If you don’t get your new addition spayed right away, she’s going to start having heat cycles, known as estrus. Once she "gets her period," as it were, you’ll want to keep her away from other dogs. Otherwise, she's bound to get pregnant and present you with a litter.
When It Happens
The exact age your fur pal will have her first period depends on her breed. It can happen anywhere from 6 to 24 months of age. Usually it occurs at the earlier end of the range for small breeds -- Chihuahuas, Yorkies and miniature poodles, to name a few. But large-breed pups, including boxers, bull mastiffs and Great Danes, among others, get their first heat much later on. Each dog is different; your veterinarian can give you a better idea when to expect the first estrus.
Check your puppy’s hind end. Her vulva will swell dramatically as her first heat approaches, and you’ll see a bloody discharge. Before this, she may pee more than normal and seem skittish. These cues let you know her hormone balance is shifting and she is gearing up to breed.
Length of Time
The heat cycle lasts about 18 days. She’s most fertile approximately 12 days after the bleeding starts. As the cycle comes to a close, the discharge slows and turns pinkish.