The heat cycle is the hormonal engine behind everything in this area — timing, mating, even how the bitch behaves. Understanding its four stages, and the hormones driving them, is what turns “she’s in season” into an actual plan.
The most important first
- The cycle has four stages: proestrus, estrus, diestrus and anestrus.
- Rising estrogen drives proestrus; a falling-estrogen, rising-progesterone shift triggers the LH surge that starts estrus.
- Ovulation follows the LH surge by roughly 48 hours, and eggs then need another 48–72 hours to mature before they’re fertilizable.
- Progesterone rises steadily from the LH surge onward and is the hormone tracked for timing.
- Anestrus, the resting phase, varies from one to several months; the uterus needs about four months to fully recover after a cycle.
The four stages
Most bitches have their first season somewhere between 6 and 24 months of age — small breeds usually earlier — and the first one is often quiet and easy to miss. After that, seasons typically come every six to seven months, though anything from about four to thirteen months can be normal, and a few breeds such as the Basenji cycle only once a year.
Proestrus (roughly 6–11 days) is when estrogen rises and peaks, causing the classic signs of a bitch “coming into season”: vulval swelling and bloody discharge. She attracts males but isn’t yet receptive to mating. Estrus (roughly 5–9 days on average, but anything from a day or two to about three weeks can occur) begins with a surge in luteinizing hormone (LH), triggered by falling estrogen and rising progesterone — this LH surge is the pivotal hormonal event of the whole cycle, since ovulation follows it by about two days. Discharge often lightens to straw-colored, and the bitch becomes behaviorally receptive. Diestrus follows, marked by progesterone that peaks about two to three weeks after ovulation, stays high for a week or two and then falls gradually; diestrus as a whole usually lasts around two months or a little more, whether or not she is pregnant — this phase looks hormonally identical whether or not the bitch is pregnant, which is part of why pregnancy can’t be confirmed by progesterone alone — it is confirmed with a relaxin blood test or an ultrasound scan from about four to five weeks after mating. Anestrus, the resting phase between cycles, varies in length from one to several months, while the uterus undergoes involution — a repair process that prepares it for the next cycle.
Why the LH surge matters more than any other single event
Nearly everything downstream depends on when the LH surge happens, because it’s the fixed reference point the whole reproductive timeline is built around. Ovulation follows the LH surge by about 48 hours, and the released eggs then take a further 48–72 hours to mature before they can actually be fertilized. That gap between ovulation and true fertility is the reason “she stood to be mated” and “she was fertile” are not the same day, and it’s the biological basis for progesterone timing logs rather than counting days from the start of visible discharge.
In practice
A breeder assumes the fertile window starts as soon as the bitch first accepts a mating, and books natural matings for that day and the next. A progesterone test taken at the same time shows the LH surge hasn’t happened yet — meaning ovulation, and true fertility, are still several days away. Without the test, two matings could easily land entirely outside the fertile window despite looking, behaviorally, like perfect timing.
(Illustrative, composite scenario.)
(Illustrative, composite scenario.)
What this means for planning a mating
Because behavioral receptivity and hormonal fertility don’t reliably line up, timing the mating to the fertile window in practice means tracking progesterone rather than relying on calendar counting or standing-heat behavior alone. This is covered in full in the next article, but the principle follows directly from the hormone sequence above: rising progesterone after the LH surge is the signal that ovulation, and the fertile window that follows it, are underway.
When the cycle needs a vet
Book a veterinary check if a season seems to go on for more than about 40 days, if a young bitch has shown no season by around two years of age, or if seasons come much closer together than usual. Most important of all: in the one to two months after a heat, contact your veterinarian the same day if she is lethargic, off her food, vomiting, drinking or urinating much more, or has a pus-like or bloody discharge. These can be signs of pyometra, a uterine infection that can become life-threatening quickly — and when the cervix is closed there may be no discharge at all.
When to call your vet
- In the one to two months after a heat: lethargy, poor appetite, vomiting, much more drinking or urinating, or a pus-like or bloody discharge — call the same day (possible pyometra).
- A season lasts more than about 40 days.
- No season by around two years of age.
- Seasons come much closer together than usual for her.
- Swollen tissue bulges from the vulva during heat, or she cannot urinate normally.
- 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
- Could I name the four stages of the heat cycle in order and describe what’s happening hormonally in each?
- Do I understand why the LH surge, not the first day of standing heat, is the key reference point?
- Am I clear on why diestrus looks the same whether or not a bitch is pregnant?
Find out & check
- Find out roughly how long your own bitch’s proestrus and estrus stages have lasted in past cycles, if known.
- Check when in her cycle you’d need to start progesterone testing to catch the LH surge in time.
Do one thing
- Write out the four stages of the cycle in order, with one line on what’s hormonally driving each.
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Sources & further reading
- Cornell Richard P. Riney Canine Health Center — Dog estrous cycles. vet.cornell.edu
- Merck Veterinary Manual — Breeding Management of Bitches. merckvetmanual.com
- Concannon PW, Animal Reproduction Science (2011) — Reproductive cycles of the domestic bitch. doi.org
- BVNA — Reproductive system of the dog and cat – Part 3: Reproductive physiology of the bitch. bvna.org.uk
- VCA Animal Hospitals — Estrus and Mating in Dogs. vcahospitals.com
- Merck Veterinary Manual (Pet Owner Version) — Reproductive Disorders of Female Dogs. merckvetmanual.com
- Merck Veterinary Manual (Pet Owner Version) — Management of Reproduction in Dogs. merckvetmanual.com
Where evidence is limited, recommendations rest on established practice and expert judgment, and may be updated as knowledge develops.
Next steps
- Read next: Timing the mating: finding the fertile window.
- Related: The bitch’s reproductive system explained.
- Return to the area hub: Reproduction and fertility.
- Related emergency: Pyometra in the breeding bitch: signs, timing and what to do.