LMP meaning in pregnancy (plus EDD, EDC and other dating terms)

LMP stands for last menstrual period — and in pregnancy it always means the first day of that period, the day the bleeding started. It’s the single most important date in early pregnancy care, because almost every other date on your notes is worked out from it: your due date, how many weeks pregnant you are, and when your scans and screening tests should happen.

What “LMP” means on a form

When a clinic form asks for your LMP or date of LMP, it wants the calendar date your last period began. Not the last day of bleeding, and not the day you think you conceived. Light spotting that’s different from your usual period usually doesn’t count; if you’re unsure which day to give, say so — forms often have a box for LMP certain or LMP uncertain, and that matters for how much weight your clinician gives it. You may also see LNMP (last normal menstrual period), which means the same thing with an emphasis on a typical period.

Why pregnancy is counted from the LMP

The exact day of conception is rarely known. The first day of the last period usually is — people tend to remember it, and many track it. So obstetrics counts every pregnancy from that point. That has a surprising consequence: in a typical 28-day cycle you ovulate around day 14, which means you are already “two weeks pregnant” on the day you conceive, and “week 1” of pregnancy happens before the pregnancy has begun. This count is your gestational age; counting from conception instead gives the fetal age, about two weeks less. The difference is explained in gestational age vs. fetal age.

How the LMP becomes your due date (EDD)

The standard rule is to add 280 days (40 weeks) to the first day of the LMP. This is Naegele’s rule, and the result is your EDD — estimated date of delivery. If your period started on 1 January, your EDD is 8 October. Because the rule assumes a 28-day cycle, many calculators adjust it for your actual cycle length: a 32-day cycle means you probably ovulated about four days later, so the EDD moves to 12 October. You can do this instantly in the due date calculator, and the full rule is covered in Naegele’s rule and your due date.

When the LMP isn’t used

LMP dating is only as reliable as the date and the regularity of your cycle. It’s less dependable if you don’t remember the date, have irregular cycles, recently stopped hormonal contraception, were breastfeeding, or had bleeding in early pregnancy. That’s why an early ultrasound is used to confirm or correct it. Under ACOG guidance, if a first-trimester scan disagrees with the LMP by more than 5 days before 9 weeks, or by more than 7 days between 9 and 13 weeks 6 days, the due date is changed to match the scan. Once set this way, the due date normally isn’t changed again. The scan measurement used is the crown-rump length; you can convert one with the CRL calculator.

EDD, EDC and the other dating abbreviations

Pregnancy notes are full of shorthand. These are the ones you’re most likely to meet around dating:

Reading “12+3” and other GA notation

Gestational age is usually written as completed weeks plus days: 12+3 or 12w3d means 12 weeks and 3 days since the LMP. You are then in your 13th week. If you know your LMP or EDD, the gestational age calculator gives you today’s figure, and the OB wheel lists every milestone date from it.

The bottom line

LMP is simply the first day of your last period, and it anchors the whole pregnancy timeline. Your EDD (or EDC, or EDB — they’re the same date) is 40 weeks on from it, adjusted for your cycle, and an early ultrasound may fine-tune it. If a date on your notes doesn’t match what you expected, ask your midwife or doctor which method they used; a few days’ difference between methods is normal.