Pregnancy Due Date Calculator
An estimated due date from the first day of your last period, from a known conception date, or from an IVF transfer — with the cycle-length adjustment most calculators leave out. It also gives your gestational age today, the trimester dates, and the window a term birth actually falls in, which is five weeks wide.
One date, and how to read it
All three end up counting from the same day. A known conception or transfer date is more precise than a remembered period.
The first day of bleeding, not the last. This is the date a midwife will ask for.
First day of one period to first day of the next. Naegele’s rule assumes 28; every day above that pushes the due date a day later.
Forty weeks from October 23, 2025. Today that is 10 weeks and 0 days, in the first trimester. About 1 baby in 25 arrives on the estimated date — any birth between 37 and 42 weeks is a normal term birth.
- Gestational age today
- 10 weeks 0 days
- Trimester
- First
- Days until the due date
- 210
- Dating counted from
- Oct 23, 2025
- Estimated conception
- Nov 6, 2025
- Second trimester begins
- Jan 29, 2026
- Third trimester begins
- May 7, 2026
- Term window, 37w0d to 40w6d
- Jul 9, 2026 to Aug 5, 2026
- The due date is the middle of a distribution, not an appointment. Roughly half of babies arrive after it.
- A dating scan in the first trimester is more accurate than any of these methods, and it is the estimate your clinician will use.
A due date is the middle of a distribution, not an appointment
About 4% of babies are born on their estimated due date. One in twenty-five. Every other pregnancy ends on some other day, and the spread is not measured in hours — birth anywhere from 37 weeks 0 days to 41 weeks 6 days is a normal term birth, which is a window five weeks wide. Roughly half of babies arrive after the estimated date, which is why the last fortnight of a pregnancy is so often spent feeling overdue by a calculator’s reckoning while being entirely on time by the body’s.
The single date exists because clinical care needs a fixed reference point. Screening tests have windows measured in days, growth is plotted against gestational age, and decisions about induction and monitoring are made relative to a number. That number has to be one number. It was never meant to be a prediction of when labour would start, and treating it as one is the single most common misreading of everything on this page.
So: use the date to know where you are, not when you will finish. The term window in the result panel is the honest version of the answer, and the calendar dates for each trimester are what the date is actually useful for.
This is general information, not medical advice
This page does date arithmetic and explains where the arithmetic came from. It is general educational information. It is not medical advice, it is not antenatal care, and it cannot see anything about you or your pregnancy that a single date does not carry — which is nearly everything that matters.
Your midwife, obstetrician, or GP will date your pregnancy from an ultrasound, and their date is the one your care is built on. If it differs from the number here, theirs is right and this one is not. Please do not use this page to decide anything about monitoring, travel, work, or induction on your own.
If you have bleeding, severe pain, reduced fetal movements, or any symptom that worries you, contact your maternity unit now rather than reading a calculator. That advice does not change with how many weeks the number above says you are.
How each of the three methods works
All three locate the same day — the day gestational age starts counting from, which by convention is the first day of the last period even when no period was involved. Once that day is fixed, every other date on the page is a fixed number of days after it, so the three methods can never quietly disagree with each other.
LMP + 280 + (cycle − 28) • conception + 266 • transfer + 266 − embryo age- 280 days
- 40 weeks from the last period — Naegele’s rule, published in 1812
- cycle − 28
- The cycle-length adjustment: a 32-day cycle ovulates around day 18, so everything shifts four days later
- 266 days
- 38 weeks from conception — the same 280 days minus the two weeks before ovulation
- embryo age
- Days since fertilisation at transfer: 3 for a cleavage-stage embryo, 5 or 6 for a blastocyst
The trimester dates are 14 weeks 0 days and 28 weeks 0 days from that same start, and the term window is 37 weeks 0 days to 40 weeks 6 days. Every date is treated as a plain calendar date in UTC, so daylight saving never moves an answer by a day.
A last period on 1 January 2026, step by step
With a 28-day cycle, add 280 days: 1 January 2026 plus 280 days is 8 October 2026. The shortcut clinicians use gets there faster — take three months off, add a year and seven days: 1 January 2026 → 1 October 2026 → 8 October 2026. Same answer, and it is the same answer because a three-month step backwards happens to be 280 days short of a year for most starting dates.
Now change one thing. If the cycle is 32 days rather than 28, ovulation falls around day 18 instead of day 14, dating starts on 5 January, and the due date moves to 12 October 2026 — four days later. That single adjustment is the difference between this calculator and most of the others, and four days matters when a 41-week appointment is being booked against it.
The other two methods land in the same place from different starting points. Conception on 15 January 2026 is 14 days after that 1 January, so it gives 8 October again. A day-5 blastocyst transferred on 20 January was fertilised on 15 January, so it also gives 8 October; the same transfer of a day-3 embryo gives 10 October, because that embryo was made two days later.
Reading the 1 January case on 30 July 2026: that is 210 days of gestation, which is exactly 30 weeks 0 days, written 30+0 on a maternity note, and firmly in the third trimester. The second trimester began on 9 April, the third on 16 July, and the term window runs from 17 September to 14 October 2026.
What a two-hundred-year-old rule cannot know
Naegele’s rule was published in 1812, and Franz Naegele credited it to Boerhaave’s earlier work rather than claiming it himself. It has survived because it is arithmetic anyone can do on paper. It has not survived because it is accurate.
- It assumes a regularity most cycles do not have. Twenty-eight days, ovulation on day 14, every cycle the same. Real cycles vary between people and between months in the same person. The cycle-length field here fixes the first half of that problem and cannot touch the second.
- It assumes you remember the date. The first day of a period that happened weeks ago, recalled without a written record, is one of the least reliable inputs in obstetrics. Bleeding in early pregnancy is easily mistaken for a period, which shifts the estimate by a whole month.
- It assumes every pregnancy is the same length. They are not. Gestation varies naturally by more than a week between healthy pregnancies, and 280 days is a population average being applied to one person.
- An early scan beats all of it.A crown-rump length measured in the first trimester dates a pregnancy more accurately than any date you can supply, because at that stage embryos of the same age are nearly the same size. That is why ACOG’s guidance is to redate from the scan when it disagrees with the LMP estimate by more than five days before 9 weeks. After a scan has dated a pregnancy, the scan date wins permanently.
None of that makes the calculation useless. Before a first scan it is the only estimate available, and it is what tells you whether you are at 7 weeks or 11 and therefore when to book. It is a starting point that gets replaced, and knowing it will be replaced is part of using it correctly.
What this calculator assumes
- A luteal phase of 14 days — the gap from ovulation to the next period. This is the stable part of a cycle, but it does vary by a day or two between people.
- A gestation of 280 days from the first day of the last period. That is an average, not a law.
- One baby. Twins and higher multiples are usually born earlier, and the term window here does not apply to them.
- A cycle length you can name. If your cycles are irregular, the LMP method has nothing solid to stand on and an early scan is the only sensible answer.
- Trimester boundaries at 14 weeks 0 days and 28 weeks 0 days. Other sources place them a week either way, and none of them is wrong.
- Dates are plain calendar dates. The tool never applies a time zone or a time of day, so a due date does not move when you travel.
Due date FAQ
How accurate is a due date?
As a single date, not very. Only about 4% of babies are born on their estimated due date — roughly one in twenty-five. Spontaneous labour spreads across weeks rather than days, and a birth any time from 37 weeks 0 days to 41 weeks 6 days is a normal term birth. That is a five-week window, and the due date sits near the middle of it rather than at the end. Treat it as the centre of a distribution, which is what it is, and expect roughly half of babies to arrive after it.
How is a due date worked out if my cycle is not 28 days?
By moving the ovulation assumption, not the pregnancy. Naegele's rule adds 280 days to the first day of the last period, which only works if you ovulated on day 14 of a 28-day cycle. The luteal phase — from ovulation to the next period — is the stable part of a cycle at roughly 14 days whatever the total length, so a 32-day cycle ovulates nearer day 18 and everything shifts four days later. This calculator applies that adjustment; a great many do not, which is why a long-cycle due date here can differ from one elsewhere by the better part of a week.
Why is my clinic's due date different from this one?
Almost certainly because they measured the baby instead of asking you for a date. A first-trimester ultrasound measurement of the crown-rump length is the most accurate dating method there is, and ACOG's guidance is to replace an LMP-based estimate with the scan date when the two disagree by more than five days before 9 weeks 0 days, or by more than seven days between 9 weeks 0 days and 15 weeks 6 days. Once a pregnancy has been dated by a scan, that date is the one everything else is measured against, and it does not get revised again at a later scan.
How do I calculate a due date from an IVF transfer?
Count forward 266 days from fertilisation, not from transfer, and the embryo's age at transfer is the whole difference between them. A day-5 blastocyst was fertilised five days before it was put back, so the due date is transfer date plus 261 days; a day-3 embryo gives transfer plus 263. Those two dates are two days apart from the same transfer, which is why the clinic letter's wording matters. IVF is the one situation where the date of conception is known exactly, so an IVF due date is more reliable than any LMP-based estimate.
How many weeks pregnant am I?
Gestational age is counted from the first day of your last period, not from conception, so it includes about two weeks before there was a pregnancy at all. At the moment of conception you are already “two weeks pregnant” by this convention. It is not a mistake — the last period is the only event with a date anyone reliably remembers, and every chart, screening window and clinical guideline is drawn against it. Weeks are written with the leftover days after a plus sign, so 30 weeks and 4 days is “30+4” on a maternity note.
When does each trimester start?
Under the convention this calculator uses, the first trimester runs to 13 weeks 6 days, the second from 14 weeks 0 days to 27 weeks 6 days, and the third from 28 weeks 0 days. Other sources put the boundaries a week either side, because trimesters are a way of talking about a pregnancy rather than a biological event: nothing changes at midnight on the boundary. If a form or a clinic uses different weeks, use theirs.
What actually counts as full term?
ACOG narrowed the definition in 2013, and a lot of material still uses the old one. Term is now split four ways: early term is 37 weeks 0 days to 38 weeks 6 days, full term is 39 weeks 0 days to 40 weeks 6 days, late term is 41 weeks 0 days to 41 weeks 6 days, and post-term is 42 weeks 0 days and beyond. The change was made because outcomes at 37 and 38 weeks are measurably worse than at 39, and calling both “term” hid that. The window this calculator labels 37w0d to 40w6d covers early term plus full term.
Sources and review notes
- American College of Obstetricians and Gynecologists, Committee Opinion 700 — Methods for Estimating the Due Date, the source of the 280-day rule, the IVF adjustments, and the redating thresholds against a first-trimester scan
- ACOG Committee Opinion 579 — Definition of Term Pregnancy, for the early / full / late / post-term split at 37, 39, 41 and 42 weeks
- NHS — the 12-week dating scan, for how a pregnancy is actually dated in practice and what happens when the scan disagrees with the dates you gave
The arithmetic on this page is addition and has nothing behind it to go stale. The conventions do move: the definition of term was redrawn in 2013, and trimester boundaries were never agreed on in the first place. Where your maternity notes differ from this page, the notes are the ones your care is based on.