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If your young female dog is suddenly attracting lots of male attention at the dog park and leaving blood spots on your sofa, there’s a good chance they’re in heat. Learning about the cycle and signs of dogs in heat helps you provide proper care, prevent unwanted pregnancies, and prepare for responsible breeding.
When Do Dogs First Go Into Heat?
Dr. Margret Casal, DVM, a Professor of Medical Genetics, Reproduction, and Pediatrics at the University of Pennsylvania School of Veterinary Medicine, explains that the timing of when a dog first goes into heat depends on their breed and size.
“Smaller dogs tend to experience their first heat by 6 to 7 months, whereas for middle-sized dogs, it’s between 7 and 9 months of age, and for large breed dogs, it’s by 12–16 months of age,” she says. However, some dogs can experience their first cycle as early as 4 months, and others don’t cycle until they’re over 2 years of age.
How Often Do Dogs Go Into Heat?
Unlike humans, whose reproductive cycle averages 28 days, Dr. Casal explains that most dogs experience a heat cycle approximately every seven months. “However, Basenjis, Tibetan Mastiffs, and Dingoes cycle only once a year,” she says.
She points out that the first heat cycle can be irregular, and while senior dogs don’t experience menopause the way human females do, it can become less prominent as a natural part of the aging process.
Stages of the Heat Cycle in Dogs
There are four distinct stages of a dog’s reproduction cycle. However, there can be variation in the timing and signs exhibited during each stage depending on the individual dog.
Anestrus
You don’t have to worry about your dog getting pregnant or exhibiting concerning external signs during the long anestrus (or anoestrus) phase of the heat cycle.
“Anestrus is the phase of hormonal quiescence, meaning that nothing is going on in terms of hormones. Estrogen, progesterone, and luteinizing hormone (LH) are low,” Dr. Casal explains. “This phase lasts 4 to 6 months (longer in the Basenji).” During this time, your dog’s body will be resting and performing uterine repairs, readying it for the next phase of the cycle.
Proestrus
“During this phase, the bitch begins to go into heat, and estrogen begins to rise, but progesterone and luteinizing hormone are still low,” Dr. Casal says. The rising estrogen levels are important for preparing the dog’s eggs for ovulation. While male dogs may show interest in your dog during this phase, your dog won’t be receptive to mating.
According to Dr. Casal, dogs can be in this phase from three to 21 days, but it lasts an average of nine days. During this time, you may notice some vulvar swelling and bloody discharge because of the increased estrogen levels.
Estrus
Estrus (or oestrus) is the stage often referred to as your dog being “in season” or “on standing heat,” as this is the time they’re most fertile and behaviorally receptive to mating from a male. Dr. Casal explains dogs can also be in this phase for three to 21 days, but it typically lasts nine days.
“During this phase, the estrogen begins to drop, and there is a spike in luteinizing hormones (LH), at which time the progesterone begins to rise,” Dr. Casal says. “LH is needed for ovulation and progesterone to get the uterus ready for a potential pregnancy.”
If ovulation occurs, this happens two days after the luteinizing hormone peaks. “Unlike many other species, dog ova have to mature for another two days after ovulation before they can be fertilized,” Dr. Casal says.
Your dog’s vulva will remain swollen during estrus, but the discharge typically lessens and changes from bloody-red to pinkish or yellowish.
Diestrus
Diestrus lasts 57 days in pregnant females and slightly longer for those who are not pregnant.
Dr. Susanne Hughes is a retired small animal veterinary practitioner with a strong interest in canine reproductive physiology and a dedicated AKC Breeder of Merit. She explains that, while your dog is no longer fertile during this time, unlike many other species, dogs are unique in that progesterone (the pregnancy-supporting hormone) continues to rise during diestrus, whether pregnant or not. “This is the actual cause of what we refer to as false pregnancy,” she says. “Every bitch experiences this but may or may not show behaviors such as nesting, lactation, etc.”
During diestrus, your dog’s vulva should return to its normal size, and any bloody discharge should stop.
What Are the Signs of Dogs in Heat?
It pays to be vigilant and understand the external signs of dogs in heat—not just the vulvar swelling and discharge during the proestrus and estrus phases. Every dog is different, with some exhibiting obvious signs or behaviors and others only showing subtle clues.
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More frequent genital licking and urination (especially in front of male dogs)
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While male dogs will start showing interest during proestrus, your female dog may react defensively if they get too close
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Flagging is when female dogs hold their tail to the side during the estrus phase to signify that they’re ready to allow mating
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During estrus, your dog’s recall may become less reliable when they’re looking for potential mates
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Hormonal changes can result in increased anxiety, vocalization, nesting or mounting behaviors, and a loss of appetite
How to Care for a Dog in Heat
Consider using dog diapers to keep your house clean during your dog’s discharge phase, and monitor for any changes in behavior and body language. If your dog is more anxious and needy while in heat, sticking with a routine and being around to offer extra reassurance can help. If they want their own space, ensure the whole family (including other pets) respects their boundaries.
“There are no health concerns during heat, but during diestrus, there is the risk of developing a pyometra (pus-filled uterus), which can be life-threatening,” Dr. Casal says.
Pyometra is more common in intact bitches over the age of 6, typically occurring within 12 weeks of a heat cycle. Knowing the signs of pyometra ensures you can get prompt veterinary assistance should the need arise. These can include loss of appetite, lethargy, vomiting, an increase in drinking and urinating, foul-smelling vaginal discharge, and licking of the genitals.
How to Prevent Pregnancy in Dogs in Heat
Avoiding direct contact with intact males is the best way to prevent pregnancy when your dog is in heat. “However, do not underestimate the wiles of an estrous bitch or intact dog,” Dr. Hughes warns.
Consider tracking your dog’s heat cycles on an app or calendar to help with planning, avoid dog parks or doggy daycares, walk your dog on a leash in quieter areas, and never leave them unsupervised in the yard. Though doggy diapers might help keep your home clean, they can’t act as a protective barrier against mating.
If you’re worried about coming into contact with off-leash males on your dog walks, offering more enrichment and brain games around the home and yard can be a safer alternative.
Contact your vet straight away if an accident does happen. There are injections that can prevent a pregnancy from developing. However, administration needs to occur within a few days of mating.
When to Spay Your Dog
Spaying your dog after they’ve had at least one heat cycle eliminates the risk of unwanted pregnancies. However, Dr. Laurence Sawyer, DVM, an assistant teaching professor in the department of clinical sciences and medical director of the Lerner Spay/Neuter Clinic at Cummings School of Veterinary at Tufts University, explains that the topic of whether and when to spay is complex.
She recommends the decision be made individually based on a conversation with your veterinarian, your lifestyle, and your commitment and ability to manage the circumstances responsibly. “Additional factors that affect the decision about the timing of spaying the dog are age, breed, size, and propensity for certain cancers and joint issues,” she says.
When to Mate Dogs in Heat
Responsible breeders will wait until a bitch is fully mature and has had all appropriate health screening tests done before mating. Dr. Hughes says that 2 years of age is generally considered a minimally appropriate age.
“Many bitches respond with obvious behavioral clues when they’re hormonally ready for breeding by flagging their tail and standing to allow the dog to mount and breed during estrus,” Dr. Hughes says. “Others may have more subtle outward signs, necessitating progesterone testing to pinpoint the receptive period.”
According to Dr. Hughes, the optimal breeding frequency has been reevaluated fairly recently. “We know that repeated cycling, specifically progesterone influences, can predispose bitches to cystic endometrial hyperplasia, which may lead to pyometra,” she says. “Most theriogenologists agree that breeding two litters back to back is probably safer than extending a bitch’s breeding life by skipping heats.”
Responsible breeders will consider the breed and overall fitness when determining the appropriate breeding life of a bitch. However, generally, they’re at their most fertile and at least risk of complications between the ages of 2 and 4. “Some older bitches may be predisposed to a condition called uterine inertia,” Dr. Hughes says. “In this case, the uterus is unable to contract and deliver puppies. A cesarean section would be necessary to save the dam and pups in this situation.”