Ever noticed when you take your black underwear out of the washing machine, there is a ‘bleach’ stain in the crotch area? Well, you have your vagina to thank.

That’s right, women’s bits can reportedly have the same corrosive, colour-changing properties as the chemicals we use to clean our toilets.

Not caused by menstruation or lack of cleanliness, this ‘bleached’ area – which often appears orange or bright white in colour – is similar to what happens if you spill small amounts of bleach on clothing.

Still unconvinced you would have gone your whole life without knowing this if it is the case? You aren’t the only one.

But Emma Soos, managing director at The Women’s Health Clinic confirms that our vaginas are totally capable of doing this (even if it seems shocking).

“It can cause a reaction, and stain, in different types of underwear,” says Soos.

So how does our vagina do this?

In short, this is all about acidic discharge.

The pH balance in the vagina is acidic – a healthy value is somewhere in the range between 3.8 and 4.5 – to stop bad things happening down there.

The NHS says: “Bacteria called lactobacilli help keep the vagina’s pH balance at its normal low level, which prevents the growth of other organisms.

“If the pH of the vagina increases – it gets less acidic – the quality or amount of lactobacilli can fall and other bacteria can multiply.”

And because, unlike male genitalia, the vagina is a self-cleaning machine, it produces discharge that has the same acidic qualities.

Dr Vanessa Mackay, spokesperson for the Royal College of Obstetricians and Gynaecologists, says: “This mucus is produced naturally from the neck of the womb, known as the cervix.

“The amount of vaginal discharge varies throughout a women’s menstrual cycle, and most pregnant women will get a pregnancy discharge.”

And then this acidic discharge ends up on our knickers when we wear them.

So is this something to worry about?

Obviously feeling like you have to replace your underwear regularly is an inconvenience (and an expense) but the discharge causing this corrosion and colour change is essential to your body’s healthy function.

MacKay says: “It’s normal and healthy for a woman to produce a clear or white discharge from her vagina. The vagina is designed to clean itself with natural secretions.”

“If these bacteria are disturbed it can lead to infection, such as bacterial vaginosis or thrush, and inflammation,” she adds.

Can anything be done to stop this bleaching?

Not really. You should see it as a sign that your vagina is operating normally and doing everything it should be.

Soos says “keep air circulating” by wearing cotton underwear rather than other more synthetic materials and perhaps consider switching out from black underwear where the staining is most obvious.

You also want to avoid doing anything too drastic to try and stop your vagina doing this, Mackay says: “Women should avoid using perfumed soaps, gels and antiseptics to clean their vaginas.

“Women are advised to use plain, un-perfumed soaps to wash the area around the vagina (the vulva), not inside it, gently every day.”

She also warns against panty liners every day as they can increase your chance of yeast infection or irritation.