Three or four full cups a cycle already means a heavy period
How do you know if your period is heavy? A doctor will usually ask how often you change your pad or tampon: if it's soaked through every hour or two, that's a warning sign. Heavy periods are common, affecting up to a third of people who menstruate, by the authors' estimate. The medical threshold is more than 80 mL of blood per cycle.
Researchers at Oregon Health & Science University measured how much the products actually hold. Manufacturers usually test with water or saline; this team used expired donor red blood cells and tested 21 products: tampons, pads, cups, discs and period underwear. Tampons, heavy pads and cups held similar amounts, roughly 20–50 mL; the cups themselves, 22 to 35 mL. Discs held 61 mL on average, period underwear under 3 mL.
So filling a cup to the brim three or four times in a cycle is enough to pass 80 mL. Using "soaked through every hour or two" as the yardstick, the authors write, greatly underestimates both blood loss and how common heavy periods are. For people who use cups or discs, a heavy period can go unnoticed. Then again, each product was tested only once, outside the body, and blood from a bag still isn't menstrual blood.
The authors advise doctors to ask what you use and how you use it. You can say it first, without waiting to be asked. "I fill my cup to the brim" tells a doctor more about your cycle than "I don't change it often."
Sources
- DeLoughery E, Colwill AC, Edelman A, Samuelson Bannow B. Red blood cell capacity of modern menstrual products: considerations for assessing heavy menstrual bleeding. BMJ Sexual & Reproductive Health, 2024; 50(1): 21–26. doi:10.1136/bmjsrh-2023-201895