What is the LMP?
LMP stands for last menstrual period. In pregnancy dating it always means the first day of your most recent period before you became pregnant — the day full bleeding started. It is the starting point for your due date (LMP + 280 days), for counting weeks of pregnancy, and for many screening and appointment schedules. You will see it on forms and scan reports as LMP or “first day of LMP”.
Why pregnancy is counted from the LMP
Conception happens out of sight and on a day most people do not know. The first day of a period, on the other hand, is a clear, memorable event. Counting from it gives everyone — you, your midwife, the sonographer — the same reference point. The trade-off is that the first two weeks of “pregnancy” happen before conception. Read more in How Pregnancy Weeks Are Calculated.
How to work out your LMP
- Check a period-tracking app or calendar notes.
- Think back to events — a trip, birthday or exam — and whether your period was before or after.
- Count back from when your next period was due: your LMP is about one cycle length before that.
- Count back from your first positive pregnancy test: tests usually turn positive about four weeks after the LMP.
Common mix-ups
| Use | Don’t use |
|---|---|
| The first day of real bleeding | Spotting in the days before the period |
| Your last period before the pregnancy | Bleeding during early pregnancy (for example around implantation) |
| The start of the period | The day it ended |
When the LMP isn’t reliable
The LMP is less useful if your cycles are irregular, very long or short, if you recently stopped hormonal contraception, were breastfeeding, or became pregnant soon after a previous pregnancy. In these cases an early ultrasound is the best way to date the pregnancy. If you only know the month, the Pregnancy Estimator gives a range; if you know the date, the Pregnancy Calculator does the rest.