LMP Meaning in Pregnancy
LMP stands for last menstrual period. In pregnancy it means the first day of your last period, and it is the date that dating usually starts from. This guide explains what the abbreviation means, why that date is used, and what happens when it is not known.
What Does LMP Stand For?
LMP is short for last menstrual period. It means the first day of bleeding in your most recent period, not the last day of it. A period is the part of the menstrual cycle when bleeding happens for a few days, usually around every 28 days, ranging from every 21 days to every 35 days.
The menstrual cycle is the time from the first day of a period to the day before the next period. Regular cycles that are longer or shorter than 28 days, from 21 to 35 days, are normal.
Sources: NHS, Periods, https://www.nhs.uk/conditions/periods/; NHS, Periods and fertility in the menstrual cycle, https://www.nhs.uk/conditions/periods/fertility-in-the-menstrual-cycle/
Why LMP Is Used to Date a Pregnancy
Pregnancy is counted from the first day of your last period rather than from the day of conception. This is a convention that goes back about 200 years, and it is still the standard method used by healthcare providers.
The reason is practical. Most people can name the first day of their last period. Very few can name the day conception happened, because ovulation usually passes unnoticed. Counting from a date that can be named gives a consistent starting point.
The convention creates a fixed two-week offset. The calendar starts before conception can happen:
- At the moment of conception, you are already considered 2 weeks pregnant.
- At the time of a missed period, you are considered 4 weeks pregnant.
- Your due date is 40 weeks from your LMP, which is about 38 weeks from conception.
The offset exists because the calendar starts on a date you can usually name. It does not mean the pregnancy is two weeks older than the baby.
Sources: NHS, Due date calculator, https://www.nhs.uk/pregnancy/finding-out/due-date-calculator/; NHS, Periods and fertility in the menstrual cycle, https://www.nhs.uk/conditions/periods/fertility-in-the-menstrual-cycle/
How LMP Becomes a Due Date
EDD = LMP + 280 days
This is the standard method. It assumes a 28-day cycle with ovulation on day 14. The NHS due date calculator works from the first day of your last period, and it asks for your cycle length separately so it can adjust for cycles that are not exactly 28 days.
Cycle-corrected LMP = LMP + (cycle length - 28)
For example, if your LMP was January 1 and your cycle is 32 days, the correction is 4 days, and the due date moves 4 days later. The correction matters most when cycles are longer than 28 days or vary from month to month.
Sources: NHS, Due date calculator, https://www.nhs.uk/pregnancy/finding-out/due-date-calculator/
LMP vs Conception Date vs Fetal Age
Three dates get used in pregnancy, and they differ by about two weeks:
| Measurement | Counted from | At conception | At missed period | At due date |
|---|---|---|---|---|
| Gestational age | LMP | 2 weeks | 4 weeks | 40 weeks |
| Fetal age | Conception | 0 weeks | 2 weeks | 38 weeks |
Healthcare providers use gestational age. When someone says "I am 20 weeks pregnant," they mean gestational age. Conception dating is used only when the conception date is known, which is most often the case after IVF.
Sources: NHS, Periods and fertility in the menstrual cycle, https://www.nhs.uk/conditions/periods/fertility-in-the-menstrual-cycle/; NHS, Due date calculator, https://www.nhs.uk/pregnancy/finding-out/due-date-calculator/
Why LMP Dating Is Not Always Accurate
LMP dating is a reasonable estimate, but it depends on three things being true: that you remember the date, that your cycles are regular, and that ovulation happened around day 14. If any of those is off, the date is off too.
Recall is the most common problem. The first fallacy of the LMP method is that a woman may not accurately recall the first day of her menstrual cycle.
The method also assumes a cycle of exactly 28 days with ovulation on day 14, which does not hold for cycles that are shorter or longer. It cannot separate menstrual bleeding from other bleeding in early pregnancy either.
Irregular cycles are the next problem. A woman's periods are irregular if the gap between them is less than 21 days or more than 35 days. NHS advice is that irregular periods are not usually a sign of a problem, but to speak to a GP if your periods are irregular or your normal pattern changes.
Sources: StatPearls, Estimated Date of Delivery, https://www.ncbi.nlm.nih.gov/books/NBK536986/; NHS, Irregular periods, https://www.nhs.uk/conditions/irregular-periods/
What Happens If You Cannot Remember Your LMP
You will still be dated. The midwife will ask for the date of your last period at your booking appointment, which is usually the first appointment at 8 to 12 weeks. If you do not know it, the dating moves to ultrasound.
The NHS offers a dating scan at 11 to 14 weeks. At that scan the sonographer estimates the due date from the baby's measurements, and your due date, originally calculated from the first day of your last period, may be adjusted according to those measurements.
ACOG states that the gestational age and the estimated due date should be determined and documented as soon as the LMP date, the first accurate ultrasound, or both are available, and that later changes should be reserved for rare circumstances.
Sources: NHS, Antenatal care and appointments, https://www.nhs.uk/pregnancy/your-pregnancy-care/your-antenatal-care-and-appointments/; NHS, Ultrasound scans in pregnancy, https://www.nhs.uk/pregnancy/your-pregnancy-care/ultrasound-scans/; ACOG, Methods for Estimating the Due Date (Committee Opinion 700), https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2017/05/methods-for-estimating-the-due-date
LMP and the Two-Week Offset
At the moment of conception, you are already considered 2 weeks pregnant. At the time of a missed period, you are considered 4 weeks pregnant. Your due date is 40 weeks from your LMP, which is about 38 weeks from conception.
The first two weeks of a dated pregnancy are weeks you were not pregnant. It is a counting convention, not a claim about the baby. The day of conception usually cannot be named, and the first day of a period usually can.
Sources: NHS, Periods and fertility in the menstrual cycle, https://www.nhs.uk/conditions/periods/fertility-in-the-menstrual-cycle/; NHS, Due date calculator, https://www.nhs.uk/pregnancy/finding-out/due-date-calculator/
LMP and Ovulation
Ovulation is the release of an egg, and it is the other half of the cycle. You are more likely to get pregnant around the time you ovulate, which is usually about 14 days before the start of your next period.
That timing depends on how long your cycle is. In a 28-day cycle, ovulation falls around day 14, which is the assumption the LMP calculation makes. In a longer or shorter cycle, ovulation happens later or earlier, and the LMP-based due date needs the cycle correction to catch up.
Sources: NHS, Trying for a baby, https://www.nhs.uk/pregnancy/trying-for-a-baby/trying-to-get-pregnant/; NHS, Due date calculator, https://www.nhs.uk/pregnancy/finding-out/due-date-calculator/
LMP and a Missed Period
A missed period is the most common early sign of pregnancy. NHS guidance lists symptoms you may notice, including a missed period and a bleed similar to a very light period, with some spotting or only a little blood, around the time your period is due. That lighter bleed is known as implantation bleeding.
If your period is late, the first day of your last period is the date you will be asked for at your first appointment, and it is the date your due date will be worked out from. If you are unsure of the date, say so. The midwife will note it and the dating will be confirmed or revised at the dating scan.
Sources: NHS, Signs and symptoms that might mean you're pregnant, https://www.nhs.uk/pregnancy/trying-for-a-baby/signs-and-symptoms-that-might-mean-youre-pregnant/; NHS, Antenatal care and appointments, https://www.nhs.uk/pregnancy/your-pregnancy-care/your-antenatal-care-and-appointments/
When to Talk to Your Clinician
Contact your healthcare provider if:
- You cannot remember the first day of your last period
- Your periods are irregular or your normal pattern has changed
- Your periods stopped and you are not sure why
- Your ultrasound dating differs from your LMP dating
- You have bleeding in early pregnancy and are unsure what it means
Heavy bleeding, stomach pain and pelvic pain are listed by the NHS as symptoms to look out for, because they may be a sign of something more serious. Contact maternity services at any time during pregnancy if you are worried about your health or your baby's health.