Dog Heat Cycle Stages Explained
A dog's heat cycle has four stages — proestrus, estrus, diestrus and anestrus — and only one of them is the fertile window. Here is how long each lasts, how to recognize it, and how the stages connect to pregnancy planning.
Proestrus (about 9 days): the announcement
Bleeding and a swollen vulva appear; her scent changes and males become very interested — but she is not receptive and will typically refuse or repel them. Hormones are rising toward ovulation but it has not happened. This is day 1 of the heat for calendar purposes: the date the heat cycle calculator counts from.
Estrus (about 9 days): the fertile window
The bleeding lightens (often straw-colored), the vulva softens, and she becomes receptive — standing, flagging her tail to the side. Ovulation happens in this stage, most often around days 9–14 of the overall cycle, and this is the only stage in which a mating can produce puppies. It is also the stage where management matters: an unplanned mating needs only a minute of inattention.
Diestrus (about 2 months): pregnant or pretending
Whether or not she was mated, her body runs the progesterone program of pregnancy for roughly the length of a gestation — which is why false pregnancies bloom in this stage. Behavior settles, the vulva returns to normal, and if she is genuinely pregnant, this stage is the pregnancy, ending in whelping around day 63 from ovulation.
Anestrus (the quiet months): rest and reset
Several months of hormonal quiet while the uterus recovers. Nothing to see and nothing to manage — but the length of anestrus is what sets her cycle length, and it varies by size and individual: small breeds often cycle about every 5–6 months, giants closer to yearly. Her own history, recorded heat by heat, is the best predictor of her next one.
Turn stages into dates: the dog heat cycle calculator estimates her next heat, fertile window and — if bred then — a projected due date.
If you are not planning to breed
Many owners have their dog spayed once they decide not to breed. The right timing depends on her breed, size and general health, so it is a decision to make with your vet — ideally planned during the quiet anestrus months rather than in the middle of a heat. The calculator above can tell you when her next heat is due, which makes that conversation easier to time.
One question deserves a straight answer of its own: yes, she can conceive on that very first heat. Read can a dog get pregnant on her first heat? for the risks, and what to do if it has already happened.
Frequently asked questions
How long does a dog's heat last?
The visible heat — proestrus plus estrus — lasts about 2–3 weeks in total, averaging roughly 9 days per stage. The whole cycle, including the quiet diestrus and anestrus that follow, runs until the next heat about six months later (normal range roughly 5–11 months).
When is a dog most fertile?
During estrus, and most often around days 9–14 counted from the first day of bleeding. Individual timing varies widely — some females ovulate earlier, some much later — which is why progesterone testing, not day-counting, is the standard for planned breedings.
At what age does a dog have her first heat?
Usually between 6 and 24 months. Small breeds often start at the early end (around 6 months) and giant breeds at the late end (18–24 months is not unusual). A first heat before any planned breeding is normal — most breeders wait until she is fully grown.