What to Expect When Your Dog Is in Season: The Estrus Cycle Explained ❤️🐶

By Dr Duncan Houston

Quick Answer

When a dog is “in season,” she is going through her heat cycle. Most dogs cycle about every 6 to 7 months, though this varies with breed and individual. The cycle has four stages: proestrus, estrus, diestrus, and anestrus. During this time, you may see vulval swelling, discharge, behaviour changes, and attraction from male dogs, and careful supervision is essential to prevent accidental mating. (Merck Veterinary Manual)

If your female dog comes into season, it can feel a bit like living with a tiny furry chaos coordinator. One day she is normal, the next there is spotting on the floor, neighbourhood males are acting like they got a calendar invite, and everyone is suddenly very invested in your fence line.

Understanding the cycle makes it much easier to manage safely, whether you are planning to breed responsibly or simply trying to avoid surprises.

🩸 What does “in season” mean?

“In season” refers to your dog’s estrous cycle, also called her heat cycle. Unlike humans, dogs do not have monthly periods. Most dogs come into heat roughly every 6 to 7 months, although some cycle more or less often depending on breed, size, age, and individual variation. Smaller breeds may cycle earlier and sometimes more often, while giant breeds can start later and may cycle less frequently. (Merck Veterinary Manual)

The four stages are:

• Proestrus
• Estrus
• Diestrus
• Anestrus

🐾 When does the first heat happen?

The first heat usually occurs between about 6 and 24 months of age. Smaller breeds often start earlier, while larger and giant breeds often start later. A dog that has not had a heat by around 2 years of age should be discussed with your veterinarian. (Merck Veterinary Manual)

1️⃣ Proestrus

This is the beginning stage.

Common signs include:

• swollen vulva
• bloody vaginal discharge
• frequent urination
• licking the area more often
• attracting male dogs but not allowing mating
• clinginess, restlessness, or irritability

Proestrus usually lasts about 3 to 21 days, with 7 to 10 days being common. This is often the stage owners notice first because of the swelling and discharge. (Merck Veterinary Manual)

2️⃣ Estrus

This is the fertile stage, when your dog is receptive to mating.

Common signs include:

• lighter or more straw-coloured discharge
• tail flagging to one side
• standing for male dogs
• increased interest in going outside
• escape behaviour or restlessness

Estrus averages about 9 days but can range from 3 to 21 days or even 2 to 24 days depending on the source and the individual dog. Ovulation occurs during estrus, so this is the highest-risk time for an accidental pregnancy. (Vca)

3️⃣ Diestrus

This is the winding-down phase.

During diestrus:

• she is no longer receptive to males
• discharge stops or becomes minimal
• the vulva gradually returns to normal
• her hormones shift whether or not she is pregnant

Diestrus lasts around 60 days. If she has been mated and conceived, pregnancy develops during this stage. Canine gestation is typically around 58 to 63 days. (American Kennel Club)

4️⃣ Anestrus

This is the resting phase between heats.

During anestrus:

• there is no discharge
• hormone levels are relatively quiet
• behaviour usually returns to normal

This stage commonly lasts around 4 to 6 months, though the timing can vary. (Merck Veterinary Manual)

⚠️ What are the risks while a dog is in heat?

The biggest immediate risk is accidental mating and pregnancy.

Other important concerns include:

• dogs escaping or being injured while trying to find or avoid mates
• unwanted attention from male dogs
• later risk of pyometra in unspayed dogs
• increased mammary tumour risk with repeated heat cycles

Pyometra is a serious and potentially life-threatening uterine infection seen in unspayed females, and it is one of the major reasons many owners choose spaying. Merck notes that ovariohysterectomy or ovariectomy is the best way to prevent estrous cycling. (Merck Veterinary Manual)

🏠 How to manage a dog in season safely

Practical tips:

• keep her indoors unless supervised
• use a lead even in fenced areas
• double-check gates, doors, and screens
• avoid dog parks and off-leash areas
• use washable bedding or dog diapers for mess management
• remember diapers do not prevent mating

A determined male dog can be astonishingly committed, and a determined female in estrus can suddenly become an escape artist with Olympic ambitions.

🩺 When should you call the vet?

Contact your veterinarian if you notice:

• unusually heavy bleeding
• foul-smelling discharge
• lethargy, vomiting, or fever
• a very prolonged heat cycle
• first heat delayed beyond expected age
• possible accidental mating
• signs of false pregnancy or illness after the heat

Any abnormal discharge or sickness after a heat cycle needs prompt attention because pyometra can develop after estrus.

💉 What about spaying?

Spaying prevents future heat cycles and helps prevent pyometra. The ideal timing depends on breed, size, orthopaedic considerations, cancer risk, and your dog’s overall situation, so it is worth discussing individually with your vet rather than using a one-size-fits-all rule. Merck confirms that surgical sterilisation is the best method to prevent the estrous cycle. (Merck Veterinary Manual)

🐾 Final Thoughts

A dog in season needs a little extra management, a lot of supervision, and zero trust in fences that “should be fine.”

Knowing the stages of the cycle helps you predict what is normal, spot what is not, and keep her safe through the process. Whether you plan to breed responsibly or avoid pregnancy altogether, understanding heat cycles is a big part of protecting her health.

❓ FAQ

How often do dogs go into heat?
Most dogs cycle about every 6 to 7 months, but this varies by breed and individual. (Merck Veterinary Manual)

How long is a dog fertile for?
She is fertile during estrus, which often lasts about 9 days, though the fertile window can vary. (Vca)

Can my dog get pregnant even if the bleeding has slowed down?
Yes. In fact, fertility often peaks after the early bloody stage, when discharge becomes lighter and she becomes receptive.

Do dog diapers prevent pregnancy?
No. They help manage discharge, not mating.

Is it normal for behaviour to change during heat?
Yes. Many dogs become clingier, more restless, more vocal, or more interested in roaming.

If you want help tracking your dog’s cycle, symptoms, or behaviour changes, the ASK A VET™ app can help you keep everything organised and get guidance when you need it.