two consecutive tests below roughly 5 ng/mL — usually means the LH surge hasn't fully happened yet, not that the cycle has stalled. Keep testing every 1–2 days rather than assuming the number will stay flat.
• A borderline number right at a threshold (say, 4.8 ng/mL) is far less useful on its own than the same number compared to yesterday's reading. A rapid rise across two tests points toward ovulation happening now; a slow creep suggests you still have a day or two.
• Switching labs or machines mid-cycle makes borderline readings especially unreliable, since the same blood sample can read differently on different analyzers. If your usual vet is unavailable, ask the new clinic which machine they use before comparing the number to your prior results.

When in doubt, retest sooner rather than later — an extra $50–$100 test is inexpensive next to a mistimed breeding.

Vet guidance still matters

This guide explains what published reference ranges typically mean. It is not a substitute for review by a veterinarian who can see your dog's specific curve, cytology results, and breeding history. Progesterone trends, not single numbers, are what a reproductive vet actually uses to time a breeding — bring your full series of readings to that conversation, not just the latest one.

Frequently asked questions

What progesterone level means my dog is ready to breed?

As a general reference, ovulation occurs around 5.0–10.0 ng/mL, with optimal breeding typically 48–72 hours after that reading. But the exact number that matters for your dog depends on the trend leading up to it and the reference range used by your specific analyzer — a single value in isolation is not enough to time breeding precisely.

Why isn't ovulation the same as the best time to breed?

A dog releases immature eggs at ovulation. Those eggs need roughly two more days to mature before they can be fertilized, so the fertile window opens a few days after ovulation, not at the moment progesterone first crosses the ovulation threshold. Breeding too early risks the eggs not yet being ready.

Does my progesterone reading change my puppy's due date?

Yes. Gestation is most accurately counted from ovulation (about 63 days) or from the LH surge (about 65 days) — not from the date of mating. A single mating date can sit anywhere from 57 to 72 days before whelping because sperm can survive over a week inside the female and eggs take time to mature, so progesterone-confirmed ovulation gives a far tighter due-date estimate.

Why do my vet's progesterone numbers look different from what I read online?

Different progesterone analyzers (in-house machines vs. reference labs) report different absolute numbers for the same blood sample. The ranges in this guide are commonly cited reference points, not a universal standard — always interpret your results against the reference range for the specific machine or lab your vet uses, and stay with the same one for the whole cycle.

How do I convert progesterone from nmol/L to ng/mL?

Many labs — especially outside the United States — report canine progesterone in nmol/L rather than ng/mL. To convert nmol/L to ng/mL, divide by 3.18; to go the other way, multiply ng/mL by 3.18. So the key breeding thresholds translate as: about 2 ng/mL ≈ 6 nmol/L (LH surge), 5 ng/mL ≈ 16 nmol/L (ovulation), and 10 ng/mL ≈ 32 nmol/L. Always check which unit your report uses before comparing a number to any reference range.

How much does progesterone testing cost, and how many tests will I need?

A single progesterone test commonly runs about $75 to $195, with in-house analyzers usually cheaper and faster than send-out reference labs. Most cycles need roughly three to five tests: you start every two to three days as the surge approaches, then tighten to every one to two days to pin down ovulation. Budgeting for the full series — rather than a single test — is what makes the timing reliable, and it is inexpensive next to a missed breeding.

Can a progesterone test tell me when my dog will whelp?

Yes. Late in pregnancy, progesterone falls sharply, dropping below about 2 ng/mL roughly 24 to 48 hours before labor begins — the same event that triggers the well-known pre-whelping temperature drop. A vet can run this "reverse progesterone" test near the due date to help decide whether whelping is imminent or a cesarean should be scheduled. It is less reliable in very small litters, where the drop can be muted, so it is used alongside temperature monitoring rather than alone.

Related Tools

Sources: Concannon PW. Reproductive cycles of the domestic bitch. Animal Reproduction Science (2011) 124:200–210; Linde-Forsberg C. Achieving canine pregnancy by using frozen or chilled extended semen. Veterinary Clinics of North America: Small Animal Practice (1991) 21(3):467–485; Merck / MSD Veterinary Manual (Canine Reproduction and Estrous Cycle Management; Whelping and Queening); Michigan State University Veterinary Diagnostic Laboratory — Assessment of Reproduction (progesterone unit conversion); Johnston SD, Root Kustritz MV, Olson PNS. Canine and Feline Theriogenology. Saunders; England GCW, von Heimendahl A (eds). BSAVA Manual of Canine and Feline Reproduction and Neonatology. Progesterone units convert as nmol/L = ng/mL × 3.18. This article is for educational purposes and does not replace professional veterinary care. Breeding timing should be confirmed with a veterinarian reviewing your dog's full progesterone trend.