when do dogs go into heat

Female dogs usually go into their first heat between about 6 and 12 months of age, but this can be as early as 4 months or as late as around 2 years depending on size and breed. After that, most dogs come into heat about every 6 months, with small breeds sometimes cycling 3 times a year and very large breeds sometimes only once a year.

Quick Scoop

• Most first heats: around 6–12 months old, earlier for toy/small breeds, later (up to 18–24 months) for large and giant breeds.

• Frequency: roughly twice a year; small dogs may cycle more often, giant breeds less often.

• Duration of each heat: about 2–3 weeks on average, often quoted as 12–20 days.

• Lifelong cycles: unlike humans, dogs do not go through menopause; intact females keep cycling for life, though intervals can lengthen as they age.

• Spayed females: once spayed, they no longer go into heat.

What “in heat” looks like

Owners often first notice a swollen vulva and bloody vaginal discharge, plus changes in behavior.

Common signs:

• Swollen vulva and spotting or bloody discharge.
• More frequent urination and “marking” on walks or even indoors.

• Attracting male dogs, with males showing intense interest in her.

• Possible behavior changes: clinginess, restlessness, or irritability.

A typical cycle has four stages (proestrus, estrus, diestrus, anestrus); the “heat” most people notice is mainly proestrus and estrus, together lasting around 2–4 weeks.

Mini timeline example

For a medium mixed‑breed dog:

1. Around 8–10 months old: first heat may appear.
2. Bleeding and swelling for about 1–2 weeks, then discharge lightens as she becomes fertile.

3. Total “messy” period: about 2–3 weeks, during which she can become pregnant.

4. Next heat comes roughly 6 months later, though it can take up to 2 years for cycles to become regular.

Simple HTML table (timing overview)

html

<table>
  <thead>
    <tr>
      <th>Dog size</th>
      <th>Typical age of first heat</th>
      <th>How often after that</th>
      <th>Notes</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>Small breeds</td>
      <td>~4–7 months[web:3][web:5][web:7]</td>
      <td>2–3 times per year[web:1][web:4][web:9]</td>
      <td>Often start younger and cycle more frequently.[web:1][web:3][web:4]</td>
    </tr>
    <tr>
      <td>Medium breeds</td>
      <td>~7–12 months[web:3][web:5][web:7]</td>
      <td>About every 6 months[web:1][web:4][web:9]</td>
      <td>Most “average” pet dogs fit this pattern.[web:1][web:4]</td>
    </tr>
    <tr>
      <td>Large/giant breeds</td>
      <td>~12–24 months[web:3][web:7][web:9]</td>
      <td>About every 6–12 months[web:1][web:4][web:9]</td>
      <td>May only come into heat once a year.[web:1][web:4][web:9]</td>
    </tr>
  </tbody>
</table>

Quick “what should I do?” checklist

If you think your dog is going into heat:

1. Keep her away from intact male dogs (even through fences or briefly at the park).
2. Use doggy diapers or washable blankets to manage discharge if needed.

3. Expect behavior and bathroom habit changes for a couple of weeks.
4. Call your vet if she seems very unwell (lethargy, foul‑smelling discharge, heavy bleeding, fever), as this can signal infection like pyometra.

5. If you don’t plan to breed, talk to your vet about the best timing for spaying, especially around the first or second heat.

Information gathered from public forums or data available on the internet and portrayed here.