What Is LMP?

What if you don't know your LMP?

If you don't know the first day of your last menstrual period (LMP), a calculator that starts from that date cannot give a useful estimate. A doctor or midwife can date the pregnancy another way, usually with an ultrasound scan early in pregnancy.1

By whatislmp.com · Last updated October 3, 2026

Doctor writing on a form on a clipboard, with a patient's hands resting on the desk opposite
Photo: Vitaly Gariev on Unsplash

Not knowing the date is common

It has been reported that only about half of women accurately recall their LMP.2 Periods are easy to lose track of, especially if they are irregular or a pregnancy was not planned. A doctor or midwife has other ways to date a pregnancy; what not knowing the date mainly limits is what a calculator can do.

Ways to pin down the date

Before deciding the date is unknown, it can help to check:

  • a period-tracking app or a fertility app, which may hold the exact date
  • a calendar, diary or planner where you noted it
  • messages, photos or receipts from around that time that remind you when it started

If you find a date you are confident about, the calculators can use it. If you can only narrow it down to a week or two, a calculator result will be off by the same amount, so treat it as a rough guide at most.

Why the calculators need the date

An estimated due date is calculated from the first day of the LMP.1 The three calculators on this site all start from that one date: the Due Date Calculator adds 280 days, the Pregnancy Weeks Calculator counts from it to today, and the Conception Date Calculator adds 14 days.

A guessed date gives a guessed result. Every day the entered date is wrong moves the result by the same number of days, and the calculators cannot tell that the date was a guess.

How a doctor or midwife can date the pregnancy

When the date of the LMP is uncertain, an ultrasound exam may be done during the first trimester to estimate the due date.1 If you are unsure of your LMP, ACOG guidance is to base the dating on ultrasound, ideally at or before 13 weeks 6 days, with the earliest measurement treated as the most reliable.2 The scan measures the baby's size and compares it with population data to estimate the due date.3

Pregnancy, Birth and Baby lists not knowing your LMP, a very long or irregular cycle, and becoming pregnant while taking the contraceptive pill among the situations in which an early ultrasound may be strongly recommended. It suggests a first antenatal visit before 10 weeks, ideally at about 6 to 8 weeks.3 In England, the dating scan happens at around 10 to 14 weeks and is used to work out how many weeks pregnant you are and your due date.4

For how a scan estimate is compared with an LMP date, and how accurate it is at different stages, see ultrasound dating vs. LMP dating.

What to tell your care team

  • That you don't know, or aren't sure of, the first day of your last period.
  • Any record you do have, such as a calendar note or an app entry, and how sure you are of it.
  • Whether your periods are usually regular, and roughly how long your cycles are. Irregular cycles also make LMP dating less reliable.
  • Whether you became pregnant while using hormonal contraception such as the pill.

Your care team decides how your pregnancy is dated. Only a healthcare professional can date your pregnancy, and this site cannot interpret scans or records.

Found the date?

If you have pinned down the first day of your last period, the Pregnancy Weeks Calculator shows how far along that makes you today.

Count your weeks

Common questions

How far along am I if I don't know my last period?

That can't be worked out reliably without the date. A dating scan can check how many weeks pregnant you are.4

Can I use my due date instead?

Yes, if you already have one from your care team. Counting 280 days back from it gives the start of the week count; see how to work out your LMP from your due date.

Is it unusual not to remember the date?

No. Only about half of women are reported to recall their LMP accurately.2 When the LMP is uncertain, ACOG guidance is to date the pregnancy by ultrasound instead.2

All articles on the blog

Sources

  1. American College of Obstetricians and Gynecologists (ACOG). How Your Fetus Grows During Pregnancy. patient FAQ, reviewed 2026.
  2. American College of Obstetricians and Gynecologists (ACOG). Methods for Estimating the Due Date (Committee Opinion No. 700). 2017; reaffirmed 2025.
  3. Pregnancy, Birth and Baby (Healthdirect Australia, funded by the Australian Government). Working out your due date. reviewed August 2024.
  4. NHS (England). 12-week scan. reviewed November 2023.