What Is LMP?

How to work out your LMP from your due date

An estimated due date is set 280 days after the first day of the last menstrual period (LMP), so the LMP a due date implies is 280 days, or exactly 40 weeks, before it.1 This article explains that reverse count in words and examples; there is no calculator for it on this site.

By whatislmp.com · Last updated October 3, 2026

Orange highlighter resting beside the 31st on a paper calendar page
Photo: Eliza Diamond on Unsplash

Counting back 280 days from a due date

By convention, an estimated due date is 280 days after the first day of the LMP.1 Reversing that, the LMP a due date implies is 280 days before it.

  1. Start from the estimated due date.
  2. Count back 40 weeks on a calendar. 280 days is exactly 40 weeks, so you land on the same day of the week as the due date.
  3. That date is the implied first day of the LMP.
Due dates and the LMP each one implies (−280 days)
Estimated due dateImplied first day of the LMP
Thursday, October 8, 2026Thursday, January 1, 2026
Wednesday, January 20, 2027Wednesday, April 15, 2026
Saturday, May 8, 2027Saturday, August 1, 2026

If a due date was worked out with Naegele's rule (nine months and seven days) rather than by counting 280 days, going back 280 days can land up to three days later than the LMP that was actually used, because months have different lengths. How to calculate your due date from your LMP explains the difference between the two methods.

Finding how many weeks along you are from a due date

Once you have the implied LMP, you can count weeks from it in the usual way. The count follows the same 40-week convention as the due date it came from.

Worked example

Estimated due date: May 8, 2027. Implied LMP: August 1, 2026.

On October 3, 2026, that is 63 days after the implied LMP: 9 weeks, 0 days. The due date is 217 days away, and 63 + 217 = 280.

To count to today, enter the implied LMP in the Pregnancy Weeks Calculator. How pregnancy weeks are counted explains the weeks-and-days notation.

What the implied LMP means

The result is the starting point of the 40-week count that the due date is based on. It is not necessarily the day your period actually started:

  • Your due date may have been set or changed after an ultrasound scan.2 An ultrasound of the embryo or fetus in the first trimester is the most accurate method to establish or confirm gestational age,1 and in that case the due date reflects the scan, not your period.
  • The 280-day convention assumes a regular 28-day cycle with ovulation on day 14.1 If your cycle is different, the date your period started and the date the count implies can differ.

Gestational age is counted from a first day of gestation that is usually based on the LMP if an early ultrasound confirms it, or on the early ultrasound where the two differ.3 The implied LMP is that first day, wherever it came from. Ultrasound dating vs. LMP dating explains when a scan date replaces an LMP date.

Can you work out conception from a due date?

Only as a convention. The standard count assumes ovulation on day 14 of a 28-day cycle,1 so on paper conception sits 14 days after the implied LMP, or 266 days before the due date. For a due date of May 8, 2027, that is August 15, 2026.

That date is an assumption, not a record of when conception happened, because the timing of ovulation varies. LMP vs. conception date explains why.

An estimate of an estimate

A due date is itself an estimate, so a date counted back from it is too. Only a healthcare professional can date your pregnancy, and your care team's dates are the ones that apply to your care.

Count the weeks from the implied LMP

Enter the date you worked out and the Pregnancy Weeks Calculator shows how many weeks and days that is today.

Count your weeks

Common questions

Is the implied LMP the day my period actually started?

Not always. If your due date came from a dating scan, or your cycle is not 28 days long, the implied LMP is the start of the 40-week count, not necessarily the start of your period.1

My due date came from a scan. Can I still count back from it?

Yes. Counting back 280 days gives the start of the week count your care team is using, which is useful for following the weeks. It just does not tell you when your period began.

Why is there no calculator for this on the site?

The site's three calculators all start from the first day of the LMP. The reverse count is a single subtraction of 280 days, which is easiest to follow written out, as above.

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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). Ultrasound scans in pregnancy. reviewed November 2023.
  3. World Health Organization (WHO). ICD-11 Reference Guide. accessed October 2026.