Dating a pregnancy from what you actually know
1. Enter the first day of your last period
The first day of bleeding, not the last, and not the day you think you conceived. This convention is universal in obstetrics, and it is why a pregnancy is counted as two weeks along before conception has happened. If you are unsure of the date, say so at your booking appointment rather than guessing here — an uncertain LMP is exactly when a scan becomes the better source.
2. Adjust the cycle length if yours is not 28 days
The standard rule assumes a 28-day cycle and ovulation on day 14. If your cycle runs 35 days, ovulation is later and the due date moves later with it, day for day. The tool accepts 20 to 45 days and now tells you when a value has been brought inside that range, rather than silently adjusting it as the previous version did.
3. Add your scan date and measurement if you have had one
This is the most useful thing on the page. A first-trimester ultrasound is more accurate than any calculation from dates, and where it disagrees with your LMP by more than a defined margin, obstetric guidance says the scan wins. Enter the scan date and the gestational age it gave, and the tool applies the published redating rules.
4. Read the estimate as a date the tool will not overstate
An estimated due date is the midpoint of a distribution, not an appointment. The page gives the date, the current gestational age in weeks and days, and the term classification — and it stops making confident predictions once the dates entered stop making sense.
5. Check the term bands, which are not what most people assume
Thirty-seven weeks is early term, not full term. Full term begins at 39 weeks. That distinction was formalised in 2013 and it changed clinical practice, and it is still not widely known outside maternity care.
6. Use the appointment timeline as a prompt, not a schedule
The milestones shown are typical rather than universal — antenatal schedules differ between the UK, the US, India and elsewhere, and vary with your own circumstances. Your midwife or obstetrician sets your actual schedule.
Naegele’s rule, and the two weeks you were not pregnant
The standard calculation is Naegele’s rule, named after the German obstetrician Franz Naegele who popularised it in the early nineteenth century: add 280 days — forty weeks — to the first day of your last menstrual period. Adjusted for cycle length, it becomes 280 days plus (your cycle length minus 28).
The part that confuses almost everyone is that gestational age is counted from the last period rather than from conception. In a textbook 28-day cycle, ovulation occurs around day 14 and conception within roughly a day of it, so by the time an embryo exists you are already recorded as two weeks pregnant. Fetal age — time since conception — is therefore about two weeks less than gestational age throughout.
The convention persists for a practical reason: the date of your last period is something you may well know, and the date of conception almost certainly is not. It is a proxy, and it is only as good as the assumption that you ovulated on day 14 of a 28-day cycle.
How wide the distribution really is
Fewer than one in twenty babies arrive on their estimated due date. That statistic is usually offered as reassurance; the underlying research is more interesting than that.
Jukic and colleagues published a study in Human Reproduction in October 2013 that measured this properly. Working with natural conceptions where the date of ovulation was actually known from hormone measurements rather than estimated from dates, they found that the length of pregnancy varied across a span of 37 days — and that was after excluding pregnancies with complications and those that ended preterm.
Five weeks of natural variation in normal, uncomplicated pregnancies. That is the honest context for a single date. The estimate is genuinely useful for scheduling care, deciding when monitoring should begin and knowing when induction becomes a discussion. It is not a prediction of a birthday.
Most births occur between 37 and 42 weeks. Roughly half of first pregnancies go past the due date, and first-time mothers tend to deliver slightly later than those who have given birth before.
Why a scan overrules the dates you entered
Early in pregnancy, embryos of the same gestational age are remarkably similar in size, which makes crown-rump length measured by ultrasound a reliable clock. That similarity fades as pregnancy progresses and genetic and environmental differences take over, so a scan at 30 weeks tells you about this baby’s growth rather than about dating.
The American College of Obstetricians and Gynecologists set out the redating rules in Committee Opinion 700. The principle is that ultrasound replaces LMP dating when the two disagree by more than a threshold that widens as pregnancy advances, because scan accuracy declines:
- Up to 8 weeks 6 days — redate if the difference is more than 5 days
- 9 weeks 0 days to 15 weeks 6 days — more than 7 days
- 16 weeks 0 days to 21 weeks 6 days — more than 10 days
- 22 weeks 0 days to 27 weeks 6 days — more than 14 days
- 28 weeks 0 days and beyond — more than 21 days
In the UK, NICE guideline NG201 on antenatal care, published on 19 August 2021, takes the same approach: it offers a dating scan between 11 and 14 weeks and uses crown-rump length to establish the due date. Where the two disagree, the scan is used.
The practical consequence is simple and worth stating plainly: if you have had a dating scan, its date is your due date. Anything a calculator produces from your last period is superseded. That is why the scan input was added, and why the previous version of this page — which had no way to enter one — was giving many of its visitors a number their own maternity notes had already replaced.
Early, full, late and post term
Until 2013, anything from 37 weeks onward was called “term”, as though the four weeks between 37 and 41 were interchangeable. They are not, and a joint workgroup convened with the American College of Obstetricians and Gynecologists and the Eunice Kennedy Shriver National Institute of Child Health and Human Development replaced the single label with four:
- Early term — 37 weeks 0 days to 38 weeks 6 days
- Full term — 39 weeks 0 days to 40 weeks 6 days
- Late term — 41 weeks 0 days to 41 weeks 6 days
- Post term — 42 weeks 0 days and beyond
The change was made because outcomes differ measurably across those bands. Babies born at 37 or 38 weeks have higher rates of respiratory problems, feeding difficulty and admission to neonatal care than those born at 39 or 40. That evidence is the reason elective deliveries without a medical indication are generally not scheduled before 39 weeks.
It matters for how the number on this page is read. Reaching 37 weeks is a real milestone, and it is not the same as being full term.
When this tool stops answering
The previous version had no boundaries, and the results were not subtle. An LMP from a year earlier produced “week 52”, a countdown of “0 days to go” and the standard past-due wording about babies usually arriving within a week or two. Two years earlier gave week 139. A date in the future was floored to zero and reported as a pregnancy of nought weeks and nought days.
Those are not obscure edge cases — they are what a mistyped year produces, and a mistyped year is common. The tool now refuses dates that cannot describe a current pregnancy and says why, rather than performing arithmetic on them.
The date handling was also rebuilt. Subtracting two local midnights gives 23 hours rather than 24 across a spring-forward daylight-saving boundary, which rounds down to the wrong number of days — a defect that would appear once or twice a year, in a tool where being one day out is the entire point. All date differences now go through UTC-normalised calendar days, which cannot drift.
What a due date cannot tell you
It cannot tell you when labour will start, whether you will need induction, or whether your baby is growing normally. Those are clinical questions answered by examination, scans and monitoring — not by arithmetic on a calendar.
The estimate is also only as good as its inputs. It assumes a regular cycle and ovulation at a predictable point in it. If your cycles are irregular, if you conceived while using hormonal contraception or shortly after stopping it, if you were breastfeeding, or if you simply are not certain of the date, an LMP-based estimate can be out by a fortnight or more. In IVF pregnancies the dates are known precisely from the transfer and the standard rule does not apply at all.
Nothing on this page is medical advice or a substitute for antenatal care. Please contact your midwife, doctor or maternity unit straight away — regardless of what any calculator says — if you have bleeding, severe or persistent abdominal pain, severe headache with visual changes, sudden swelling of the face or hands, a fever, fluid loss, or reduced or changed fetal movements after 24 weeks. Reduced movements in particular should never be left until the next appointment.
Nothing you enter is transmitted
Your dates and scan details are used locally in your browser to compute the estimate. Nothing is sent, stored or logged; there is no account and no upload, and the page keeps working with the connection off once loaded. For something this personal, that is the only arrangement worth offering.
Questions people ask about due dates
How accurate is a due date?
As a midpoint, useful; as a prediction, weak. Fewer than one in twenty babies arrive on the date, and among natural conceptions with a known ovulation date the length of normal pregnancy varies across 37 days. Most births fall between 37 and 42 weeks.
Why does the pregnancy start two weeks before I conceived?
Because gestational age is counted from the first day of your last period, which is a date you may know, rather than from conception, which you almost certainly do not. Fetal age is about two weeks less than gestational age throughout.
My scan gave a different date from this calculator. Which is right?
The scan, in almost every case. A first-trimester ultrasound is more accurate than dating from a last period, and obstetric guidance formally redates the pregnancy when the two differ by more than a set margin. If you have had a dating scan, use its date.
Does my cycle length really change the due date?
Yes, day for day. A 35-day cycle pushes ovulation about a week later than a 28-day cycle, so the due date moves about a week later too. This is one of the more common reasons a calculator and a scan disagree.
Is 37 weeks full term?
No. Thirty-seven weeks is early term. Full term begins at 39 weeks. The distinction was introduced in 2013 because outcomes at 37 and 38 weeks are measurably different from those at 39 and 40.
What happens if I go past 40 weeks?
Very commonly nothing unusual — around half of first pregnancies do. Monitoring generally increases after 41 weeks, and induction is usually discussed somewhere between 41 and 42 weeks. Your own maternity team will set out their approach.
Can I use this after IVF?
Not reliably. IVF dating is calculated from the transfer date and the embryo’s age at transfer, which is known precisely, so your clinic’s date is exact and this calculation is not applicable.
What if I do not know my last period date?
Say so at your booking appointment. An early dating scan is the standard answer, and it is more accurate than a guessed date would be.
Do you store my dates?
No. Nothing is saved or transmitted.
Sources behind these numbers
- Jukic AM, Baird DD, Weinberg CR, McConnaughey DR, Wilcox AJ, “Length of human pregnancy and contributors to its natural variation”, Human Reproduction, October 2013, 28(10):2848–2855 (PMID 23922246) — the 37-day span of normal variation.
- American College of Obstetricians and Gynecologists — Committee Opinion 700, “Methods for Estimating the Due Date”, for the ultrasound redating thresholds, and the ACOG/NICHD term classification.
- NICE NG201 — Antenatal care, published 19 August 2021, for the UK dating-scan approach at 11 to 14 weeks.
- NHS — pregnancy, for antenatal schedules and when to seek urgent advice.
- WHO — maternal health.
Tools people open next to this one
- Period Calculator — cycle tracking assessed against the FIGO 2018 normal limits.
- Water Intake Calculator — fluid needs rise meaningfully in pregnancy.
- Sleep Calculator — for the trimester when sleep becomes its own project.