Due Date Calculator
Estimate your delivery date from your last period, a dating scan, conception or an IVF transfer — with the week-by-week schedule it implies.
Every figure here is an estimate. Fewer than one baby in twenty arrives on the estimated due date, and most births fall within a fortnight either side of it.
| Week | Dates | Trimester | Milestone |
|---|
Dates run from the first day of your last period, which is how pregnancy is counted clinically — so the first fortnight precedes conception. Milestones are typical averages, not a schedule any individual pregnancy has to follow. These figures describe a single pregnancy; twins and other multiples run differently throughout.
| Based on | Due date is |
|---|---|
| Last period | +280 days, adjusted for cycle length |
| Conception date | +266 days |
| IVF transfer | +266 days, less the embryo's age |
| Ultrasound | scan date + the time still to run |
| Due date | taken as given |
A dating ultrasound in the first trimester is the most accurate of these. Last-period dating assumes ovulation on day 14, which is why the cycle-length field matters if yours is not 28 days.
| Trimester | Weeks |
|---|---|
| First | 1 – 12 |
| Second | 13 – 27 |
| Third | 28 – 42 |
Full term runs from week 37. A birth before 37 weeks is preterm and after 42 is post-term, both of which are managed rather than simply waited out.
Read this as reference material rather than clinical guidance — it is written for general understanding and cannot stand in for the care you receive. Should the estimate below sit differently from the one recorded in your notes, defer to your notes: whoever wrote them has seen your scans.
The sum behind the date
An estimated due date is 280 days — forty weeks — from the first day of your last period. That single line carries an assumption worth knowing about: it takes ovulation to have happened on day 14, and conception with it.
The rule dates back to Franz Naegele in the early nineteenth century and has survived largely because it is easy and roughly right. Its weakness is exactly where the assumption sits. Cycles are not all 28 days, ovulation is not reliably on day 14 even when they are, and the fortnight of uncertainty that introduces lands directly on the date. That is why this calculator asks for your average cycle length instead of assuming, and why a scan beats arithmetic.
Everything downstream inherits that uncertainty. The week-by-week schedule below is exact arithmetic built on an estimated starting point — precise, but no more accurate than the date it grew from.
Four starting points, one answer
Whichever date you have, the calculator converts it to the same estimate.
Conception date · +266 days
IVF transfer · +266 days − the embryo's age at transfer
Dating ultrasound · scan date + however much of the 280 days remained
The gap between the first two is the same fortnight the Naegele rule assumes: conception dating drops the two weeks before it, so it counts 266 rather than 280.
IVF is the most precise of the four and the one most often got wrong elsewhere. Fertilisation happened on a known day in a laboratory, so there is no ovulation guesswork — but a day-5 blastocyst had already been growing for five days when it was transferred. Subtracting that is what keeps the date honest; skipping it runs five days late.
A dating scan works backwards from measured size. In the first trimester embryos grow so consistently that size genuinely indicates age, which makes an early scan the most reliable input on this list.
The odds of arriving on the day
About 4% of babies are born on their estimated due date. Somewhere near three in five land inside a seven-day band around it, and nearly nine in ten inside a fortnight. Spontaneous labour clusters somewhere around 40 weeks but spreads across several weeks in both directions.
So the useful mental model is a four-week landing zone with your date in the middle of it, not an appointment. Plan for the window: have the bag ready a fortnight early, and expect the last fortnight to feel longer than it is because a specific date has been sitting in your calendar for months.
It also explains a common frustration. Being "overdue" at 40 weeks and 3 days is not late in any medical sense — it is squarely inside the normal distribution. Only after 41 to 42 weeks does the timing itself start to matter clinically.
When a due date gets moved
Due dates are revised more often than people expect, and a revision is usually better information rather than an earlier mistake.
What usually prompts it is an early scan that lands five to seven days or more away from what the period-based sum predicted. Guidelines in most countries say to adopt the scan date at that point and stop using the old one. The larger the disagreement and the earlier the scan, the stronger the case.
What does not happen is re-dating from a late scan. By the third trimester babies vary enough in size that a large measurement means a large baby, not an older one — so a growth scan at 34 weeks measures growth, and your dates stay as they were. If a clinician has already moved your date, enter that revised date's underlying information here rather than recalculating from your last period.
Early, full, late and post term
"Term" is not one block. It was formally split in 2013 because outcomes differ measurably across it.
- Preterm — before 37 weeks.
- Early term — 37 weeks to 38 weeks 6 days.
- Full term — 39 weeks to 40 weeks 6 days.
- Late term — 41 weeks to 41 weeks 6 days.
- Post term — 42 weeks and beyond.
The reason for the split is that early-term babies, despite counting as term, have slightly higher rates of breathing difficulty and feeding problems than full-term ones. That evidence is why elective delivery before 39 weeks without a medical reason fell out of favour — those last two weeks do measurable work.
Going past the date
Passing your due date is ordinary. Around half of first-time mothers do.
Monitoring typically steps up after 41 weeks, and induction is usually discussed somewhere between 41 and 42. The reasoning is that the placenta gradually becomes less efficient as a pregnancy runs long, while the baby keeps growing and needing more — so the margin narrows. The exact threshold, and whether it is offered or recommended, varies by country and by your own circumstances.
This is also where a wrong due date does real damage. If dating was off by a week, "42 weeks" may actually be 41, and decisions get made against a date rather than a pregnancy. It is the strongest argument for getting an early scan and for using its date afterwards.
What this calculator does not cover
- Twins and higher multiples. The sum is the same; the expectation is not. Multiples usually arrive weeks earlier and are monitored differently throughout.
- Whether anything is going well. This counts days. It is not a screening test and cannot tell you about the health of a pregnancy.
- Cycles that vary or have stopped. Where no typical cycle exists to count from, the period route is guesswork before it begins, and a scan carries the whole answer.
- Induction and delivery planning. Timing decisions belong with your care team, who have your history alongside the date.
- Dates already revised. Once a scan has reset your dates, feed the scan in above; going back to the period sum reintroduces the error the scan removed.
For the fuller picture week by week — trimesters, milestones and typical size — the pregnancy calculator works from the same dates. The conception calculator and ovulation calculator work backwards from them.
Frequently asked questions
How is a due date worked out?
By adding 280 days — forty weeks — to the first day of your last period, adjusted if your cycle is not 28 days. The other three routes reach the same place from a different starting point: 266 days from conception, the same less the embryo's age for an IVF transfer, or whatever is left of the 280 days at the time of a dating scan.
How likely am I to give birth on my due date?
Roughly one time in twenty-five. Around 60% of births fall within a week of the estimate and close to 90% within two weeks. The date is a midpoint of a wide distribution, not a prediction, which is why clinicians talk about a due window rather than a due day.
Which estimate should I trust if two disagree?
The earlier ultrasound, generally. Embryos grow at nearly the same rate in the first trimester, so a scan before about 14 weeks measures age well. If a scan and your last period differ by more than about five to seven days, most guidelines say to adopt the scan date and stop using the period-based one.
Why does an IVF due date use a different sum?
Because the starting point is known rather than estimated. Fertilisation happened on a recorded date, so the calculation counts 266 days from conception and then subtracts however many days the embryo had already been developing when it was transferred — three, five or six. Skipping that subtraction pushes the date out by the same number of days.
Does my cycle length really change the date?
It can move it by a week or more. The standard sum assumes ovulation on day 14 of a 28-day cycle. On a 35-day cycle ovulation usually falls around day 21, so the pregnancy began a week later than the standard assumption — and the due date moves back accordingly. If your cycles are long, short or irregular, set the length here and treat a dating scan as the better answer.
What is the difference between early, full, late and post term?
They are the four bands inside and beyond the term window. Early term is 37 weeks to 38 weeks 6 days, full term 39 weeks to 40 weeks 6 days, late term 41 weeks to 41 weeks 6 days, and post term from 42 weeks. Outcomes are best in the full-term band, which is why induction is not usually offered simply for passing the due date.
What happens if I go past my due date?
Usually nothing dramatic. Most units start monitoring more closely after 41 weeks and discuss induction somewhere between 41 and 42, because the placenta becomes less efficient as a pregnancy runs long. The specific point and the options vary by country and by your own circumstances, so it is a conversation to have with your care team rather than a fixed rule.
Can I use this if I am expecting twins?
The date it produces is calculated the same way, but the expectation around it is not. Twins are usually delivered well before 40 weeks — often between 36 and 38 — and are monitored on a different schedule throughout. Use the dates and plan your obstetric team gives you rather than this page.
What this tool offers is arithmetic and context, nothing clinical. It makes no assessment of how a pregnancy is progressing, identifies no condition, and stands outside the care you are already receiving; the figure it returns describes a population, not the person reading it, and assumes one baby rather than two. Where your notes and this page disagree, your notes decide. Anything that unsettles you — blood, pain, a quieter baby than usual, or a worry you cannot quite articulate — warrants a call to your maternity team the same day, and none of those calls are a waste of their time. Learn more about how we build these tools or read our privacy policy.