What does LMP mean?
LMP stands for last menstrual period. In pregnancy dating, “date of LMP” always means the first day of the last period — the first day of bleeding, not spotting and not the day it ended. Every pregnancy week, from the first scan to the due date, is counted from that day.
LMP to EDD and EDD to LMP
| From | To | Formula |
|---|---|---|
| LMP | EDD (due date) | LMP + 280 days |
| EDD | LMP | EDD − 280 days |
| Gestational age on a date | LMP | date − gestational age (in days) |
| Conception date | LMP | conception − 14 days |
Choose what you know above and the calculator gives the matching LMP. If your due date came from an ultrasound, the LMP it implies may differ from your actual period date — that “equivalent LMP” is the one your care team uses from then on.
If you don’t remember your last period
That is common. An early ultrasound can date the pregnancy instead; enter the scan date and the gestational age shown to get the equivalent LMP and due date. To predict future periods from a known LMP, use the Menstrual Calculator. To find how far along you are from your LMP on any date, use the Gestational Age Calculator.
First day of the last period
Doctors always ask for the first day of the last period — the 1st day of the last menstrual period, not the last day. If you are unsure of the first day last period started, check a diary or period app; even an approximate date gives a useful estimate until your first scan.