Is the menstrual cycle supposed to be 28 days from the first day of one period to the start of the next, or is it 28 days from the last day of one period to the start of the next?

--T.

This is a good question, as menstrual cycle length is frequently misunderstood. We count from the beginning of one period to the beginning of the next. Cycle day 1 is the first day of bleeding. A normal cycle length is between 23 and 35 days; most of the time, cycles outside this range indicate one is not ovulating, although other problems must be ruled out, especially in cycles shorter than 23 days.

It is important to chart one's cycle accurately for many reasons. First, it is helpful if doctor and patient are talking about the same thing; for example, if a patient tells me her cycles are only 17 days, but she is counting from the end of one to the beginning of the next, and I assume she means from beginning of one to the beginning of the next, I may think there is something seriously wrong and begin doing a lot of tests; if she is having seven-day periods, her cycle length is really a normal 24 days (17 + 7). If a woman is trying to get pregnant, she must keep accurate track of her cycles in order to time intercourse appropriately (or, conversely, avoid intercourse if she is using the "rhythm method" -- to avoid pregnancy).