The most likely reason your female dog is bleeding is that she’s in heat. This is the natural reproductive cycle that unspayed female dogs go through, typically twice a year. Bloody vaginal discharge during heat is completely normal and usually lasts six to eleven days. But heat isn’t the only possibility, and some causes of bleeding need prompt veterinary attention.
The Heat Cycle: The Most Common Cause
If your dog hasn’t been spayed, the bleeding you’re seeing is almost certainly from her heat cycle, also called the estrus cycle. The bleeding happens during the first stage, called proestrus, when rising estrogen levels cause bloody vaginal discharge and noticeable swelling of the vulva. This stage lasts an average of six to eleven days. During this time, male dogs will be very interested in her, but she won’t be receptive to mating yet.
After proestrus ends, the discharge typically changes from red to a straw or pinkish color as she enters the next stage, when she becomes receptive to breeding. This stage lasts another five to nine days on average, though it can stretch up to twenty days in some dogs. So from the first drop of blood to the end of her fertile window, you’re looking at roughly two to three weeks total.
Most dogs go into heat about twice a year, though this varies by breed and individual. Small breeds may cycle more frequently, while some larger breeds cycle only once a year. If your dog is young (anywhere from six months to two years old, depending on her size), this could be her very first heat, which can catch owners off guard.
What Normal Heat Bleeding Looks Like
During a normal heat cycle, you’ll notice bright red drops of blood, a swollen vulva, and your dog licking herself more than usual. She may seem restless or clingy, and she’ll attract attention from male dogs in the area. The amount of bleeding varies a lot between individual dogs. Some bleed heavily enough to leave spots on furniture, while others keep themselves so clean you barely notice.
The bleeding should gradually taper off and shift in color over a week or two. If it stays heavy, turns dark or foul-smelling, or lasts significantly longer than two to three weeks, something else may be going on.
Pyometra: A Dangerous Uterine Infection
Pyometra is a serious, potentially fatal infection of the uterus that typically develops in unspayed dogs within a few weeks after a heat cycle. It happens when bacteria enter the uterus during heat and multiply, filling the uterus with pus. The discharge from pyometra looks different from normal heat bleeding. It ranges from thick and yellowish to dark and bloody, and it often has a foul smell.
In what’s called “open” pyometra, infected discharge drains out through the vagina, so you’ll see it. In “closed” pyometra, the cervix seals shut and nothing drains. The infection builds up inside, and the dog becomes severely ill very quickly. Signs include loss of appetite, lethargy, vomiting, diarrhea, drinking excessive amounts of water, and a swollen or painful abdomen. Without treatment, the uterus can rupture and spill infection into the abdomen, which is fatal.
Any unspayed female dog who seems unusually sick, is drinking far more water than normal, and has vaginal discharge (or a distended belly without discharge) should be evaluated immediately. Pyometra is a veterinary emergency.
Urinary Tract Infections
Sometimes what looks like vaginal bleeding is actually blood in the urine. Urinary tract infections are common in female dogs and can produce pink or red-tinged urine that’s easy to mistake for reproductive bleeding. The key difference is in the other symptoms: a dog with a UTI will strain to urinate, pee in small amounts more frequently, have accidents in the house, and lick her genital area excessively. The urine may also smell unusually strong or foul. Bladder stones can cause similar symptoms and make dogs more prone to recurring infections.
Vaginitis
Vaginitis, or inflammation of the vagina, can produce a discharge that ranges from clear to bloody. It occurs in both puppies (juvenile vaginitis) and adult dogs. You’ll typically notice your dog licking herself more and may see irritation or discharge around the vulva. Juvenile vaginitis in puppies often resolves on its own after the first heat cycle, but adult-onset vaginitis usually needs veterinary evaluation to rule out underlying causes like urinary infections or structural abnormalities.
Vaginal Tumors
Tumors in the vaginal area can cause bloody or blood-tinged discharge, particularly a type called transmissible venereal tumor (TVT) that spreads between dogs through direct contact. TVT tends to appear in younger dogs, typically between two and five years old, unlike most genital tumors that occur in dogs over ten. These growths start as small reddish bumps and can develop into larger, cauliflower-like masses that bleed easily and produce foul-smelling discharge. They’re most common in dogs that have had contact with strays or unfamiliar dogs. Other types of vaginal or uterine tumors are possible in older dogs and also cause abnormal bleeding.
Bleeding Disorders
Less commonly, vaginal bleeding can be a sign of a systemic problem with blood clotting. One important cause: rat poison. Dogs that ingest rodenticide lose the ability to form blood clots properly because the poison disrupts clotting proteins made by the liver. This can show up as unexplained bleeding from the vagina, gums, or nose, along with bruising, black stools (from internal bleeding), or prolonged bleeding from minor wounds. Severe liver disease and inherited conditions like von Willebrand disease can also impair clotting.
If your dog is bleeding and also showing bruises, pale gums, weakness, or bleeding from multiple sites, a clotting disorder is a strong possibility that needs urgent care.
Bleeding in Spayed Dogs
If your dog has been spayed and is showing signs of heat, including bloody vaginal discharge and vulvar swelling, the most likely explanation is ovarian remnant syndrome. This happens when a small piece of ovarian tissue is accidentally left behind during the spay surgery. That tissue fragment continues producing hormones and can trigger heat-like symptoms, including bleeding and behavioral changes like flagging her tail. The dog won’t get pregnant, but the leftover tissue can lead to complications over time, including tumor development and infection of the remaining uterine stump. A second surgery to remove the remnant is the standard fix.
How to Tell What’s Causing the Bleeding
Context matters a lot when figuring out why your dog is bleeding. Ask yourself these questions:
- Is she spayed? If yes, heat is off the table (unless ovarian remnant syndrome is in play), and the cause is more likely medical.
- How old is she? A young, unspayed dog bleeding for the first time is almost certainly in heat. An older dog with new or unusual discharge warrants more concern.
- What does the discharge look like? Bright red blood with a swollen vulva points to heat. Thick, yellowish, or foul-smelling discharge suggests infection. Pink-tinged urine with straining suggests a urinary issue.
- How is she acting? A dog in heat may be restless but otherwise healthy. A dog who is lethargic, not eating, drinking excessively, or vomiting needs veterinary attention right away.
- How long has it lasted? Heat bleeding that resolves within two to three weeks is normal. Bleeding that persists beyond that, or occurs outside the expected twice-yearly cycle, is worth investigating.
For a young, otherwise healthy unspayed dog showing her first signs of bleeding with a swollen vulva and normal energy, you’re most likely witnessing a normal heat cycle. For anything that doesn’t fit that pattern, or if the bleeding is accompanied by lethargy, foul odor, excessive thirst, or pain, a vet visit is the right call.