A fishy smell during your period usually signals that something has shifted in the balance of bacteria in your vagina, most commonly a condition called bacterial vaginosis (BV). Normal period blood has a metallic or slightly musky scent from the iron it contains. If you’re noticing something distinctly fishy, that’s a different process, and it’s one of the most common vaginal complaints.
What Normal Period Blood Smells Like
Menstrual blood has its own scent, and having some odor during your period is completely expected. The iron in blood gives it a coppery, metallic smell, similar to handling pennies. You may also notice a slightly sour or tangy quality, which comes from lactobacilli, the beneficial bacteria that keep your vagina healthy by maintaining an acidic environment. These smells can intensify toward the end of your period as older blood oxidizes, but they shouldn’t smell like fish.
A healthy vaginal pH sits between 3.8 and 4.5, acidic enough to keep harmful bacteria in check. Right before and during your period, though, pH naturally rises because menstrual blood is more alkaline. This temporary shift is normal, but it creates a window where the bacterial balance is more vulnerable to disruption.
Bacterial Vaginosis: The Most Likely Cause
BV is the most common reason for a fishy vaginal odor, and it affects roughly 1 in 3 women at some point. It’s not a sexually transmitted infection. It happens when the protective lactobacilli bacteria lose ground to other types of bacteria that don’t belong in large numbers.
Those overgrown bacteria produce specific chemicals called volatile amines, particularly trimethylamine and dimethylamine. These are the same compounds responsible for the smell of rotting fish. In more severe cases, the bacteria also generate putrescine and cadaverine (named, quite literally, for the smells of decay). Women with a healthy vaginal balance have only trace amounts of these compounds, while women with BV have dramatically elevated levels.
The fishy smell from BV often gets stronger during your period because menstrual blood raises vaginal pH, which helps release those volatile amines into the air. Many women also notice the smell intensifies after sex for the same reason: semen is alkaline. Other signs of BV include a thin, white or grayish discharge with a milklike consistency. Some women have no symptoms beyond the odor itself.
Other Causes Worth Knowing
Trichomoniasis
This sexually transmitted infection can also produce a fishy smell, sometimes with a clear or white discharge. It’s caused by a parasite rather than a bacterial imbalance, so it requires different treatment. Some women with trichomoniasis also experience itching, burning during urination, or irritation that BV doesn’t typically cause. A simple swab test at your doctor’s office can distinguish between the two.
A Forgotten Tampon or Menstrual Product
It sounds unlikely, but retained tampons are more common than you’d think, especially when a second tampon is inserted without removing the first, or when the last tampon of a period is simply forgotten. The smell in this case is often described as overwhelmingly foul rather than mildly fishy, and it gets worse with each passing day. Other warning signs include unusual discharge that may be yellow, green, or brown, along with pelvic pain, fever, or discomfort while urinating. If you suspect this, a doctor can remove it quickly and check for any secondary infection.
Why Your Period Makes the Smell Worse
Even if you’ve had a mild bacterial imbalance for days or weeks without noticing, your period can unmask it. Menstrual blood raises vaginal pH above 4.5, and at that higher pH, the fishy-smelling amines become volatile, meaning they escape into the air more easily. Think of it like uncorking a bottle. The bacteria and their byproducts may have been present before your period started, but the chemical conditions during menstruation make the odor suddenly noticeable.
Wearing pads can also concentrate the smell in a way that internal products like menstrual cups don’t. Blood sitting on a pad is exposed to air and warmth, which accelerates the breakdown of any compounds present. This doesn’t cause the smell, but it can make you more aware of it.
How BV Is Diagnosed and Treated
Doctors typically diagnose BV using a combination of signs: the characteristic thin discharge, a vaginal pH above 4.5, the presence of certain cells under a microscope, and a positive “whiff test” where a chemical is added to a discharge sample to check for that fishy amine release. You don’t need all four markers, but at least three confirm the diagnosis.
BV is treated with a short course of antibiotics, usually taken orally or applied as a vaginal gel. Most women notice the odor resolving within a few days of starting treatment. The frustrating part is recurrence: BV comes back within a year for a significant number of women, sometimes because the underlying bacterial balance was never fully restored.
Habits That Protect Your Vaginal Balance
Douching is one of the strongest risk factors for developing BV. Women who douche at least once a month have about 1.4 times the risk of BV compared to those who don’t, and that risk jumps to 2.1 times for women who douched within the past week. Douching washes away the hydrogen peroxide-producing lactobacilli that keep harmful bacteria suppressed. Your vagina is self-cleaning, and water on the outside is all that’s needed.
A few other practices help maintain a healthy balance:
- Change period products regularly. Pads and tampons left in place too long create a warm, moist environment that encourages bacterial overgrowth.
- Avoid scented products near the vagina. Fragranced soaps, sprays, and wipes can disrupt pH and irritate the vaginal lining.
- Wear breathable underwear. Cotton or moisture-wicking fabrics reduce the trapped warmth and humidity that bacteria thrive in.
- Wipe front to back. This prevents introducing bacteria from the rectal area into the vagina.
If the fishy smell is new, persistent, or accompanied by unusual discharge, getting a proper diagnosis matters. BV, trichomoniasis, and a retained object all produce similar smells but require completely different responses. A quick office visit or telehealth appointment can sort it out and get you the right treatment within days.