Dog's First Heat: Timing, Signs & Why Birth Dates Matter

Dog's First Heat: Timing, Signs, and Why Your Puppy's Birth Date Matters

The short answer: Most dogs have their first heat between six and twelve months of age — small breeds usually earlier, large breeds later. The signs are easy to recognize once you know them. What the timing can't tell you is how old your puppy actually is, and that's the part that caught me off guard.

I bought a puppy online. The seller told me she was "a little over two months." Three and a half months later, she started spotting. By my math she should have been five and a half months old at most — and everything I'd read said a first heat usually shows up closer to six.

So either the reading was wrong, or the age was. Only one of those turned out to be true.

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What a first heat actually is

A heat — vets call it estrus, and in the UK and EU you'll more often hear a season — is the fertile phase of a female dog's reproductive cycle. The visible part usually runs two to four weeks. After the first one, most dogs cycle again roughly every five to eight months, though individual variation is wide.

It's worth separating two ideas that get blurred together: a first heat is a puberty milestone, not a sign of physical maturity. A dog can be capable of breeding months before she's finished growing. For a medium-sized dog, that gap is significant.

When do dogs go into heat for the first time?

There is no fixed date, and anyone who gives you one is oversimplifying. The commonly reported window is six to twelve months, and adult body size is the strongest single factor:

Two caveats matter more than the table. First, breed and bloodline influence timing as much as size does, and the spread within a single size band is wide — a first heat at five months and a first heat at eighteen months can both fall inside normal range for the right dog. Second, these are typical ranges, not deadlines. If nothing has happened by around eighteen to twenty-four months, that's a question for your vet, not a forum.

Signs of a first heat

The first heat is often quieter than later ones, which is why it gets missed.

• Vulvar swelling — frequently the first sign, sometimes a week before any discharge appears
• Discharge — bloody or straw-colored; can be light, and some dogs keep themselves remarkably clean
• More frequent urination, and more marking than usual
• Restlessness or clinginess, or sudden intense interest in every dog on the street
• Flagging — holding her tail off to one side when you touch her back or hindquarters
• Appetite changes, or a softer stool in the first few days

Light discharge on a dark floor is genuinely easy to overlook. That's exactly how it started here — a small amount, once, and a dog who otherwise behaved like nothing had happened.

What happened with my dog

Qiqi is a wǔhēi — a "five blacks": black coat, black skin, black nose, black paw pads, black tongue. She's a Chinese rural dog, bought online, with no paperwork, no birth date, and no health record. Just a number a seller gave me over a chat window.

That number was "a little over two months." Three and a half months later, she's five and a half months old at the very most.

Then came the spotting, and a simple piece of arithmetic that stopped working. If she really was around nine weeks when I bought her, a heat now would be early — early enough to be worth questioning. If she was closer to three months at purchase, everything fits comfortably, and she's nearer six and a half or seven months now.

A month of error sounds small. At that age it isn't. A month was roughly a third of her life at the time — and it's the difference between a vaccine schedule that lands correctly and one that doesn't.

Why you can't date a puppy by her first heat

Two reasons, and both are worth internalizing before you try:

1. The normal range is too wide. Five to eighteen months covers most of it. A single heat tells you almost nothing about age in absolute terms.
2. One data point can only create doubt, not correct a number. Her heat didn't prove the seller wrong. It moved "possibly wrong" from vague suspicion to something worth verifying — which is a different, and more honest, conclusion.

I'd rather say that plainly than pretend the timing was conclusive. It wasn't. It was a prompt to go check.

How to get a better answer than the seller's

If you're in the same position, these are the checks that actually help:

• Ask for the birth date in writing. A message, an invoice, a health record — something dated. "About two months" is not a birth date, and it's not something you can build a schedule on.
• Look at the teeth. Puppy teeth start being replaced around three to four months, and most permanent teeth are in by six to seven. A full adult set with little wear is a reasonable proxy for seven months or older.
• Ask your vet to estimate. Teeth wear, weight curve, and overall development give a range — imperfect, but far better than a guess from a chat window.
• Request the weight history. A growth curve with dates attached beats any single number anyone quotes you.
• Start your own record. Weigh her on day one, then monthly. It's five minutes a month and it's the only data you'll fully trust.

Once you have a plausible range rather than an exact number, build the vaccination and deworming schedule around that range with your vet — not around the seller's figure.

What we did, and what we didn't

We called our vet before we started reading forums. That order mattered more than anything else we did.

I'm deliberately not writing a how-to here. Care during a heat depends on the individual dog — her age, her size, her health, where you live, whether there are intact males nearby — and a blog post has no business prescribing it. What I will say is narrower:

• Write down the first day you saw discharge. That date is useful to your vet later, and easier to recall than you'd think.
• An intact female can become pregnant during her first heat, not only in later ones. If that's a concern for you, it's a conversation with your vet about management.
• Spaying timing is genuinely debated. Before a first heat, or after? The answer depends on breed, size, and individual risk profile. Get an opinion about your dog, not a general rule from the internet.
• Call rather than search if you see heavy bleeding, lethargy, vomiting, or she seems unwell in the weeks after a heat. Those need a professional. They are not a comment-section problem.

The last point isn't caution for its own sake: a heat is the point in the cycle when uterine infection becomes a real possibility, which is one more reason the dates are worth writing down.

Buying a puppy without papers: three things to get in writing

1. Exact date of birth — in a message you keep and can point to later.
2. Vaccination record — with dates and product names, not just "vaccinated."
3. Deworming dates — same standard.

If a seller can't produce the first one, treat the age as unknown rather than approximate, and tell your vet that's the situation. Breeders and sellers who do have records will send them without friction. The ones who won't are telling you something.

Qiqi is fine, for what it's worth. She's asleep on my foot, unbothered, while I'm the one who aged five years in a week.

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FAQ

How long does a dog's first heat last?
Usually two to four weeks of visible signs, with the fertile window falling in the middle-to-later part rather than at the start. Bleeding can continue lightly after the fertile period ends.

Can a dog get pregnant on her first heat?
Yes. Sexual maturity arrives with the first heat, so pregnancy is possible from that point on.

Is five months too early for a first heat?
It's early, but not unheard of, particularly in small breeds. For a medium or large dog, a heat at five months is unusual enough to be worth mentioning to your vet.

Do all dogs bleed during their first heat?
Most do, but volume varies enormously — some have obvious discharge for two weeks, others have very little. A dog who keeps herself clean can make a light heat almost invisible.

Can a vet tell how old a puppy is?
Approximately. Teeth development and wear, weight, and physical maturity together give a usable range, especially under about a year old. It's an estimate, not a measurement — but far better than a seller's recollection.

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*This article is general information, not veterinary advice. Every dog is different; for anything specific to your dog, talk to your vet. Mention of a health condition here is for informational purposes only and is not a claim about any product.