A heat cycle is the reproductive cycle that female mammals go through when their bodies prepare for potential pregnancy. Most people searching this term are dog or cat owners, and in that context, “heat” refers to the period when a female animal is fertile and receptive to mating. A dog’s full heat cycle lasts several months from start to finish, though the window where she can actually become pregnant is much shorter, typically five to nine days.
The scientific name for this cycle is the estrous cycle. Unlike the human menstrual cycle, which sheds the uterine lining each month, the estrous cycle reabsorbs most of that lining internally. The visible signs you notice, like swelling or discharge, are driven by the same core hormones (estrogen and progesterone) but follow a different pattern and timeline depending on the species.
The Four Stages in Dogs
A dog’s heat cycle has four distinct phases, and understanding them helps you recognize what’s normal and when your dog is actually fertile.
Proestrus is the opening stage, lasting about 6 to 11 days. This is when you’ll first notice vulvar swelling and a bloody discharge. Rising estrogen levels attract male dogs, but the female won’t accept mating yet. She may seem more clingy or restless than usual.
Estrus is the fertile window. It typically lasts 5 to 9 days, though it can range anywhere from 1 to 20 days. The discharge lightens in color, the vulva softens, and the female becomes willing to mate. Estrogen peaks and then drops sharply, triggering a surge of luteinizing hormone that causes ovulation. This is the only stage where pregnancy can occur.
Diestrus follows whether or not the dog has mated. Progesterone rises, peaking about two to three weeks after ovulation and staying elevated for another one to two weeks before gradually declining over the next 10 to 30 days. If the dog is pregnant, this hormone sustains the pregnancy. If not, her body still goes through many of the same hormonal shifts, which is why some dogs develop “false pregnancy” symptoms like nesting or swollen mammary glands.
Anestrus is the resting phase. It lasts roughly four months, during which the uterus repairs itself and the reproductive system resets for the next cycle. Your dog won’t show any signs of heat during this time.
When the First Heat Happens
Dogs can have their first heat anywhere between 6 and 24 months of age. Smaller breeds tend to cycle earlier, sometimes as young as six months, while giant breeds may not have their first heat until they’re closer to two years old. Most dogs then cycle about twice a year, or roughly every 5 to 11 months. A few breeds break this pattern: Basenjis and Tibetan Mastiffs typically cycle only once a year.
Cats reach sexual maturity on a similar timeline, with most having their first heat between seven and ten months. But cat cycles work very differently after that.
How Cats Differ From Dogs
Cats are seasonally polyestrous, meaning they cycle repeatedly throughout a breeding season rather than on a fixed schedule. In the Northern Hemisphere, that season runs from roughly January through late fall, driven by daylight length. Cats are sometimes called “long day breeders” because increasing daylight triggers their cycles. Indoor cats exposed to artificial light, and cats in tropical climates, may cycle year-round.
Each heat in a cat lasts an average of seven days, with a range of 1 to 21 days. If the cat doesn’t mate, she’ll typically come out of heat for a short period and then cycle back in again, sometimes within two weeks. This rapid cycling is why unspayed cats can seem to be in heat almost constantly during spring and summer. Cats are also induced ovulators: they don’t release eggs unless mating occurs, which is why the cycles keep repeating until breeding happens or the season ends.
Behaviorally, cats in heat are hard to miss. They vocalize loudly (especially at night), rub against furniture and people more than usual, adopt a crouched posture with their hindquarters raised, and may try to escape outdoors. Dogs in heat are generally less dramatic but will flag their tail to the side, urinate more frequently to spread pheromones, and become more interested in male dogs.
Signs to Watch For
In dogs, the earliest sign of heat is usually vulvar swelling followed by a bloody discharge. Some owners first notice small spots of blood on bedding or the floor. As the cycle moves from proestrus into estrus, the discharge often shifts from bright red to a lighter pink or straw color. The dog may lick herself more frequently, which can make the discharge less obvious.
Not every dog shows clear signs. Some go through what’s called a silent heat, where ovulation occurs normally but the typical swelling and discharge are minimal or absent. This is more common in a dog’s first cycle and can catch owners off guard if they’re watching for visible cues.
Cats rarely have noticeable discharge during heat. Their signs are almost entirely behavioral: loud, persistent vocalization, rolling on the floor, increased affection, and the raised-hindquarter posture when you touch their lower back.
Managing a Pet in Heat
If your dog is in heat and you don’t want puppies, supervision is the single most important thing. Never leave her unsupervised outdoors, and keep her on a leash during walks. Male dogs can detect a female in heat from a surprising distance, and both males and females become more motivated to roam during this time.
For the mess indoors, restricting your dog to uncarpeted areas makes cleanup easier. Dog diapers or washable belly bands designed for heat cycles are widely available and work well for managing discharge, though you’ll need to change them regularly to prevent irritation.
For cats, the biggest challenge is preventing escape. A cat in heat will actively try to get outside, so secure windows and doors carefully. The persistent vocalization can be disruptive, but it’s a normal part of the cycle and will stop once heat ends.
Spaying and the Heat Cycle
Spaying (removing the ovaries and uterus) permanently ends heat cycles. The timing of that surgery involves some tradeoffs, particularly for dogs.
Every heat cycle a dog goes through increases her lifetime risk of mammary cancer. Intact females are also at risk of pyometra, a serious uterine infection that requires emergency surgery and can be fatal. For maximum cancer and infection prevention, spaying before seven months of age, before the first heat, offers the greatest benefit.
However, some evidence suggests that very early spaying in certain large breeds, particularly German Shepherds and Golden Retrievers, may delay the closure of growth plates in long bones. This can lead to slightly altered proportions and may increase the risk of hip or joint problems. Some retrospective studies in these breeds have also found a slightly higher rate of certain blood and lymph cancers after early spaying. For large and giant breeds, the decision often comes down to weighing joint health against reproductive cancer risk, and the best timing varies by breed and individual.
For cats, the calculus is simpler. There’s no known benefit to allowing a cat to go through even one heat cycle. Early spaying reduces mammary cancer risk, eliminates the chance of uterine infection, and stops the behavioral disruptions that come with repeated cycling.