When Do Dogs Go Into Heat? First Heat Age by Breed Size

Most female dogs have their first heat between six and fifteen months of age — but that window swings dramatically with adult size. If you're raising a future breeding prospect (or just an intact puppy), knowing when to expect the first season keeps it from catching you off guard.

First Heat Age by Size

The pattern is simple: the smaller the dog, the earlier the first heat.

• Toy and small breeds (Chihuahuas, Yorkies, Poodles under 15 lbs): commonly 6–9 months, occasionally as early as five.
• Medium breeds (Spaniels, Border Collies, Bulldogs): typically 9–12 months.
• Large breeds (Labs, Goldens, German Shepherds): usually 12–18 months.
• Giant breeds (Great Danes, Mastiffs, Newfoundlands): often 18–24 months — a two-year-old giant girl having her first season is not unusual.

Nutrition, body condition, health, and even the presence of other cycling females can nudge timing. Genetics matters too: a dam line where first heats reliably arrive at eight months will tend to repeat that pattern in daughters, which is one more reason multi-generation records earn their keep.

What the First Heat Looks Like

Physically, a first heat runs the same stages as any other — swelling, bloody discharge, then the receptive fertile phase; the complete heat cycle guide covers the sequence. But first heats are notorious for being messy in format:

• They're often shorter, longer, or fainter than the textbook two to four weeks.
• Silent first heats are common — a cycle so subtle it's missed entirely, which is why a "late" first heat sometimes already happened. Our silent and split heats article covers how to catch them.
• Split heats — where the cycle starts, stalls, and resumes weeks later — show up most often in young bitches on early cycles. Usually normal, always confusing.

Expect a young girl's cycles to take two or three seasons to settle into her adult rhythm and interval. Once they do, she'll likely hold that pattern for years — see how often dogs go into heat.

Fertile Doesn't Mean Ready: Why You Don't Breed the First Heat

A first heat is a fertile heat, and to intact males a six-month-old in season is exactly as compelling as an adult. Physiologically, pregnancy is possible. Responsibly, it's a different story:

• She's not done growing. Pregnancy loads a skeleton and metabolism that are still developing — especially in large breeds that won't be mature for another year.
• Health testing isn't possible yet. Hip and elbow certifications (OFA) can't be finalized until 24 months of age, and most breed clubs' recommended tests can't be completed before her second birthday. Breeding before testing means breeding blind.
• Maturity matters for mothering. Experienced breeders will tell you maiden bitches bred too young are more likely to be overwhelmed dams.

Most reputable programs wait until at least the second or third heat and completed health clearances — for many breeds, in practice, around two years old. The flip side of the timing question — how to hit the right days once you do breed — is covered in When Do You Breed a Dog?

Between that first heat and the first breeding, your only real job is containment and record-keeping. Which brings us to:

Start the Records at Heat One

The single most useful habit you can build with a young prospect: log the first day of every heat from the very first one. Two or three cycles in, her personal interval emerges, and from then on you can project seasons months ahead — when to schedule progesterone testing, when to book the stud, when to plan around travel.

Our free dog heat cycle calculator does the projection from a last-heat date and typical interval. Breeders Companion goes further: heat histories logged per dog, forever, next to health records, litters, and your breeding calendar — so by the time she's cleared for her first breeding, her cycle pattern is already documented instead of reconstructed from memory.

This article is for general education, not veterinary advice. If your dog hasn't cycled by around 18–24 months, or anything about her first season concerns you, talk to your veterinarian.