What Is LMP?

Pregnancy dating basics

What Is LMP?

LMP stands for last menstrual period. In pregnancy, it means the first day of your most recent period: the day bleeding started, not the day it stopped.12

Pregnancy is counted from that date, and it is the starting point for estimating a due date. By convention, the estimated due date is 280 days (40 weeks) after the first day of the LMP.1

Calculate your due date How pregnancy weeks are counted

Updated October 3, 2026 · Sources are listed at the end of this page

Calculators

What LMP means in pregnancy

In everyday speech, "your last period" can mean any part of it. In pregnancy dating, LMP refers to one specific day: the day your most recent period started.1 The NHS also dates pregnancy from the first day of your last period.2

You may see it written as "LMP" in pregnancy notes, often next to "EDD", the estimated due date.

Why the first day matters

The first day of bleeding is a clear, specific date, and using it gives everyone the same starting point. Gestational age, the number of weeks and days used to describe how far along a pregnancy is, is counted from that day.5

This means the count starts before conception. The standard calculation assumes a 28-day cycle with ovulation on day 14,1 so conception is assumed to happen about two weeks after the first day of the LMP.5 At the moment of conception, a pregnancy is therefore already counted as about two weeks along.

  1. Week 0 First day of the LMP The first day of bleeding of your last period. The count starts here.
  2. About week 2 Ovulation and conception (assumed) The standard calculation assumes ovulation around day 14 of a 28-day cycle.
  3. Week 40 Estimated due date 280 days after the first day of the LMP, by convention.

What EDD means

EDD stands for estimated due date. It is the date a pregnancy reaches 40 weeks when counted from the LMP.3

By convention, it is calculated by adding 280 days to the first day of the LMP.1 If your cycle is usually longer or shorter than 28 days, calculators commonly adjust the date. The NHS due date calculator, for example, asks for your cycle length,4 and a longer cycle moves the estimate later.6

Worked example (28-day cycle)

First day of the LMP: August 1, 2026

Add 280 days: estimated due date May 8, 2027

A quicker hand method, known as Naegele's rule, adds nine months and seven days to the LMP.7 Depending on the months involved, it lands on the same date as the 280-day count or within a few days of it.

How to calculate a due date by hand, including Naegele's rule

Work out your estimated due date

Enter the first day of your last period. The calculator adds 280 days and shows your estimated due date.

Calculate your due date

Related terms

LMP
Last menstrual period. In pregnancy dating, the first day of your most recent period.1
EDD
Estimated due date. 280 days (40 weeks) after the first day of the LMP, by convention.1
Gestational age
How far along a pregnancy is, in weeks and days counted from the LMP, for example "8 weeks, 6 days".5
Trimesters
The first trimester runs to 13 weeks, 6 days; the second from 14 weeks, 0 days to 27 weeks, 6 days; the third from 28 weeks.5
Full term
39 weeks, 0 days to 40 weeks, 6 days. Early term begins at 37 weeks, late term at 41 weeks and post-term at 42 weeks.3
Naegele's rule
A hand method for estimating the due date: add nine months and seven days to the LMP.7

Why the due date is an estimate

The 280-day convention assumes a regular 28-day cycle with ovulation on day 14. It does not account for an LMP 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.4 In a study of 34,249 pregnancies, about half of births (49.5%) happened within 7 days either side of a due date estimated from the LMP.8

Your care team may give you a different date

A due date is first worked out from the LMP and may be adjusted after a dating scan.9 If you have been given a due date, your doctor or midwife can answer questions about it. This site cannot interpret your scans or records.

From the blog

All 8 articles on the blog

Common questions

Is the LMP the first or last day of your period?

The first day. LMP refers to the first day of bleeding of your most recent period, not the day it ended.1

How many days are there between the LMP and the due date?

280 days, or 40 weeks, by convention. The calculation assumes a 28-day cycle.1

Does cycle length change the due date?

It can. A longer regular cycle moves the estimate later.6 Irregular cycles make an LMP-based estimate less reliable.1 See how irregular periods affect your due date.

What if I don't know the date of my last period?

An LMP-based estimate needs that date. Your doctor or midwife can date the pregnancy in other ways, for example with a dating scan.10 See what to do if you don't know your LMP.

Will the baby arrive on the due date?

Usually not exactly. The due date is an estimate. In one large study, about half of births happened within a week either side of a due date based on the LMP.8

Is this site medical advice?

No. whatislmp.com explains published conventions and offers calculators that give estimates. It does not diagnose anything or replace care from a doctor or midwife. See the medical disclaimer.

Ready to estimate your due date?

Enter the first day of your last period and the Due Date Calculator does the counting.

Calculate your due date

Sources

  1. American College of Obstetricians and Gynecologists (ACOG). Methods for Estimating the Due Date (Committee Opinion No. 700). 2017; reaffirmed 2025.
  2. NHS (England), Best Start in Life. Week 4 (week-by-week guide to pregnancy). accessed October 2026.
  3. American College of Obstetricians and Gynecologists (ACOG). Definition of Term Pregnancy (Committee Opinion No. 579). 2013; reaffirmed 2025.
  4. NHS (England). Due date calculator. reviewed August 2024.
  5. American College of Obstetricians and Gynecologists (ACOG). How Your Fetus Grows During Pregnancy. patient FAQ, reviewed 2026.
  6. Pregnancy, Birth and Baby (Healthdirect Australia, funded by the Australian Government). Working out your due date. reviewed August 2024.
  7. 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.
  8. 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.
  9. NHS (England). Ultrasound scans in pregnancy. reviewed November 2023.
  10. NHS (England). 12-week scan. reviewed November 2023.