It’s tough to go with the flow when you see all that blood.

Most girls lose less than three ounces each cycle, says Dr. Diane Merritt, director, pediatric and adolescent gynecology, Washington University School of Medicine, St. Louis, Mo. “The average patient will soak a pad in two to three hours on her heaviest day,” she says. More than that could be too much blood.

So when should you head to the doc? Dr. Merritt says to ask yourself these questions: 1) Are you bleeding through more than one pad or tampon an hour? 2) Are you bleeding longer than 10 days? 3) Are your periods less than 20 days apart? If the answer to any of these is “yes,” see a gyno.