2 A cycle's length not only varies from woman to woman, but from month to month.

"An average cycle is typically 28 days, though it's normal for it to be anywhere between 21 and 35 days," says Jennifer Wider, MD, spokeswoman for the Society of Women's Health Research and author of The Savvy Woman Patient. When calculating the length of your cycle, day one is the first day of your period, and the last day is the day before your next period arrives. According to Dr. Elizabeth Lyster of Holtorf Medical Group in California, it's the first phase of the menstrual cycle that varies the most from woman to woman. "The time from day one of your cycle to ovulation can be anywhere from one to three weeks," she says, "while the second phase, which occurs post-ovulation, lasts about 14 days for everyone." It's also normal for your cycle's length to vary from month to month—stress, diet and a plethora of other factors play a part. A common remedy for irregular or heavy periods that don't have a more serious underlying cause is birth control pills, which can help regulate your cycle.