What Is LMP?

How to calculate your due date from your LMP

To calculate a due date from your last menstrual period (LMP), take the first day of that period and count forward 280 days, which is exactly 40 weeks.1 Naegele's rule, adding seven days and then nine months, is a hand method that lands on the same date or a few days later.2 Either way the result is an estimate, not the day the baby will arrive.

Count the 280 days with the Due Date Calculator

By whatislmp.com · Last updated October 3, 2026

Monthly paper calendar with red push pins on several days and the 30th circled in red
Photo: Towfiqu barbhuiya on Unsplash

What you need: the first day of your last period

Both methods start from one date: the first day of your last menstrual period, meaning the day bleeding started, not the day it stopped.1 Everything else in the calculation is fixed, so the result is only as good as that date. If you enter a date that is three days off, the due date is three days off too.

If you are not sure of the date, read what to do if you don't know your LMP before relying on any due date worked out from it.

Method 1: count 280 days from the LMP

By convention, the estimated due date is 280 days after the first day of the LMP.1 ACOG describes pregnancy as lasting an average of 40 weeks (280 days) from the first day of the LMP to the estimated date of delivery.3

  1. Write down the first day of your last period.
  2. Count forward 40 weeks. Because 280 days is exactly 40 weeks, the due date always falls on the same day of the week as the first day of your LMP.
  3. The date you land on is the estimated due date.

Counting in weeks on a calendar is easier than counting 280 single days, and the weekday gives you a quick check: if your LMP started on a Thursday, the due date should be a Thursday too.

Examples of the 280-day count
First day of the LMPEstimated due date (+280 days)
Thursday, January 1, 2026Thursday, October 8, 2026
Wednesday, April 15, 2026Wednesday, January 20, 2027
Saturday, August 1, 2026Saturday, May 8, 2027

Method 2: Naegele's rule (nine months and seven days)

Naegele's rule works from the same first day of the LMP: add seven days, then add nine months.2 Pregnancy, Birth and Baby describes the same steps for a regular 28-day cycle.4

  1. Take the date your last period started.
  2. Add 7 days.
  3. Add 9 months.

Worked example

First day of the LMP: January 1, 2026

Add 7 days: January 8, 2026. Add 9 months: October 8, 2026

You may also see the rule written as "subtract 3 months, add 7 days and add 1 year". It reaches the same date, because going back 3 months and forward 1 year is the same as going forward 9 months.

The 40-week idea is attributed to the German obstetrician Franz Naegele (1778–1851).2 Some authors have argued that his rule was meant to start from a different day of the period, but a re-examination of his original textbooks found that he advised counting from the first day of menstruation.5

Why the two methods can give different dates

Months have different lengths, so nine months and seven days is not always exactly 280 days. Checked for every start date from 2025 to 2028, Naegele's rule lands on the same day as the 280-day count or one to three days later, never earlier.

Worked example where they differ

First day of the LMP: March 1, 2026

Naegele's rule: March 8, then 9 months later, December 8, 2026

280 days: December 6, 2026

Neither date is more "correct": both are estimates built on the same assumptions. The Due Date Calculator always uses the 280-day count, so for some start dates its answer is a day or a few days earlier than a hand calculation with Naegele's rule.

What the 280 days assume

The 280-day convention assumes a regular 28-day cycle with ovulation on the 14th day. ACOG notes that it therefore 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

Cycle length matters because ovulation tends to come later in a longer cycle and earlier in a shorter one, which moves the estimated due date later or earlier.4 Neither method above adjusts for that; irregular periods and your due date explains what that means in practice.

The count also starts before conception. Pregnancy is assumed to begin about two weeks after the first day of the LMP, so those two weeks are part of the 40.6 How pregnancy weeks are counted goes through the convention in detail.

How accurate is a due date from the LMP?

A due date is the middle of a range, not a prediction of the birthday. Most babies are born between 37 and 42 weeks of pregnancy.4 In a study of 34,249 pregnancies with dates the mothers were sure of, 49.5% of births happened within 7 days either side of the LMP-based due date, and 64.1% within 10 days.7

Some of that spread is natural. Even when the day of ovulation was known, pregnancy length in a study of 125 naturally conceived pregnancies still varied over a range of 37 days, after excluding preterm births.8 A review of the length of pregnancy concluded that Naegele's rule should be treated as a guideline, not a definite date.2

Your care team may use a different date

An ultrasound of the embryo or fetus in the first trimester is the most accurate way to establish or confirm how far along a pregnancy is,1 and a due date worked out from the LMP may be changed after a dating scan.9 See ultrasound dating vs. LMP dating for how the two are compared.

Let the calculator count the days

Enter the first day of your last period and the Due Date Calculator adds the 280 days for you.

Calculate your due date

Common questions

Is a due date 40 weeks from my last period?

Yes, by convention: 280 days, or 40 weeks, counted from the first day of the LMP.1 About two of those weeks come before conception is assumed to happen.6

Do I count from the first or the last day of my period?

From the first day: the day bleeding started.1 Counting from the last day would make the due date several days too late.

Does a longer cycle change my due date?

It can. With a longer cycle the estimated due date will be later, and with a shorter cycle earlier.4 The 280-day count and the Due Date Calculator do not adjust for this.

Why is the due date from my doctor or midwife different?

A due date first worked out from the LMP may be changed after a dating scan.9 The date your care team gives you is the one that applies to your care.

All articles on the blog

Sources

  1. American College of Obstetricians and Gynecologists (ACOG). Methods for Estimating the Due Date (Committee Opinion No. 700). 2017; reaffirmed 2025.
  2. 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.
  3. American College of Obstetricians and Gynecologists (ACOG). Definition of Term Pregnancy (Committee Opinion No. 579). 2013; reaffirmed 2025.
  4. Pregnancy, Birth and Baby (Healthdirect Australia, funded by the Australian Government). Working out your due date. reviewed August 2024.
  5. Sexual & Reproductive Healthcare. Loytved CA, Fleming V. Naegele's rule revisited. 8:100–1. 2016.
  6. American College of Obstetricians and Gynecologists (ACOG). How Your Fetus Grows During Pregnancy. patient FAQ, reviewed 2026.
  7. 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.
  8. Human Reproduction. Jukic AM, Baird DD, Weinberg CR, et al. Length of human pregnancy and contributors to its natural variation. 28(10):2848–55. 2013.
  9. NHS (England). Ultrasound scans in pregnancy. reviewed November 2023.