I was curious about bleach-like stains in my underwear. A midwife explained that they're a sign of a healthy vagina.

• I noticed that my black underwear had what seemed like bleach stains in the crotch area.
• A midwife explained that a healthy vagina is mildly acidic to defend itself from harmful organisms.

I've always been impressed with my vagina, an organ capable of cleaning itself and alerting me when something's wrong.

But I noticed a curious trend: My black underwear had a white splotch in the crotch that seemed to look a bit bleached. Concerned, I did some investigating and reached out to a gynecological expert to confirm that all was OK down there.

The pH of a healthy vagina is mildly acidic

I talked to Rebecca Ingalls, a midwife and women's-health nurse practitioner in New Hampshire. Ingalls told me that in premenopausal years, a healthy vagina tends toward acidity.

"The acid is created by helpful microorganisms called lactobacilli, which defend against harmful microorganisms," Ingalls said.

I knew that every substance with water has a pH level indicating how acidic or basic it is. One way the vagina stays fresh is by maintaining a pH of 3.8 to 4.5, meaning healthy vaginal discharge is slightly acidic.

I'd noticed the gussets — the cotton flap that covers the synthetic material of panties, designed to provide extra ventilation — in my undies were changing color after long-term wearing. I learned this is because cotton is sensitive to acid dyeing.

My perfectly acidic vagina was secreting fluids that encourage good bacteria to grow and keep bad bacteria at bay. And the evidence was right there in my underwear.

Many things can alter the pH

Ingalls said a few things that can cause pH changes in vaginal discharge include sex, periods, and hygiene practices. Even douching with water, which has a neutral pH of 7, can cause an imbalance in the vagina.

"While a higher pH is not always a cause for concern, it can be a sign that the lactobacilli are few or absent and that the vaginal environment is out of balance or is experiencing an infection," Ingalls said.

She added that vaginas should never be cleaned inside — only outside, with water and mild, fragrance-free soap — to keep the lactobacilli plentiful so the vagina can clean itself with the acidity.

I have an entire ecosystem at work in my vagina keeping it clean and balanced.

My discharge consistency and scent change throughout my menstrual cycle, but if it were ever to take on a new color or strong odor, or if things got itchy, I'd consider visiting a medical professional who specializes in gynecological health. A midwife or doctor could rule out things like a yeast infection or bacterial vaginosis, which has a different pH than healthy discharge.

Since I wasn't experiencing any symptoms aside from bleaching in my underwear, Ingalls' input reassured me it was safe to conclude that things are healthy down there. And I learned my vagina is so good at staying healthy that it's in its prime when just left alone.

If my undies are looking a little off-colored, I can take comfort in knowing it's a sign my vagina is going strong.