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Last Menstrual Period (LMP) Medically Reviewed

Last Updated: June 3, 2020

Definition - What does Last Menstrual Period (LMP) mean?

Last menstrual period (LMP) refers to the start date of the most recent menstrual period. A typical menstrual cycle is around 21-35 days in length, with the first day of bright red menstrual bleeding signifying day 1 or the start of the cycle.

For example, if the first day of bright bleeding in a most recent menstrual cycle occurred on January 1st, the LMP would be January 1st.

FertilitySmarts explains Last Menstrual Period (LMP)

Because some people have a relatively predictable pattern when menstruation occurs, the start date and duration of the last menstrual cycle can provide important insight, including:

When the next menstrual period will occur.

Many people will get their period every 21-35 days. To calculate the length of a cycle, use the number of days between the first day of bleeding to the day before the next day of bleeding. It is likely that the tracking of 3 or more cycles is needed to establish a pattern.

The time frame in which ovulation may occur.

The length of a cycle can be used to estimate the timing of ovulation, which for most people is at the midpoint of their cycle. For people with a 28-day cycle, this is around day 14. If the egg is not fertilized in 24 hours, it disintegrates.

If pregnant, the approximate due date and gestational age of the fetus.

LMP is also used to calculate an estimated due date where spontaneous labor may occur during pregnancy. This date is estimated by adding 280 days to the first day of the last menstrual period. This method is not as accurate as estimates calculated with ultrasound findings.

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