Vaginal discharge is a normal part of having a vagina. But sometimes the stuff you see on your underwear can be a sign that something is off with your health.

Discharge is a fluid — made up largely of water — that contains good bacteria to maintain the vagina’s pH balance and protect against infection. It carries away bad bacteria and dead cells from the lining of the vagina to keep the area healthy.

These secretions naturally clean the vagina, much like saliva does for the mouth or tears do for the eyes, according to Dr. Sherry Ross, an OB-GYN and the author of “She-Ology: The Definitive Guide to Women’s Intimate Health. Period.”

Normal vaginal discharge is typically clear or white in color (though when it dries on your underwear, it may appear slightly yellow), and it is mostly odorless. If it has “some amount of a smell, it shouldn’t be unpleasant or offensive,” added Dr. Charis Chambers, the OB-GYN behind the @theperioddoctor account on Instagram.

The consistency and amount of discharge changes throughout the menstrual cycle. When you’re about to ovulate, for example, your discharge becomes somewhat clear, stretchy and slippery — similar to raw egg whites. This type of cervical mucus helps facilitate a sperm’s journey through the reproductive tract so an egg can be fertilized in the fallopian tube.

And while discharge tends to be wet and slippery around the time of ovulation, you may notice it’s drier or stickier at other points in your cycle.

But there may be times when your discharge seems off. Dr. Johana D. Oviedo, an OB-GYN and a clinical assistant professor at New York University’s Grossman School of Medicine, offered a rule of thumb to determine if it might be a sign of an infection or other health issue.

“The most important question to ask yourself is: Is this discharge associated with discomfort? Is it itchy, does it have a very foul odor, has it changed significantly from what I am used to?” Oviedo told HuffPost.

If the color, odor, amount or consistency is unusual for you, then it’s worth calling your medical provider.

Here are the signs you should generally look out for and what they might mean:

This type of discharge may be indicative of a yeast infection, which is caused by an overgrowth of healthy yeast in the vagina.

It’s “often associated with an increased amount of discharge as well as vulvovaginal irritation, itching and redness,” Chambers said.

The discharge may be thickened like curdled milk or cottage cheese, Oviedo noted.

Discharge that fits this description can be a sign of bacterial vaginosis, a common condition caused by an imbalance of the bacteria naturally found in the vagina.

“Bacterial vaginosis is not often as itchy or irritating as a yeast infection,” Chambers said. “The greatest discomfort is often the amount and odor of the discharge.”

Neither bacterial vaginosis nor yeast infections are sexually transmitted diseases, “but having sex can sometimes shift your vaginal pH and vaginal flora,” Oviedo said. “And this may result in one of these infections.”

A thin, frothy discharge with a foul smell can also be a sign of trichomoniasis, a common but curable sexually transmitted infection.

Discharge that’s yellow or green in color may be a sign of a sexually transmitted infection like gonorrhea or chlamydia.

This type of discharge may also be associated with “a foul odor or pain with urination,” Chambers said. “But many bacterial infections are asymptomatic.”

Some people regularly have a day or two of brown discharge right before or after their period. If that’s the case for you, then it probably shouldn’t be a concern, Dr. Pamela D. Berens, a professor at McGovern Medical School at the University of Texas Health Science Center at Houston, told Women’s Health in 2018. It’s likely just bit of old period blood mixed in with discharge. (Blood that’s exposed to oxygen turns deep red or brown.)

But if brown or bloody discharge is present at other times of the month or is atypical for you, give your OB-GYN a call. It could potentially be related to a vaginal infection, polycystic ovary syndrome, uterine polyps or other issues.

Brown or blood-tinged discharge can also be associated with early pregnancy spotting, a miscarriage or even an ectopic pregnancy. “So if it’s possible you might be pregnant, it’s important to see your doctor as soon as possible,” Berens told Women’s Health.

So if you aren’t already, start paying attention to what your discharge looks and smells like at baseline so you’ll be able to detect any changes. And remember that it will fluctuate a bit at different points in your menstrual cycle.

“Any sustained changes of amount, odor or consistency warrant discussion with your physician,” Chambers said. “This is especially warranted if these changes are associated with bothersome symptoms like itching, burning or pain with urination.”

You may experience something that doesn’t fit descriptions like these. But if it’s concerning to you, have a conversation with your medical provider.

“Your doctor or qualified health care professional can perform an exam or obtain simple testing to rule out infection,” Chambers said.