The LMP Method: The Primary Clinical Approach
The first question asked at nearly every first prenatal appointment is: “What was the first day of your last menstrual period?” The reason is straightforward: the LMP date is the most reliable starting point most people can provide for estimating gestational age, and it anchors the entire clinical calendar of prenatal care.
The LMP-based method for calculating a due date is based on Naegele’s Rule, a formula described in the early 19th century by German obstetrician Franz Karl Naegele. The original rule adds 9 months and 7 days to the LMP date. In modern form, this is equivalent to adding exactly 280 days.
The rule reflects an important convention: gestational age is counted from the LMP, not from conception. Because conception typically occurs around day 14 of a 28-day menstrual cycle — approximately two weeks after the LMP — a newly conceived embryo is already “2 weeks” by the gestational age convention. A full-term pregnancy is 40 weeks (280 days) from LMP, which corresponds to approximately 38 weeks from fertilisation.
How the Calculator Works
Step 1: Identify LMP
Enter the first day of your last menstrual period. This is typically the first day of bleeding, not any spotting that may precede the main flow.
Step 2: Enter cycle length
The default is 28 days. For cycles shorter or longer than 28 days, the calculator adjusts the EDD by the difference from the 28-day baseline.
Step 3: Calculate EDD
EDD = LMP + 280 + (cycle length − 28) days
For a 28-day cycle: EDD = LMP + 280 days. For a 30-day cycle: EDD = LMP + 282 days (+2 days, because ovulation likely occurred on day 16 rather than day 14). For a 25-day cycle: EDD = LMP + 277 days (−3 days, because ovulation likely occurred on day 11).
Worked Example: LMP January 1, 2026, 28-Day Cycle
LMP = January 1, 2026 Cycle length = 28 days → cycle adjustment = 0 days
EDD = January 1, 2026 + 280 days = October 8, 2026
Gestational age (if today is June 14, 2026): Days from LMP to today = 164 days Gestational weeks = 164 ÷ 7 = 23 weeks, 3 days
Trimester boundaries:
- End of 1st trimester: LMP + 91 days = April 2, 2026
- End of 2nd trimester: LMP + 182 days = July 2, 2026
- Start of 3rd trimester: LMP + 183 days = July 3, 2026
How Accurate Is the LMP Method?
The LMP method provides an estimate, and its accuracy depends on two factors: the reliability of the LMP date and the regularity of the menstrual cycle.
LMP date recall is frequently imprecise. Retrospective studies consistently find that 20–30% of people are uncertain about the exact date of their LMP. Dates that are off by a week or more are common, particularly if the last period was lighter or shorter than usual.
Cycle regularity is the second source of error. The cycle-length adjustment in the calculator assumes that ovulation always occurs exactly 14 days before the next period. In practice, the luteal phase (from ovulation to the next period) is relatively constant at approximately 14 days, but ovulation timing within a cycle can vary by several days. For irregular cycles, the LMP estimate may be off by a week or more.
First-trimester ultrasound is more accurate than LMP dating. Crown-rump length (CRL) measured between 8 and 14 weeks gestation can estimate gestational age to within 5–7 days. The ACOG recommends using ultrasound dating when it differs from LMP dating by more than 7 days in the first trimester. When ultrasound and LMP dates disagree significantly, most providers use the ultrasound date.
In a population with reliable LMP dates and regular cycles, the LMP method produces an EDD accurate to within one week for the majority of pregnancies. Only about 4–5% of births occur exactly on the EDD. Most occur within two weeks on either side.
Gestational Age and Trimester Boundaries
Gestational age is universally measured from the LMP date in clinical practice. The convention affects how prenatal tests are scheduled, how growth is monitored, and how preterm vs. post-term birth is defined.
First trimester (weeks 0–13, ending approximately 91 days after LMP): The embryo develops from a single cell to a fetus approximately 7 cm long. Major organ systems form during this period. First-trimester screening for chromosomal conditions (nuchal translucency ultrasound, cell-free DNA screening) is typically scheduled between 10 and 13 weeks.
Second trimester (weeks 14–26, roughly days 92–182 from LMP): Often called the “comfortable trimester,” when nausea typically subsides and the pregnancy becomes more visible. Anatomy scan ultrasound is typically scheduled around 18–20 weeks.
Third trimester (weeks 27–40): Rapid fetal growth and maturation. Group B Streptococcus (GBS) screening and labour preparation occur in this trimester.
When the LMP Date Is Unknown
If the LMP date is not known or is unreliable, ultrasound dating becomes the primary method. The earlier the ultrasound is performed during the first trimester, the more accurate it is. After 14 weeks, ultrasound-based dating becomes less precise because fetal size variation between individuals increases.
For people undergoing assisted reproduction (IUI, IVF), the date of egg retrieval or embryo transfer provides more precise gestational age information, and the LMP method is replaced by procedure-based dating.
Frequently Asked Questions
What if my period is irregular? For cycles outside the 20–45 day range, the LMP calculator’s accuracy decreases substantially. Irregular cycles mean ovulation timing is less predictable, and the EDD estimate may be off by two weeks or more. A first-trimester ultrasound is particularly important for accurate dating when cycles are irregular.
Does the cycle-length adjustment actually improve accuracy? Research on this is mixed. Some studies find that the cycle-length adjustment modestly improves EDD accuracy for people with predictably non-28-day cycles. Others find no significant improvement over the unadjusted estimate, in part because LMP date recall errors dominate. The adjustment is included because it has theoretical support from the biology of ovulation timing.
Why is gestational age measured from LMP and not from conception? The LMP date is observable and consistently recorded; the conception date is almost never known precisely. The convention of measuring from LMP predates modern understanding of the menstrual cycle and was established when Naegele’s Rule was developed. Today it persists because it is universally understood and all prenatal care calendars are built around it.
What is considered post-term? A pregnancy is considered post-term at 42 weeks from LMP (294 days, or 14 days past the EDD). Post-term pregnancies carry elevated risk of stillbirth and other complications, which is why providers typically discuss induction options starting around 41 weeks.