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 methods, but they suit different situations. This guide explains how each works and when to use them.
The LMP Method: Standard Practice
The last menstrual period method is the default in obstetric care worldwide. It is used because:
- Most women can recall the first day of their last period more reliably than the exact date of conception.
- It provides a consistent, standardised baseline for all pregnancies.
- The formula — LMP + 280 days — is simple and requires only one piece of information.
The limitation is that it assumes ovulation on day 14 of a 28-day cycle. For women with irregular cycles, longer cycles (common in polycystic ovary syndrome, for example) or cycles significantly shorter than 28 days, the estimated due date may be off by days or even a week.
The Conception Date Method: When You Know More
The conception date method is particularly useful for:
- Women who track ovulation (using ovulation test kits, basal body temperature or cervical mucus monitoring)
- Women who conceived through intrauterine insemination (IUI) or in vitro fertilisation (IVF), where the date of fertilisation is known precisely
- Women who had intercourse on a known single occasion and want a more direct estimate
The formula — conception date + 266 days — assumes the same 38-week fetal development period regardless of cycle length. This can be more accurate than the LMP method for women whose cycles differ significantly from 28 days.
Where the Two Methods Converge
In a woman with a regular 28-day cycle, both methods typically produce the same or very similar due dates. This is because conception in a 28-day cycle occurs approximately 14 days after the LMP, and:
- LMP + 280 days
- Conception date (LMP + 14 days) + 266 days
Both arrive at the same point.
When the Two Methods Diverge
If your cycle is 35 days long, ovulation likely occurs around day 21. This means:
- LMP method: LMP + 280 days (assumes ovulation at day 14 — slightly early for your cycle)
- Conception date method: If you know ovulation was at day 21, conception date + 266 days gives a result approximately 7 days later than the LMP method
A calculator that allows you to enter your cycle length (as ours does) adjusts the estimated conception window accordingly, even if the base 280-day calculation remains the standard reference.
The Gold Standard: Early Ultrasound
Both methods are estimates. The most accurate way to establish a due date is an ultrasound scan in the first trimester. At 11–14 weeks, the crown-rump length (CRL) measurement is highly consistent between babies and can date the pregnancy to within a few days. This is why early ultrasounds are part of standard antenatal care.
If your scan-based date and your LMP or conception-based date differ significantly, your healthcare provider will discuss which date to use as your official estimated due date.
This guide is for general information only. Always discuss your pregnancy timeline with your midwife or doctor.