Of everything covered in this area, timing has the single biggest effect on whether a mating results in a pregnancy at all. This article covers how to actually find the fertile window, rather than guess at it.
The most important first
- Behavioral receptivity and true fertility do not reliably occur on the same days.
- Progesterone testing is the standard, reliable method for timing a mating — not calendar counting.
- Ovulation follows the LH surge by about 48 hours; eggs then need 48–72 hours more to mature.
- Fresh and chilled semen generally allow a wider timing window than frozen semen.
- A progesterone timing log turns test results into a clear breeding-day decision.
- Before any mating, both dogs should have a current negative Brucella canis test; arrange this well before the expected season.
Why counting days from the start of heat isn’t reliable
Proestrus and estrus vary enough in length between individual bitches — and even between cycles in the same bitch — that counting a fixed number of days from the first sign of heat is an unreliable way to predict ovulation. Two bitches showing identical external signs on day one can ovulate several days apart. This variability is exactly why the heat cycle and reproductive hormones — specifically the LH surge and the progesterone rise that follows it — are what’s actually tracked, rather than the visible signs of heat.
How progesterone testing works
Progesterone starts low and rises as the LH surge approaches, then continues climbing after ovulation. Serial blood tests, typically starting around five or six days after you first notice signs of heat — before she is willing to stand — and repeated every one to two days, track this rise and let a vet identify the LH surge and predict ovulation from the trend, rather than from a single reading. After ovulation, the released eggs take a further 48–72 hours to mature before they’re actually fertilizable — so ovulation is the reference point the fertile window is calculated from, not necessarily the mating day itself.
In practice
A breeder plans a natural mating based on the bitch “standing” for the male, and a single progesterone reading taken that day. The reading alone, without a prior baseline to show the trend, makes it hard to tell whether ovulation has already happened or is still days away. A repeat test 48 hours later — now showing a clear rising trend — is what actually pins down the timing, illustrating why serial testing beats a single snapshot.
(Illustrative, composite scenario.)
(Illustrative, composite scenario.)
Use the same laboratory or the same machine for every test in a cycle. Studies comparing in-clinic analyzers with laboratory methods have found that the in-clinic results can read noticeably higher, which may make ovulation look one to two days earlier than it really is — enough to mistime a frozen-semen insemination. If progesterone rises and then stalls for several days, or never rises, ask your vet about further checks such as an ovarian ultrasound.
Matching the window to the type of semen used
The fertile window that matters in practice depends on what’s being used to mate the bitch. With natural mating or fresh semen, sperm can survive for some days inside the reproductive tract, giving a wider effective window — matings are commonly timed around one to four days after ovulation, and natural matings from the LH surge onward can still result in a pregnancy. Chilled semen survives a little less well, so it is usually timed slightly later, around two to four days after ovulation. Frozen semen survives for a much shorter time once thawed, so it’s generally timed more precisely, around three to four days after ovulation, once the eggs are fully mature. This is one of the main reasons semen and artificial insemination decisions and timing decisions are made together, not separately.
The day your vet identifies the LH surge also gives you a dependable due date: puppies are usually born about 64 to 66 days later, and you should call your vet if she has not started labor by about day 66 from the LH surge. Electronic heat detectors and urine or glucose strips do not track ovulation reliably and should not replace blood testing.
When to call your vet
- Book progesterone testing to start around day five or six after the first signs of heat — before she is receptive.
- Progesterone rises and then stays flat for more than about three days, or has not risen well into the season.
- A season seems to last more than about 40 days.
- Either dog has no current negative Brucella canis test before the planned mating.
- Results come from different labs or machines and do not seem to fit together — ask before deciding on a mating day.
- She reaches about day 66 from the LH surge (about 70 days after the first mating) without labor starting.
- If in doubt, call — your vet would rather hear from you early.
Work on it
A few minutes here will teach you more than re-reading the article.
Ask yourself
- Do I understand why a single progesterone reading is less useful than a trend from repeated tests?
- Do I know how the fertile window differs between natural mating/fresh semen and frozen semen?
- Am I clear on the gap between ovulation and actual fertility (the 48–72 hour egg maturation window)?
Find out & check
- Find out where you can get progesterone testing done, and how quickly results come back.
- Check what timing approach your stud dog owner or AI provider recommends for the semen type being used.
Do one thing
- Set up a progesterone timing log before your next planned mating, ready to record results as they come in.
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Sources & further reading
- Merck Veterinary Manual — Breeding Management of Bitches. merckvetmanual.com
- UC Davis School of Veterinary Medicine — Ovulation Timing in the Dog. healthtopics.vetmed.ucdavis.edu
- England G and Concannon PW, IVIS — Determination of the Optimal Breeding Time in the Bitch: Basic Considerations. ivis.org
- Concannon PW, Animal Reproduction Science (2011) — Reproductive cycles of the domestic bitch. doi.org
- VCA Animal Hospitals — Estrus and Mating in Dogs. vcahospitals.com
- American Kennel Club — 20 Facts You Should Know About Timing of Ovulation in the Bitch. akc.org
- Zuercher J et al., Frontiers in Veterinary Science (2021) — Comparison of a Point-of-Care Analyzer With a Chemiluminescent Immunoassay for Serum Progesterone Measurement in Breeding Management of the Bitch. frontiersin.org
Where evidence is limited, recommendations rest on established practice and expert judgment, and may be updated as knowledge develops.
Next steps
- Read next: Progesterone timing log (tool).
- Related: The heat cycle and reproductive hormones · Natural mating: what to expect and how to help it go smoothly.
- Return to the area hub: Reproduction and fertility.