Signs Your Dog Is in Heat: What to Look For, Stage by Stage

Catching the start of a heat early matters twice over: it starts the clock for every breeding decision of the cycle, and it starts the containment clock if breeding isn't the plan. Here's what to watch for, in the order it usually appears.

The Early Signs (Proestrus)

The opening stage of the heat cycle announces itself physically:

• Vulvar swelling — often the very first change, sometimes a day or two before anything else. If you know your dog's normal, the difference is unmistakable.
• Bloody discharge — the classic sign. Volume varies enormously between dogs; some drip noticeably, while fastidious self-groomers may show almost nothing on the floor. Checking with a white tissue at the vulva beats waiting to spot drops on tile.
• Increased licking of the area — frequently what owners notice before they notice the discharge itself.
• Male attention — intact males become suddenly, conspicuously interested. A male dog's nose detects proestrus earlier than most humans detect anything; if your stud starts camping at a kennel door, believe him. (More on male behavior in Do Male Dogs Go Into Heat?)

Behaviorally, early heat is a mixed bag: some girls turn clingy and affectionate, others restless, irritable, or off their food. Marking with small amounts of urine often increases — her urine now advertises her status, and she distributes the news.

Crucially, she is not receptive yet. A female in proestrus will typically tuck her tail, sit, or warn off interested males. Fertility comes next — which is exactly why these early signs are your action signal, not a curiosity.

The Fertile-Stage Signs (Estrus)

After roughly a week (with wide variation — see the day-by-day timeline), the signs shift:

• Discharge lightens from red toward pink or straw and usually decreases. Counterintuitively, less blood means more fertile — many accidental litters trace to reading the fading discharge as "heat's over."
• Flagging — the giveaway behavior. Touch her back near the tail and she moves the tail aside and stands planted. That's standing heat.
• Standing for males instead of objecting — the defining sign of the fertile stage.
• The vulva softens from firm and swollen to softer and more pliable, easing mating.

For breeders, flagging plus lightened discharge means the fertile window is open or opening. It does not pinpoint ovulation day — for that, progesterone testing is the professional standard, covered in When Do You Breed a Dog?

Signs the Heat Is Ending

Swelling recedes, discharge stops, flagging fades, males lose (some) interest, and her behavior returns to baseline. Keep the containment rules going several days past "back to normal" — receptivity and risk outlast peak fertility, as explained in Can a Dog Get Pregnant When Not in Heat?

When the Signs Barely Show

Some cycles are silent heats — fully fertile, nearly invisible. Minimal swelling, negligible discharge, subtle behavior. They're the reason experienced breeders treat male attention as a primary detection tool and why a "skipped" season often wasn't. If your girl's heats tend to run quiet, our article on silent heats, split heats, and irregular cycles covers the detection playbook, from vaginal cytology to scheduled progesterone checks.

Write It Down While It's Happening

Every sign you observe is data with a shelf life. "First discharge on the 12th, first flagging on the 19th" is the raw material for timing her next season — because individual dogs repeat their own patterns with surprising fidelity. Note the dates, note the signs, and after a cycle or two her rhythm is on paper instead of in guesswork.

Project the next season with our free dog heat cycle calculator, and keep the observations themselves in Breeders Companion — per-dog heat logs beside health records, litters, and your breeding calendar, so every "when did she last…?" has an answer.

This article is for general education, not veterinary advice. Discharge outside an expected heat, discharge with a foul odor, or signs of illness during a cycle warrant a call to your veterinarian — pyometra, a serious uterine infection, can masquerade as cycle weirdness.