LMP (last menstrual period): what it means and why your doctor asks for it
In this article
LMP stands for last menstrual period, and it means the first day of your most recent period before you got pregnant. Your pregnancy weeks are counted from it. KKH's obstetrics team explains that week 1 of pregnancy starts on day 1 of your LMP, because that is a fixed date you know, while the day you conceived usually is not.
| Stands for | Last menstrual period |
|---|---|
| Which day | The first day of bleeding, not the last |
| Used for | Your week count and your estimated due date |
| If unsure | A dating scan fixes the dates instead |
Why your doctor counts from your LMP
Conception is hard to date. KKH's obstetrics team gives the reasons: couples may have sex often, it is hard to remember the timeline, and the day of sex is not always the day an egg is fertilised. Your period, on the other hand, is a day you can usually check.
So week 1 of pregnancy starts on day 1 of your LMP. At that point you are not actually pregnant yet. KKH says ovulation comes roughly 14 days later. That is why your gestational age always runs about two weeks ahead of your baby's age from conception.
Getting the date right
Your LMP is the first day you bled, not the day your period ended. The NHS due date calculator asks for exactly that date, and it lets you change the cycle length because your cycle may not be exactly 28 days. A period tracker app or a note in your calendar saves you guessing at the clinic.
Expect to be asked for it early. The NHS lists the date of your last period among the first questions at the booking appointment.
When a scan overrides your LMP
An LMP date only works well if your cycle is regular and you are sure of the day. KKH's obstetrics team says the estimated date of delivery can be corrected after the dating scan, from week 7 onwards. The NHS says a scan can estimate how many weeks pregnant you are more accurately than the period date.
If your periods are irregular or you are unsure of the day, say so at your first visit and let the dating scan settle it.
Using your LMP at home
Put your LMP into our due date calculator to see your due date and how many weeks along you are now. Then read the matching week, for example 6 weeks pregnant, to see what is happening and which check-up comes next. Bring the date to your first antenatal visit.
Frequently asked questions
What does LMP mean on a pregnancy form?
Last menstrual period. Write the date your last period started, before you became pregnant.
Is LMP the first or last day of my period?
The first day. The NHS calculator asks for the first day of your last period, and that is the date doctors count from.
What if my periods are irregular?
Tell your doctor. The NHS calculator lets you adjust for a cycle longer or shorter than 28 days, and an early scan will date the pregnancy more accurately than the period date.
Why am I four weeks pregnant if I conceived two weeks ago?
Because the count starts at your LMP, about two weeks before you ovulated. Cleveland Clinic says that by the time you test positive, two weeks after ovulation, you are already counted as four weeks pregnant.
Related terms
- EDD (Estimated Due Date): The date your baby is expected: 40 weeks from the first day of your last period, checked by a dating scan.
- Gestational Age: How far along your pregnancy is, in weeks and days, counted from the first day of your last period.
- Trimester: One of the three stages of pregnancy, each about three months long.
- Antenatal (also Prenatal): Antenatal means before birth. Antenatal care is the check-ups, scans and tests you get while you are pregnant.
- HealthXchange (KKH Obstetrics and Gynaecology) - Pregnancy week 1: here's how it's calculated (read 9 November 2026)
- NHS - Pregnancy due date calculator (read 9 November 2026)
- NHS - Antenatal (pregnancy) care and appointments (read 9 November 2026)
- Cleveland Clinic - Pregnancy: gestation, trimesters and what to expect (read 9 November 2026)
