In many cases it will be necessary to determine the date of a woman's last menstrual period, generally for medical purposes or when a woman is attempting to get pregnant or, in some cases, to avoid getting pregnant. The first day of the last menstrual period is referred to as "day one," and represents the start of the menstrual cycle. This is because it is easier to measure than the last day of menstruation, which can be more difficult to determine. Also for fertility purposes, the last day of menstruation is not as significant as the first day.
Most women have a menstrual cycle that lasts 28 days. By determining the date of her last menstrual period, a woman can count the days in her menstrual cycle, starting over again at day one, when menstruation begins again. This allows women who know the average length of their cycles to predict when the period will start again. This can also help to indicate abnormal cycles, for example, which can be very important to calculate when trying to get pregnant. Generally, actual menstruation lasts for three to seven days after the start of the menstrual period, and ovulation occurs about fourteen days after the start of the last menstrual period. This is the time of the month when a woman is capable of becoming pregnant if she has unprotected sex, since an egg has been released.
For this reason, the date of a last menstrual period is also often used to calculate the approximate date of conception if it is not known. Doctors will generally take the date of the last menstrual period, add two weeks, and consider that to be an estimated date of conception on which to base the age of the fetus. This way, the doctor can then predict the approximate due date of the baby, and allow for better planning, though this can also change for various reasons.
Some women with irregular periods find that they skip a period entirely one month, or that it will be very light. This can make it difficult to calculate the date of the last menstrual period, but it is best to try be as accurate as possible with estimations for the doctor. Some doctors will recommend keeping a chart to make the process easier, and also to notice changes or patterns over time that may indicate changes in fertility, or even certain health issues.