What Is LMP?

LMP Due Date Calculator

Enter the first day of your last menstrual period (LMP) to estimate your due date. The calculator adds 280 days, the standard convention for a due date based on the LMP.1

Updated October 2, 2026 · Your date stays on your device

Calculate your estimated due date

The day your last period started (the first day of bleeding), not the day it ended.

How the calculation works

By convention, the estimated due date (EDD) is 280 days after the first day of the last menstrual period.1 That is 40 weeks, the typical length of a pregnancy counted from the LMP.3 This calculator counts exactly 280 calendar days forward from the date you enter.

Worked example

First day of the LMP: January 1, 2026

Add 280 days: estimated due date October 8, 2026

You may also see Naegele's rule, a hand method that adds nine months and seven days to the LMP.4 It lands on the same date or within a few days of the 280-day count, depending on the months involved. This calculator always uses the 280-day count.

The calculation does not change the result for cycle length. It assumes a regular 28-day cycle with ovulation on day 14, as the standard convention does.1

Naegele's rule and the 280-day count, step by step

What the LMP date means

The LMP date is the first day of your most recent period: the day bleeding started, not the day it stopped.1 Pregnancy is dated from this day,5 so the count begins about two weeks before conception is assumed to happen.6

What is LMP? The full explanation

Why the result is an estimate

The 280-day count does not account for an LMP date that is remembered inaccurately, for cycles of irregular length, or for ovulation that happens earlier or later than usual.1 ACOG notes that about half of women are reported to recall their LMP accurately.1

Births also spread across several weeks: the NHS describes pregnancy as normally lasting 37 to 42 weeks from the first day of the last period.7 In a study of 34,249 pregnancies, 49.5% of births happened within 7 days either side of a due date estimated from the LMP.2

When to talk to a healthcare professional

Talk with your doctor or midwife about your due date if:

  • you are not sure of the first day of your last period
  • your periods are irregular, or your cycle is usually much longer or shorter than 28 days
  • you have been given a due date that is different from this estimate

A due date worked out from the LMP may be changed after a dating scan,8 and the scan can estimate how many weeks pregnant you are more accurately.7 This site cannot interpret your scans or records.

Back to What Is LMP?

Sources

  1. American College of Obstetricians and Gynecologists (ACOG). Methods for Estimating the Due Date (Committee Opinion No. 700). 2017; reaffirmed 2025.
  2. American Journal of Obstetrics and Gynecology. Mongelli M, Wilcox M, Gardosi J. Estimating the date of confinement: ultrasonographic biometry versus certain menstrual dates. 174(1 Pt 1):278–81. 1996.
  3. American College of Obstetricians and Gynecologists (ACOG). Definition of Term Pregnancy (Committee Opinion No. 579). 2013; reaffirmed 2025.
  4. Australian and New Zealand Journal of Obstetrics and Gynaecology. Lawson GW. Naegele's rule and the length of pregnancy – A review. 61(2):177–82. 2021.
  5. NHS (England), Best Start in Life. Week 4 (week-by-week guide to pregnancy). accessed October 2026.
  6. American College of Obstetricians and Gynecologists (ACOG). How Your Fetus Grows During Pregnancy. patient FAQ, reviewed 2026.
  7. NHS (England). Due date calculator. reviewed August 2024.
  8. NHS (England). Ultrasound scans in pregnancy. reviewed November 2023.