How heavy are your periods? Medically, it’s an important question—one-third of patients bleed heavily, which can cause iron deficiency or anaemia, and occasionally be so severe as to require blood transfusions or surgery. But it’s a very difficult question to answer. Bethany Bannow is an Oregon haematologist with a special interest in bleeding disorders and menstruation. “I see patients in my clinic all the time with terrible periods who have no idea they are heavy,” she says, “because their periods, and often those of their female relatives, have always been heavy.”

So doctors tend to ask a second question: how often do you have to change your sanitary product? But here, too, they’ve been flying blind—because they have scant information about the absorbency of those products. The only published studies used saline or water, not menstrual fluid or anything like it. And no one had compared the capacity of more modern products such as menstrual cups and discs, or period underwear.

In a study just published in the British Medical Journal, Bannow and her co-authors used blood—expired O+—to soak 21 period products of various types, sizes and brands. The graphic above shows the maximum each type of product could handle. Often, this was much less than advertised. Period underwear absorbed 3mL, “and quite slowly”. A ‘Ziggy’ menstrual disc, meanwhile, held 80mL. That much menstrual fluid, even if it took an entire cycle to accumulate, is enough to diagnose a patient with heavy bleeding.