An estimate, not medical advice. A dating scan is more accurate than any calculation and supersedes it. If you are pregnant, your midwife or doctor is the authority on your dates, and any concern about bleeding, pain or reduced movement should be raised with them the same day rather than researched online.

A 35-day cycle moves the due date a full week later, and almost no calculator adjusts for it. Naegele’s rule assumes a 28-day cycle with ovulation on day 14 — a pattern most people do not have. This one asks, and it is honest about what a due date can and cannot tell you.

Due Date Calculator

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Calculate from

Date

Average cycle (days)

Pregnancy

The cycle length is the number of days from the first day of one period to the first day of the next. If it varies, use the average — and treat the result as approximate.

Estimated due date

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The window most births fall in

Dates along the way

A dating scan is more accurate and supersedes this. Runs entirely in your browser — nothing you enter is sent anywhere.

A due date is a middle, not an appointment

The curve is illustrative of the shape rather than a precise distribution. What is not illustrative is the spread: planning around a single date sets up disappointment in the great majority of pregnancies.

Arriving	Roughly	Note
On the exact due date	~4%	About one in twenty-five
Within a week either side	~70%	The practically useful window
Between 37 and 42 weeks	~90%	The clinical definition of term, and the honest answer
Before 37 weeks	~10%	Preterm. Risk falls sharply with each additional week
After 42 weeks	Under 1%	Post-term. Induction is usually offered before this

Why cycle length changes the answer

Naegele’s rule adds 280 days to the first day of the last period. That works only if ovulation happened on day 14, which is true of a 28-day cycle and not of most others:

Your cycle	Adjustment	Effect on the due date
21 days	−7 days	A week earlier than the standard rule gives
26 days	−2 days	Slightly earlier
28 days	None	The assumption the rule was built on
32 days	+4 days	Later
35 days	+7 days	A full week later. At 40 weeks that is a meaningful difference

The adjustment is simply cycle length minus 28. A longer cycle means ovulation happened later, so conception happened later, so the pregnancy started later than the last period implies. It is arithmetic rather than medicine, and leaving it out is why two calculators can differ by a week on the same information.

Pregnancy is dated from the last period, not from conception. That means you are counted as two weeks pregnant at the moment of conception, and the first two of the forty weeks contain no pregnancy at all. It is a convention that survives because the last period is a date people can report and conception usually is not.

Which starting point is most reliable

Dated from	Days added	Accuracy
Dating scan	Measured directly	The most accurate, and it overrides everything else. Crown-rump length between 8 and 13 weeks is precise to a few days
IVF transfer	261 or 263	Almost exact, because the date of fertilisation is known. Day 5 blastocyst adds 261; day 3 embryo adds 263
Known conception date	266	Good, if the date is genuinely known rather than assumed
Last period, adjusted	280 ± cycle	Reasonable. Depends on remembering the date and knowing the cycle
Last period, unadjusted	280	The weakest. Off by a week for anyone whose cycle is not 28 days

If you have had a dating scan, use its date and ignore this page. Ultrasound in the first trimester measures the embryo directly, and it is the reason clinics revise dates that were calculated from a period.

The milestones that have clinical meaning

Week	Called	Why it matters
Week 12–13	End of first trimester	Miscarriage risk falls substantially. Usually when the dating scan happens
Week 24	Viability	The threshold at which intensive care is generally offered. Survival improves rapidly after it
Week 28	Third trimester begins	Outcomes for babies born from here are much better than earlier
Week 37	Early term	No longer preterm. The main development left is weight and lung maturity
Week 39–40	Full term	The best outcomes cluster here
Week 41–42	Late and post term	Induction is usually offered somewhere in this window

Definitions follow the ACOG and WHO terminology for term birth. Local practice varies — the week at which induction is offered differs between countries and between hospitals, and your own care team’s guidance is the one that applies to you.

Twins and higher multiples

Multiple pregnancies almost always arrive earlier, and the standard 40-week date is not the planning figure:

Pregnancy	Typical delivery	Note
Single	39–40 weeks	The 280-day calculation
Twins	~36 weeks	Around a month earlier. Roughly 60% arrive before 37 weeks
Triplets	~32 weeks	Two months earlier. Almost all are preterm by definition

The due date does not change; the expectation does. The calculation still runs to 40 weeks because that is how gestation is measured, but a twin pregnancy reaching 40 weeks is unusual. Care for multiples is managed differently throughout, and the schedule comes from the clinic rather than from a calculator.

Common mistakes

Ignoring cycle length. The standard rule assumes 28 days. At 35 days it is a week out, and a week matters when decisions about induction are being discussed. Enter your actual average, and say it to your midwife too.

Counting from conception instead of the last period. Gestational age is measured from the last period by convention, which puts you at two weeks at conception. Entering a conception date in the period field makes the pregnancy two weeks further along than it is.

Treating the date as fixed after a scan has revised it. A first-trimester ultrasound measures the embryo directly and is more accurate than any date arithmetic. If the clinic gave you a different date, theirs is the one that counts — for appointments, for tests with a timing window, and for decisions later on.

Planning as though the date will happen. Around 4% of babies arrive on it. Booking leave to start on the due date, or telling everyone that day, means most people spend an anxious fortnight explaining. Plan for the window instead of the point.

Frequently asked questions

How is a due date calculated?

Naegele’s rule adds 280 days — 40 weeks — to the first day of the last period. That assumes a 28-day cycle with ovulation on day 14, so this page adjusts by your cycle length minus 28. A 35-day cycle moves the date a week later.

How accurate is a due date?

About 4% of babies arrive on the exact date, roughly 70% within a week of it, and around 90% between 37 and 42 weeks. It is the middle of a distribution several weeks wide rather than a prediction of a day, and treating it as the latter causes a lot of unnecessary worry.

Why am I two weeks pregnant at conception?

Because gestational age is counted from the first day of the last period, not from conception. The convention exists because the period is a date people can report and conception usually is not. It means the first two of the forty weeks contain no pregnancy.

Which is more accurate, a scan or my dates?

The scan, by a wide margin, if it was in the first trimester. Measuring the embryo directly between 8 and 13 weeks is accurate to within a few days, which is why clinics revise dates calculated from a period. If you have a scan date, use it.

How does IVF change the calculation?

It makes it more precise, because the date of fertilisation is known rather than estimated. A day 5 blastocyst transfer adds 261 days to the transfer date; a day 3 embryo adds 263. Both options are in the dropdown above.

Will twins arrive on the due date?

Usually not. Twins typically arrive around 36 weeks and triplets around 32, so the 40-week date is calculated the same way but is not the realistic expectation. Care for multiples is managed differently throughout, and your clinic will set the schedule.

What if my cycle is irregular?

Use your average and treat the result as approximate. Irregular cycles are exactly the case where dating from a period is least reliable, and where an early scan is most worth having. Mention the irregularity when you book — it changes how much weight the clinic gives your dates.

Is anything I enter sent anywhere?

No. Everything runs in your browser with no server request, and works offline once the page has loaded. Nothing is written to disk and nothing persists after you close the tab — which matters more than usual for a page like this one.

Sources

American College of Obstetricians and Gynecologists. Committee Opinion 700, “Methods for Estimating the Due Date.” Establishes that first-trimester ultrasound is the most accurate dating method and should supersede menstrual dating where they differ.
ACOG and Society for Maternal-Fetal Medicine. The term birth definitions used above: early term 37–38 weeks, full term 39–40, late term 41, post term 42 and beyond.
World Health Organization, preterm birth. The definitions of preterm categories and the viability threshold context.
UK National Health Service, due date guidance. The dating scan window and how NHS practice handles revised dates.
Naegele F. The rule, published in 1812, adding 280 days to the first day of the last menstrual period. The cycle-length adjustment used here is the standard modification.
Formula. Due date is LMP + 280 days + (cycle − 28). From conception it is +266; from a day 5 IVF transfer +261; from a day 3 transfer +263.

Every date on this page is computed from these formulas by walking the calendar rather than dividing elapsed days, so leap years are handled correctly. Nobody involved in producing this page is a clinician — the figures are taken from the sources above, and your own care team is the authority on your pregnancy.

Related calculators

See the full list of Health calculators, or try:

Ovulation Calculator — the fertile window
Conception Date Calculator — working backwards
Pregnancy Weight Gain — the recommended ranges
Age Calculator — including corrected age for preterm babies
Date Difference Calculator — weeks and days between any two dates
BMI Calculator — and why it is read differently in pregnancy

Due Date Calculator Adjusted for Your Cycle Length