LMP dating (last menstrual period dating) is the traditional way to estimate gestational age and your due date. It counts 40 weeks, or 280 days, forward from the first day of your last period. It is quick, free and needs no scan, but it rests on assumptions about your cycle that do not hold for everyone. Here is how LMP dating works, how accurate it really is, and when an ultrasound should take over.
Quick answer: your due date is about 280 days after the first day of your last period (Naegele’s rule: add 7 days, subtract 3 months). That estimate assumes a 28-day cycle with ovulation on day 14. If your cycles are longer, shorter or irregular, or you are unsure of the date, a first-trimester ultrasound is usually more reliable.
What Is LMP Dating?
LMP dating uses the first day of your last menstrual period as day one of pregnancy. This is why doctors say you are “two weeks pregnant” at the moment you conceive: gestational age is measured from your period, not from fertilization. The method is also called menstrual dating, and it produces what clinicians call the estimated due date (EDD), or “EDD by LMP.” You can learn more about the difference in our guide to gestational age vs. fetal age.
How LMP Dating Works: Naegele’s Rule
The standard formula is Naegele’s rule: take the first day of your last period, add 7 days, subtract 3 months, and add 1 year. It is simply a shortcut for adding 280 days (40 weeks). The method assumes a 28-day cycle with ovulation around day 14, which puts conception roughly 266 days before birth.
Worked example: if your last period began on January 10, 2026, adding 7 days gives January 17 and subtracting 3 months gives October 17. Your estimated due date is October 17, 2026.
Adjusting for a longer or shorter cycle
If your cycle is not 28 days, ovulation will not fall on day 14, so the due date can be adjusted. A common rule of thumb is to add or subtract the difference between your usual cycle length and 28 days:
- 35-day cycle: add 7 days to the standard due date (the example above becomes October 24, 2026).
- 24-day cycle: subtract 4 days.
- Irregular or unknown cycle length: LMP dating becomes unreliable and an ultrasound is the better guide. See our guide to irregular cycles.
The easiest way to do this is with our due date calculator, which lets you enter your cycle length, or the gestational age calculator to see how far along you are today.
How Accurate Is LMP Dating?
LMP dating is a good starting estimate, but a due date is a midpoint rather than a deadline. Even when pregnancies are dated perfectly, natural variation is large:
- Only about 4–5% of babies are born on their estimated due date.
- In a well-known study that timed ovulation precisely, the average time from ovulation to birth was 268 days (38 weeks and 2 days), and natural pregnancy length varied by as much as five weeks among healthy pregnancies (Jukic et al., Human Reproduction, 2013).
- Ovulation does not reliably happen on day 14. The follicular phase, the part of the cycle before ovulation, varies from person to person and from cycle to cycle, which shifts the real conception date even when your LMP is known exactly.
Because of this, major guidelines treat a first-trimester ultrasound as more accurate than LMP alone when the two disagree meaningfully.
Limitations of LMP Dating
LMP dating can be off by a week or more when any of these apply:
- Irregular cycles: if you do not ovulate at a predictable time, counting from your period tells you little about when you conceived.
- Late or early ovulation: a cycle longer or shorter than 28 days moves conception away from day 14.
- An uncertain LMP date: many people cannot recall the exact first day of their last period. Light spotting at the start of pregnancy is easy to mistake for a period.
- Recent hormonal birth control or breastfeeding: cycles may not have regulated yet, so the “last period” is a poor marker of ovulation.
- Assisted conception: with IVF, the transfer or retrieval date gives a far more precise starting point. Use our IVF due date calculator instead.
LMP vs. Ultrasound Dating: When the Due Date Changes
A dating ultrasound measures the embryo or fetus and compares it with growth standards. In the first trimester, the crown-rump length (CRL) is the most accurate measurement available, because embryos grow at a very consistent rate early on. The American College of Obstetricians and Gynecologists (ACOG) recommends changing the due date to the ultrasound estimate only when the gap with the LMP estimate is larger than the thresholds below (ACOG Committee Opinion 700).
| Gestational age at ultrasound | Measurement used | Due date changes if the difference exceeds |
|---|---|---|
| Up to 8 6/7 weeks | Crown-rump length | More than 5 days |
| 9 0/7 to 13 6/7 weeks | Crown-rump length | More than 7 days |
| 14 0/7 to 15 6/7 weeks | Head, abdomen and femur measurements | More than 7 days |
| 16 0/7 to 21 6/7 weeks | Head, abdomen and femur measurements | More than 10 days |
| 22 0/7 to 27 6/7 weeks | Head, abdomen and femur measurements | More than 14 days |
| 28 0/7 weeks and beyond | Head, abdomen and femur measurements | More than 21 days |
Once a due date has been confirmed by a first-trimester scan, it generally should not be changed by later ultrasounds. Babies naturally vary in size, and ACOG warns that re-dating a small baby in the third trimester can hide a genuine growth problem. ACOG also notes that a pregnancy without an ultrasound confirming or revising the due date before 22 weeks is considered “suboptimally dated.”
If your scan date and LMP date differ, that does not mean anything is wrong. Read more in when and why gestational age gets adjusted and how doctors calculate gestational age.
What If You Are Not Sure of Your Last Period?
If you cannot remember the date, or your cycles are irregular, do not guess. Tell your provider exactly that. They will usually schedule an early ultrasound and base your due date on it. If you tracked ovulation, you can also work from the likely conception date instead: our conception calculator and ovulation calculator can help, and LMP vs. conception date explains how the two methods compare.
Tips for Getting the Most Accurate Due Date
- Note the first day of full flow (not spotting) of each period in a calendar or tracking app.
- Record your usual cycle length so the due date can be adjusted correctly.
- Mention any ovulation tests, basal temperature charts or known conception dates at your first appointment.
- Ask about a dating scan in the first trimester, ideally before 14 weeks, if your dates are uncertain.
- Treat your due date as a window: delivery between 37 and 42 weeks is common and normal.
Frequently Asked Questions About LMP Dating
Is LMP dating accurate?
It is accurate enough as a first estimate when your cycles are regular and you know your last period date. It is less accurate with irregular cycles or an uncertain date, and a first-trimester ultrasound is considered the more precise method.
Why do doctors count pregnancy from my last period instead of conception?
Most people do not know the exact day they conceived, but many know when their last period started. Counting from the LMP gives a consistent, easy-to-find starting point, which is why pregnancy is measured as 40 weeks even though the baby develops for about 38 weeks after conception.
What if my ultrasound due date is different from my LMP due date?
Small differences are normal and your provider will usually keep the LMP date. If the gap is larger than the ACOG thresholds above for your stage of pregnancy, the due date is typically moved to the ultrasound date.
Does LMP dating work if my cycle is longer than 28 days?
Not without adjustment. With a longer cycle, ovulation happens later, so the due date should be pushed back by the number of extra days. Our due date calculator does this automatically.
Can my due date change after it is confirmed?
It can, but changes are reserved for uncommon circumstances and should be discussed with you and documented. Once confirmed by a first-trimester scan, the date is normally kept.
Key Takeaways
- LMP dating counts 280 days from the first day of your last period using Naegele’s rule.
- It assumes a 28-day cycle with ovulation on day 14, so it is least reliable with irregular or longer cycles.
- Only about 4–5% of babies arrive on their due date, so treat it as an estimate.
- A first-trimester ultrasound can replace the LMP date when the difference exceeds ACOG thresholds (more than 5 days up to 8 6/7 weeks, more than 7 days from 9 0/7 to 13 6/7 weeks).
This page is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always consult your OB-GYN or midwife about your own pregnancy.
Leave a Reply