How long does a woman have periods?
Women usually continue having periods until menopause. Menopause occurs around the age of 51, on average. Menopause means that a woman is no longer ovulating (producing eggs) and therefore can no longer become pregnant. Like menstruation, menopause can vary from woman to woman and may take several years to occur. Some women have early menopause because of surgery or other treatment, illness, or other reasons.
While the length of the first part of the cycle (from onset of bleeding to ovulation) varies among women, the second part (from ovulation to onset of bleeding) is almost always 14 days long.
Sperm can live for 3–4 days (possibly up to 7 days) inside a woman, so the most fertile period (with the highest likelihood of sexual intercourse leading to pregnancy) is the time some 5 days before until 1–2 days after ovulation. In a normal four-week cycle, this corresponds to the second and the beginning of the third week of the cycle. (It is important to note that pregnancy can occur from intercourse at any time during the menstrual cycle, even during menstruation.) Various natural family planning methods of birth control attempt to determine the precise time of ovulation in order to find the fertile and infertile days in the cycle.
A somewhat common error of people who have heard about the menstrual cycle and ovulation is to assume, for contraceptive purposes, that cycles are always regular and 28 days long, and that ovulation always occurs 14 days after beginning of the menses. This may lead to unintended pregnancies.