Women's Health
Pregnancy Due Date Calculator
Estimate your delivery date from whichever starting point you have — a period, a scan, or an embryo transfer.
Pregnancy Due Date Calculator
These dates are estimates from an average cycle. Real cycles vary month to month, so treat every result as approximate and confirm anything you are relying on with a midwife or doctor.
About the Pregnancy Due Date Calculator
Estimate your due date from whichever starting point you actually have — the first day of your last period, a known conception date, a dating scan, or an IVF embryo transfer. Each method has its own arithmetic, and the calculator applies the right one.
The standard method is Naegele's rule: 280 days from the first day of your last period. This tool adjusts that for your cycle length, because someone with a 32-day cycle ovulates later than someone with a 26-day cycle and their due date shifts accordingly.
Whatever the method, the result is an estimate of a range, not an appointment. Only around 4% of babies arrive on their exact due date.
How to use the Pregnancy Due Date Calculator
- Choose what you are working from — a period, conception date, scan or IVF transfer.
- Enter that date.
- For the last-period method, set your cycle length — a longer cycle pushes the due date later.
- For a scan, enter the gestational age the sonographer measured, in weeks and days.
- Select Calculate due date to see the date plus your trimester milestones.
The four methods
Last period : due = LMP + 280 days + (cycle length − 28)
Conception : due = conception date + 266 days
Ultrasound : due = scan date + (280 − gestational age at scan)
IVF transfer: due = transfer date + 266 − embryo age in days
Worked example
Last period began 3 September, with a 32-day cycle.
Standard Naegele: 3 September + 280 days = 10 June.
Cycle adjustment: +4 days, because 32 − 28 = 4.
Estimated due date 14 June
Without the cycle adjustment you would be dated four days early — enough to matter if a decision about induction is being discussed near term.
Naegele's rule formula and calculator
A dating method published in the early nineteenth century and named after the German obstetrician Franz Naegele. The traditional form is: take the first day of your last period, add one year, subtract three months, add seven days. That works out to 280 days — exactly 40 weeks.
It assumes a 28-day cycle with ovulation on day 14, which is why the cycle-length adjustment matters. It remains the standard starting point in maternity care worldwide, and it is remarkably durable for a rule that predates ultrasound by 150 years.
Which method is most accurate?
| Method | Typical accuracy | Best used |
|---|---|---|
| First-trimester scan | ±5 – 7 days | Up to about 14 weeks — the gold standard |
| IVF transfer date | Within about a day | Whenever applicable — conception timing is known exactly |
| Second-trimester scan | ±7 – 14 days | When no earlier scan was done |
| Last menstrual period | ±1 – 2 weeks | Regular cycles and a confident date |
| Third-trimester scan | ±3 weeks | Poor for dating — growth varies too much |
IVF is the most precise, because the date of fertilisation is known rather than inferred. Otherwise an early ultrasound wins: in the first trimester, embryos grow at a very consistent rate, so measurement predicts age closely. Later on, babies grow at genuinely different rates and a scan measures size rather than age.
Professional guidance is that if a first-trimester scan differs from the period-based date by more than about 5 to 7 days, the scan date is used and the due date is formally changed. This is normal practice, not a sign anything is wrong.
Why cycle length changes the answer
Naegele's rule assumes ovulation on day 14. If your cycle is 32 days, you ovulate around day 18 — four days later — so conception happened four days later and the due date moves four days back. A 24-day cycle shifts it four days earlier.
This matters most for people with cycles well away from 28 days, where the uncorrected estimate can be out by a week. Enter your real average rather than accepting 28.
IVF and FET due date calculator
IVF removes the guesswork, because the embryo's age at transfer is known precisely. The calculation subtracts that age from the standard 266 days post-conception:
- Day-3 embryo: due = transfer date + 263 days.
- Day-5 blastocyst: due = transfer date + 261 days.
Your clinic will give you a due date directly, and it should match. If it does not, check which embryo age was used — that is nearly always the difference.
What a due date actually means
It marks 40 weeks of gestational age, which is the midpoint of a normal range rather than a deadline. Roughly:
- Only about 4% of babies are born on their exact due date.
- Most arrive within a week or so either side.
- Term covers 37 to 42 weeks, with 39–40 weeks described as full term.
To see how far along you are right now, use the pregnancy calculator. To work backwards from a due date to a likely conception date, try the conception calculator.
This is an estimate, not medical advice. It works from average cycle figures, and real bodies vary a great deal month to month. Nothing here replaces a consultation. For anything you are relying on — a pregnancy, fertility treatment, a health concern, or a decision about contraception — speak to a midwife, doctor or sexual health service who can assess your own situation.
How the NHS and doctors calculate your due date
Your first estimate comes from the first day of your last period: add 280 days (40 weeks). Then, at the dating scan — usually between 8 and 14 weeks in the UK — the sonographer measures the baby. If the scan date differs from the period date by more than a few days, the scan date usually wins. This calculator lets you try every method and compare them.
Due date methods compared
| You know | Due date = |
|---|---|
| First day of last period (28-day cycle) | + 280 days |
| Conception date | + 266 days |
| Day 3 embryo transfer (IVF or FET) | + 263 days |
| Day 5 blastocyst transfer (IVF or FET) | + 261 days |
| Day 6 blastocyst transfer | + 260 days |
A frozen embryo transfer (FET) uses the same numbers as a fresh transfer. Only the embryo's age at transfer matters.
Related guides
Gestational age calculator: EDD and LMP
Your EDD (estimated due date) and your LMP (first day of your last menstrual period) are 280 days apart. Gestational age is simply the time since your LMP. Know one and you know the others:
- EDD = LMP + 280 days
- Gestational age today = today − LMP
- Gestational age from EDD = 280 days − days left until the EDD
After IUI, the insemination date works like a conception date: add 266 days to find the due date. For twins, the due date is calculated the same way, but most twins arrive a few weeks early, so doctors plan around that.
Other ways to use this calculator
- EDD calculator — figure out your due date from your last period, conception, IVF or a scan.
- Naegele rule — the last-period method uses Naegele’s rule, the same one the NHS uses to calculate due date.
- Ultrasound due date and due date from conception calculator — choose that method above.
Frequently asked questions
Naegele's rule estimates a due date as 280 days — 40 weeks — from the first day of your last period. The traditional version adds a year, subtracts three months and adds seven days, which comes to the same thing. Published in the early nineteenth century, it assumes a 28-day cycle with ovulation on day 14, which is why adjusting for your real cycle length matters.
An IVF transfer date is the most precise, because fertilisation timing is known exactly. Otherwise a first-trimester ultrasound is the gold standard at ±5 to 7 days, since early embryos grow very consistently. A period-based date carries one to two weeks of uncertainty. Third-trimester scans are poor for dating — by then babies genuinely differ in size.
Because Naegele's rule assumes you ovulate on day 14. With a 32-day cycle you ovulate nearer day 18, so conception happened about four days later and the due date moves four days back. A 24-day cycle shifts it four days earlier. If your cycle sits well away from 28 days, entering your real average can change the estimate by a week.
The embryo's age at transfer is known, so the guesswork disappears. The calculation is transfer date plus 266 days minus the embryo age: a day-5 blastocyst gives transfer + 261 days, and a day-3 embryo gives transfer + 263. Your clinic will provide a due date too — if it differs from this one, the embryo age used is almost always the reason.
Add 266 days (38 weeks) to the conception date. If you know your ovulation date, use that as the conception date. Choose 'A known conception date' above to do it automatically.
Sources and further reading
- ACOG — Methods for estimating the due date (Committee Opinion 700) — the redating criteria and accuracy of each method
- NHS — Working out your due date — UK practice on dating scans
- NICE — Antenatal care guideline (NG201) — when dating scans are offered and how dates are set
- NICHD — Pregnancy and prenatal care — gestational age and term definitions
- NHS — Pregnancy due date calculator — how the NHS estimates and confirms due dates