Three different methods can date a pregnancy — the last period, an ultrasound, or an embryo transfer — and they do not always agree. This guide works each method in full, shows the ACOG thresholds that decide which date wins when they disagree, and explains why the IVF date, alone of the three, is never revised.

First, the anchor everything hangs on: what LMP means

Ask what does LMP mean and the short answer is “last menstrual period” — but the precision matters: in pregnancy dating, LMP always means the first day of the last menstrual period. That single day is the standard starting line of obstetric counting. Gestational age is counted from it, the due date falls 280 days after it, and it is used even though conception actually happens about two weeks later in a typical cycle, because it is a date people can know, while the conception day usually has to be inferred.

An LMP pregnancy, then, is simply a pregnancy dated from that anchor — and an LMP pregnancy calculator is any tool that takes the first day of the last period and applies the standard intervals to it. Two of those intervals do nearly all the work: 280 days from LMP to the estimated due date, and about 14 days from LMP to conception in a 28-day cycle. Both intervals are conventions built on a standard cycle, which is exactly where the method’s limits come from, as the worked examples below show.

Method 1 — Naegele’s rule: how to calculate your due date from a period

The oldest method is still the default. Naegele’s rule takes the first day of the last period, adds 7 days, subtracts 3 months and adds 1 year — which lands on the same date as simply adding 280 days. So how to calculate your due date with it, worked once in full: a last period of January 1, 2026, plus 280 days, gives an estimated due date of October 8, 2026. How to calculate pregnancy due date figures by any other route is a variation on this line — every method below either feeds a different anchor into the same 280-day frame or checks the result against it.

The rule is named for Franz Karl Naegele, the German obstetrician who popularised it in the early nineteenth century, though the underlying count — ten lunar months, 280 days, from the last period — is older still. It has survived two centuries of obstetrics for a practical reason: for a certain LMP and a standard cycle it is accurate to within days, and its errors are systematic and understood, which is more than can be said for most rules of thumb that old.

The rule prices in a 28-day cycle with ovulation on day 14. When your cycle is regularly a different length, the honest adjustment is mechanical: ovulation falls about 14 days before the next period, so the conception point — and with it the due date — shifts by cycle length minus 28. A regular 32-day cycle with the same January 1, 2026 period puts ovulation around January 19 and the adjusted due date at October 12, 2026, four days later than the unadjusted rule. A regular 24-day cycle pulls it about four days earlier. Notice the scale: cycle length bends an LMP date by days, never weeks — and when cycles are irregular rather than simply long or short, no adjustment exists, which is precisely the case the second method exists for.

Method 2 — ultrasound dating, and when it overrules the calendar

An early ultrasound measures the embryo or fetus directly — crown–rump length in the first trimester — and converts size to age. ACOG’s Committee Opinion 700, Methods for Estimating the Due Date (2017, reaffirmed 2025), states that ultrasound measurement up to 13 weeks 6 days is the most accurate way to establish or confirm gestational age, because embryos grow at a near-identical rate that early. But a scan does not move the due date merely by disagreeing with it. The date is revised only when the disagreement exceeds a threshold set for that stage of pregnancy:

ACOG Committee Opinion 700, Table 1 (2017, reaffirmed 2025). Inside the threshold, the LMP date stands; beyond it, the ultrasound date replaces it.

One worked case shows how the thresholds bite. Last period January 1, 2026; a scan on March 5, 2026 is at 9 weeks 0 days by the period. If the crown–rump length measures 8 weeks 2 days, the disagreement is 5 days — inside the 7-day threshold for that stage, so the October 8, 2026 date stands. If it measures 7 weeks 5 days, the disagreement is 9 days — beyond the threshold — and the due date is redated from the scan: March 5 plus the 226 days remaining of a 280-day count gives October 17, 2026. Two further ACOG points complete the picture: once a date is set by a good early scan it should not drift with later scans (third-trimester ultrasound is the least reliable dating tool, accurate only to about ±21–30 days, and later scans measure growth, not dates); and a pregnancy with no ultrasound confirming or revising the date before 22 weeks is classed as suboptimally dated.

Method 3 — IVF dates: the most exact of the three

An IVF pregnancy removes the inference altogether, because the fertilisation day is a recorded fact rather than an estimate. The due date is built from the transfer: transfer date + 266 days − the embryo’s age in days. A day-5 blastocyst transferred on January 20, 2026 gives January 20 + 261 days = October 8, 2026; a day-3 embryo transferred the same day gives +263 days = October 10, 2026. ACOG’s guidance is that this ART-derived date is used as assigned and is not revised by ultrasound measurements later — the one method whose answer a scan cannot overrule, and the reason IVF dates are treated as the most exact of the three.

From the date to the count: calculate weeks pregnant

Whichever method set the date, the week count runs the same way. To calculate weeks pregnant from a last period, count whole weeks from that first day and keep the leftover days: February 12, 2026 is exactly 6 weeks from a January 1 period. To calculate how far along I am from a due date instead, subtract what remains from 40 weeks: with a due date of October 8, 2026, August 20, 2026 has 49 days — 7 weeks — to run, so the pregnancy stands at 33 weeks. And if the date was redated by a scan, the count follows the new date, not the old one; that, in the end, is what redating means in practice — every “how far along” answer downstream moves with it.

The three methods at a glance

The comparison the whole guide builds towards fits in four lines:

The three dating methods side by side. All three describe the same 40-week frame; they differ in what is treated as known.

Which date wins — and what none of them can promise

The hierarchy the methods form is simple: an IVF date stands as assigned; an early ultrasound date stands once it has crossed the redating threshold; an LMP date stands when it is certain and the scan agrees within the margin. Once settled, the date is meant to stay settled: ACOG’s recommendation is that the gestational age and due date are determined as soon as the LMP and first accurate ultrasound data exist, discussed with the patient and documented clearly — with later changes reserved for rare circumstances — and that the resulting “best obstetric estimate” is the date recorded for care and on the birth certificate. What none of them can do is turn an estimate into a schedule: only about 5% of babies are born on the estimated due date by any method, and the date’s real job is to anchor care — scans, tests and leave — around the right weeks. The EDD Calculator runs all three methods from whichever single date you know.