“In heat” means a female animal is in the fertile phase of her reproductive cycle and ready to mate. The technical term is estrus, defined as the periodic state of hormonal excitement in female mammals that immediately precedes ovulation. During this window, a female’s body sends out strong signals, both physical and behavioral, to attract males and maximize the chance of pregnancy.
Most people searching this phrase are pet owners noticing unusual behavior in a dog or cat. Here’s what’s actually happening in your pet’s body, what to look for, and what it means practically.
The Biology Behind Heat
Being in heat is driven by a surge in reproductive hormones. Rising estrogen levels trigger a chain reaction: once estrogen stays high long enough, it causes a spike in luteinizing hormone (LH), which triggers ovulation roughly 10 to 12 hours later. This hormonal shift changes the animal’s behavior, body, and scent, all designed to signal fertility to nearby males.
This cycle exists in most mammals but not humans. While humans have a menstrual cycle, most animals have what’s called an estrous cycle, and the two work differently in one key way. In animals that don’t become pregnant, the uterine lining is quietly reabsorbed by the body. In humans, the uterine lining is shed as a period. So if your female dog has a bloody discharge during heat, that blood comes from the vaginal walls, not from shedding the uterine lining the way a human period does.
What Heat Looks Like in Dogs
Most dogs go into heat twice a year, roughly every 5 to 11 months. The visible signs typically last 14 to 21 days. The cycle begins with a stage called proestrus, where you’ll first notice something is different. Common signs include:
- Swollen vulva, often the first noticeable change
- Bloody or pink vaginal discharge
- Swollen nipples
- Increased urination, which spreads scent to signal fertility
- Restlessness or change in temperament, sometimes more affectionate, sometimes more aggressive
- Decreased appetite
- Excessive licking of the vulva
- Male dogs suddenly very interested in her
Not every dog shows every sign. Some dogs experience what’s called a silent heat, where ovulation still occurs but the typical swelling and discharge never appear. This can catch owners off guard, since the dog can still become pregnant without any visible symptoms.
What Heat Looks Like in Cats
Cats in heat behave quite differently from dogs, and the cycle runs on a different schedule. Cats are seasonally polyestrous, meaning they cycle repeatedly throughout the breeding season (typically spring and summer in natural light conditions). Each heat episode averages about seven days but can range anywhere from 1 to 21 days, and the full cycle from one heat to the next averages about three weeks.
The signs in cats are mostly behavioral rather than physical. You won’t typically see vaginal discharge. Instead, watch for loud, persistent vocalizing (often described as yowling or calling), dramatically increased affection toward people and other cats, restlessness, poor appetite, and sometimes inappropriate urination outside the litter box. Many cat owners first realize their cat is in heat because of the near-constant, almost distressing vocalization, especially at night. Cats may also adopt a mating posture, lowering their front end while raising their hindquarters.
How Long Heat Lasts
For dogs, the entire heat cycle lasts two to three weeks. The discharge and swelling are most obvious in the first half, but the fertile window, when the dog will actually accept mating, typically falls in the second half of the cycle. Dogs cycle roughly every six months, though this varies by breed. Larger breeds often cycle less frequently.
For cats, individual heat episodes are shorter (averaging a week), but they repeat every one to six weeks throughout breeding season if the cat doesn’t mate or become pregnant. This means an unspayed indoor cat can seem like she’s in heat almost constantly during spring and summer months.
Why Timing Matters for Spaying
If you’re planning to have your pet spayed, timing around the heat cycle matters. When a dog is in heat, the blood supply to her uterus and ovaries becomes heavily engorged in preparation for possible pregnancy. This increased blood flow raises the risk of bleeding complications during and after surgery. Veterinarians generally prefer to wait until the heat cycle has fully ended before performing the procedure.
There’s another practical concern: even after spaying during heat, it takes about two full weeks for hormone levels to drop. During that time, male dogs will still be attracted to her and may try to mount, which can cause pain at the surgical site, internal bleeding, or even a serious abdominal infection. Spaying during heat can also trigger a false pregnancy 45 to 60 days later, caused by the sudden drop in progesterone when the ovaries are removed.
If your pet is currently in heat and you want her spayed, most vets will recommend waiting two to three months after the cycle ends to let everything return to normal before scheduling surgery.
Managing a Pet in Heat
For dogs, the main concerns are containing the discharge and preventing unwanted pregnancy. Dog diapers or washable belly bands can help keep your home clean. Keep her on a leash during walks and avoid off-leash dog parks entirely, since male dogs can detect a female in heat from a surprising distance. Even well-fenced yards aren’t always enough, as determined males will dig under or climb over fences.
For cats, the priority is keeping her indoors. An indoor cat in heat will actively try to escape, and outdoor access almost guarantees pregnancy. The vocalizing can be hard to live with, but it will pass. Providing extra attention, play, and warmth (like a heated pad) can help reduce her restlessness, though nothing fully eliminates the behavioral changes until the cycle ends.