LMP vs Conception Date: Which Is More Accurate?
When calculating a pregnancy due date, you have two starting points: your last menstrual period (LMP) or your conception date. Both are valid approaches, but they suit different situations and can produce different results. This guide explains how each method works, when each is more reliable, and what happens when they disagree.
What Is LMP Dating and Why Is It the Standard?
The last menstrual period method is the default in obstetric care worldwide. It uses the first day of your most recent period as the reference point, then adds 280 days (40 weeks) to calculate your estimated due date (EDD).
This method became standard for several practical reasons:
- Most women can recall the first day of their last period more reliably than the exact moment of conception.
- It provides a consistent, standardised baseline that is applied uniformly across all pregnancies.
- The 280-day calculation (Naegele's Rule) is simple and requires only one piece of information.
- It has been used in clinical practice since the 19th century and is embedded in all major obstetric guidelines, including those from the NHS and the American College of Obstetricians and Gynecologists (ACOG).
The limitation is that LMP dating assumes ovulation occurs precisely on day 14 of a 28-day cycle. For women with irregular cycles, polycystic ovary syndrome (PCOS), or cycles that are significantly shorter or longer than 28 days, the estimated due date may be off by several days or even a week or more.
What Is Conception Date Dating?
The conception date method uses the day of fertilisation — when a sperm successfully meets an egg — as the starting point. From there, 266 days (38 weeks) are added to estimate the due date.
This method is particularly useful in these situations:
- Women who track ovulation precisely, using ovulation predictor kits, basal body temperature monitoring, or cervical mucus observation.
- Women who conceived through intrauterine insemination (IUI), where the insemination date is known.
- Women who conceived through IVF, where the egg retrieval or embryo transfer date is documented.
- Women who had intercourse on a known single occasion and want a direct estimate.
Because it starts from the actual biological event rather than a menstrual marker, this method can be more precise for women whose cycles do not conform to the 28-day model.
The 14-Day Offset Explained
In a textbook 28-day cycle, ovulation occurs on day 14. This means conception happens approximately 14 days after the LMP. As a result:
- LMP + 280 days
- Conception date (which equals LMP + 14 days) + 266 days
Both arrive at exactly the same estimated due date. The 14-day offset is built into the LMP formula — it is not a coincidence but a deliberate part of how Naegele's Rule was constructed.
The two methods only diverge when the actual cycle length (and therefore ovulation timing) differs from the assumed 28 days. For example:
| Cycle length | Assumed ovulation day | LMP method EDD | Conception method EDD | Difference |
|---|---|---|---|---|
| 28 days | Day 14 | Same | Same | 0 days |
| 35 days | Day 21 | Earlier | ~7 days later | ~1 week |
| 21 days | Day 7 | Later | ~7 days earlier | ~1 week |
A good pregnancy calculator — including ours — allows you to enter your cycle length and adjusts the estimated ovulation and conception window accordingly.
When IVF Makes Conception Date More Accurate
In natural conception, even if you know when you had intercourse, conception could have occurred anywhere from the same day up to five days later (the maximum viable lifespan of sperm). This uncertainty adds a few days of imprecision to the conception date method.
With IVF, this uncertainty disappears entirely. The egg retrieval date and fertilisation date are recorded precisely by the clinic. For a fresh embryo transfer, the embryo age at transfer is also known. This means the gestational age can be calculated with a precision that no natural conception — and no LMP-based method — can match.
ACOG guidance states that for IVF pregnancies, the ART-based (assisted reproductive technology) due date takes precedence over both the LMP and ultrasound dating. Your fertility clinic will typically provide your obstetric team with the dates required.
Worked Examples Comparing Both Methods
Example 1: Regular 28-day cycle
- LMP: 1 January 2025
- Assumed ovulation: 15 January 2025 (day 14)
- LMP method EDD: 8 October 2025 (1 Jan + 280 days)
- Conception date method EDD (using 15 Jan): 7 October 2025 (15 Jan + 266 days)
- Difference: 1 day (negligible, within rounding)
Example 2: Long 35-day cycle
- LMP: 1 January 2025
- Ovulation (tracked): 22 January 2025 (day 21)
- LMP method EDD: 8 October 2025 (unchanged — LMP method does not know about cycle length unless adjusted)
- Conception date method EDD: 14 October 2025 (22 Jan + 266 days)
- Difference: 6 days — clinically meaningful; the scan will clarify
Example 3: IVF transfer
- Egg retrieval: 5 February 2025
- Embryo transfer (Day 5 blastocyst): 10 February 2025
- Calculated due date: 10 February + 261 days = 28 October 2025
- This date takes precedence over any LMP or scan estimate
Which Method Will Your Doctor Use?
In clinical practice, your due date goes through up to three stages:
- Initial estimate: Based on LMP (with cycle-length adjustment if provided). This is what an online calculator gives you and what your GP may record at your first appointment.
- Dating scan: Performed at 11–14 weeks. The sonographer measures the crown-rump length (CRL), which is the most accurate way to date a pregnancy at this stage. If the scan date differs from the LMP date by more than 5–7 days (guidelines vary slightly by country), the scan date becomes the official EDD.
- IVF / ART exception: If conception was through assisted reproduction, the clinic-derived date is used from the outset and the scan is used to confirm rather than revise.
Once set at the dating scan, the due date is not normally changed again in later pregnancy. Third-trimester scans assess growth and wellbeing — they do not revise gestational age.
FAQ
Is the LMP method or the conception date method more accurate? Neither is definitively more accurate for everyone. The LMP method is most useful when you have a regular cycle and a reliable LMP date. The conception date method is useful if you know when you conceived. An early ultrasound scan is more accurate than either.
What if my LMP and conception date give different due dates? A small difference of a few days is normal. If the difference is more than a week, discuss it at your first antenatal appointment. The dating scan will resolve any discrepancy.
Do conception and implantation happen on the same day? No. Fertilisation occurs within 24 hours of ovulation. Implantation happens 6–12 days later. Pregnancy is dated from conception (fertilisation), not implantation.
My cycle is 35 days long — will the LMP method give the wrong due date? Possibly by about a week. With a 35-day cycle, ovulation typically occurs around day 21 rather than day 14. Use a calculator that accepts your cycle length, or rely on the dating scan.
I had IVF — which dating method should I use? For IVF pregnancies, the conception date (based on egg retrieval or embryo transfer) is the most accurate method. Your fertility clinic will provide the dates, and your obstetric team will use these rather than your LMP.
Can I use the conception date method if I only had sex once? Yes, though sperm can survive up to five days, so conception could have occurred up to five days after intercourse. An early ultrasound will confirm gestational age precisely.
Will my doctor use my LMP date or my scan date officially? Your LMP date is used initially. After the dating scan at 11–14 weeks, the scan-based date is used if it differs from the LMP date by more than a few days. The due date is not normally revised after that.
Related Guides
- How to Calculate Your Due Date
- Why Due Dates Change During Pregnancy
- Pregnancy Due Date Calculator
- Ultrasound Dating vs LMP: Which Is More Accurate? — how scan dating compares to calendar-based methods
Sources: NHS (nhs.uk), American College of Obstetricians and Gynecologists (ACOG). This guide is for general information only and does not constitute medical advice. Always discuss your pregnancy timeline with your midwife or doctor.