Three ways to date a pregnancy, and which to trust
How the last period, a dating scan and a known conception date compare in accuracy, and what happens when they disagree.
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In short
- A first-trimester scan dates a pregnancy to within about five days. The last menstrual period is accurate to within a week or two at best.
- Where a scan and the LMP differ by more than five to seven days in the first trimester, clinical practice is to redate to the scan.
- IVF gives the most accurate date of all, because the conception moment is known: 261 days from a day-5 transfer.
- Accuracy falls sharply after the first trimester. A scan at 30 weeks dates to within about three weeks.
- The date matters clinically because it decides when screening happens and when a pregnancy is considered post-term.
Three methods, three levels of accuracy. Knowing which one produced your date tells you how much weight it can carry.
Method one: last menstrual period
Add 280 days to the first day of the last period, adjusted for cycle length.
due date = LMP + 280 days + (cycle length − 28) — Naegele's rule assumes a 28-day cycle, and the adjustment is what a longer or shorter one costs.
conception = LMP + 14 days + (cycle length − 28).
The other three methods convert to an LMP first and then use the same rule: a conception date is date − 14 days, an IVF transfer is date − (14 + embryo day), and a scan is date − gestational age in days.
Accuracy: roughly ±1 to 2 weeks.
It rests on knowing the date of the last period, on a regular cycle, and on ovulation falling where the cycle length predicts. Studies of women with certain LMP dates and known ovulation find that the LMP method misdates by more than a week in a substantial minority.
It is nonetheless the starting point for almost every pregnancy, because it needs no equipment and is available from the first day.
Method two: the dating scan
Ultrasound in the first trimester measures crown-rump length, the distance from the top of the head to the bottom of the spine.
Accuracy: about ±5 days between 8 and 13 weeks.
It works because early embryonic growth is remarkably uniform. Variation between pregnancies at that stage is small, so size maps to age tightly.
That precision does not last. As the pregnancy progresses, genetics, nutrition and placental function make babies of the same age differ in size:
| Timing of scan | Accuracy |
|---|---|
| 8–13 weeks | ±5 days |
| 14–20 weeks | ±7–10 days |
| 20–30 weeks | ±2 weeks |
| After 30 weeks | ±3 weeks |
A late scan showing a small baby is far more likely to be showing a small baby than a younger one, which is why late scans are used to assess growth rather than to date.
Method three: known conception
Where the moment of conception is known, the uncertainty about it disappears.
| Situation | Calculation |
|---|---|
| IVF, day-5 blastocyst transfer | Transfer date + 261 days |
| IVF, day-3 embryo transfer | Transfer date + 263 days |
| Known ovulation or insemination | Conception date + 266 days |
Accuracy: ±1 to 2 days for IVF. This is the most precise dating available and it is not revised by a scan, because nothing a scan can measure is more certain than the transfer date.
When they disagree
The standard rule is to redate to the scan if it differs from the LMP estimate by more than five to seven days in the first trimester.
The reasoning follows the accuracy figures. In the first trimester the scan is the more precise measurement, so where the two conflict, the scan is more likely to be right.
After the first trimester the position reverses. A discrepancy at 24 weeks between an established date and a growth measurement is usually telling you about the baby's growth, not about the dates, and the original date stands while the growth is investigated.
Once a due date is established, it is not changed by later scans. Repeated redating would make it impossible to detect a baby that is growing poorly, since the measurement would simply be reinterpreted as a younger pregnancy each time.
Why the date matters clinically
It is not administrative. A number of decisions depend on it.
Screening windows are narrow. The nuchal translucency scan and first-trimester combined screening are valid between 11 weeks and 13 weeks 6 days. Outside that window, the results are not interpretable and the option is lost.
The anomaly scan at 18 to 22 weeks depends on the fetus being developed enough for structures to be visible and small enough to be seen whole.
Preterm management. Whether a baby is 33 or 35 weeks changes decisions about steroids for lung maturation and about where the delivery should happen.
Post-term management. Induction is normally offered between 41 and 42 weeks, because stillbirth risk rises beyond that. A date that is a week out means induction a week early or a week late, and both carry consequences.
Growth assessment. A baby measuring small can only be identified as small if the age is right.
What to expect at each stage
For a due date of 8 October 2026, from an LMP of 1 January with a 28-day cycle:
| Milestone | Date |
|---|---|
| End of first trimester, 12 weeks | 26 March 2026 |
| Anomaly scan window, 20 weeks | 21 May 2026 |
| Viability, 24 weeks | 18 June 2026 |
| Third trimester, 28 weeks | 16 July 2026 |
| Full term, 37 weeks | 17 September 2026 |
| Due date, 40 weeks | 8 October 2026 |
| Post-term, 42 weeks | 22 October 2026 |
The gap between full term and post-term is five weeks. Almost every birth happens inside it, and none of those five weeks is late.
Special cases
Twins and higher multiples are dated the same way and delivered earlier — the average for twins is around 36 weeks, and 37 to 38 weeks is generally considered term for them. The due date is calculated normally and the expectation around it is different.
Irregular cycles make LMP dating unreliable and raise the value of an early scan considerably. Where cycles vary by more than a week, the scan should be treated as the primary date from the outset.
Conception while on hormonal contraception, or shortly after stopping it, disrupts the cycle assumptions entirely. An early scan is the answer.
An uncertain LMP date — which is common — is a reason to say so rather than to guess. A clinician told the date is uncertain will book an early scan; one given a confident wrong date may not.
Estimated date of delivery, and the words around it
The date goes by several names, and they mean the same thing.
EDD — estimated date of delivery. EDC — estimated date of confinement, the older term. Due date, in ordinary speech.
The word doing the work in all three is estimated, and it is the word most frequently dropped.
Clinicians increasingly describe it as a 40-week point in a term window running from 37 to 42 weeks, because that framing matches the evidence and sets a more realistic expectation. A pregnancy at 41 weeks is not late; it is within the normal range, and about a fifth of pregnancies are there.
Gestational age notation
Antenatal records write gestational age as weeks + days: 32+4 is thirty-two weeks and four days.
The week you are "in" is one ahead of the week completed, which is a routine source of confusion. At 32+4 you have completed 32 weeks and are in your thirty-third. Someone saying "I'm 32 weeks" has completed 32; someone saying "I'm in my 33rd week" means the same thing.
The corrected age convention continues after birth for premature babies. A baby born at 32 weeks is eight weeks premature, and their developmental milestones are assessed against their corrected age — eight weeks behind their chronological age — until about two years, by which point the difference has usually washed out.
When the dates are genuinely unknown
Some pregnancies present with no usable LMP: irregular cycles, conception on hormonal contraception, breastfeeding amenorrhoea, or simply no record.
The answer is an early scan, and the earlier the better. A scan at 8 weeks dates to within five days; the same scan at 20 weeks dates to within ten days, and at 30 weeks to within three weeks.
Where a pregnancy first presents late, dating is done from the best available combination of measurements — head circumference, femur length, abdominal circumference — and the resulting date carries a wide margin that is documented alongside it. Clinical decisions then account for that uncertainty rather than pretending it away, which is why an estimate labelled uncertain is more useful than a confident wrong one.
What this calculator assumes
- 280 days from the last menstrual period, adjusted for cycle length, unless you supply a scan or conception date.
- Scan-based dating uses the gestational age measured at the scan, projected forward, which is the clinically standard approach.
- IVF dating uses 261 days from a day-5 transfer and 263 from a day-3 transfer.
- A single pregnancy.
- This is an estimate. Where it differs from what your clinician has recorded, theirs is the operative date.