Pregnancy Calculator
Your due date, how far along you are, and every dated milestone in between
One question at a time. The answer is a table of dates, because a due date on its own is only one row of it.
- 1Dating method
- 2The date you know
- 3Measured as of
Which date do you actually know?
Pick the one you are most confident about. They do not all carry the same weight, and the comparison below the tool explains which usually wins when two of them disagree.
Choose the first day of your last period
A cycle longer than 28 days usually means later ovulation, so the estimate moves later by the same number of days. A shorter cycle moves it earlier.
This adds 266 days rather than 280, because the 40 week count already contains about two weeks before conception. Guessing this date is worse than using your period date, so only use it if you tracked ovulation.
Copy the gestational age printed on the report, not the age you worked out yourself. A scan before 14 weeks dates from crown rump length and is the measurement clinicians trust most.
The embryo has already lived the stage number in days, so the calculator adds 266 minus that number. Day 3 adds 263, day 5 adds 261 and day 6 adds 260.
How far along as of which date?
Today by default. Change it to check where a future appointment or a past scan sits in the pregnancy.
…
| Milestone | Date | Age | When | What it means |
|---|
Counting from a date, against measuring the pregnancy
Every dating method on this page falls into one of two families, and they are not equals. One asks you to remember something. The other measures the pregnancy that is actually there.
Last period, conception date
You supply a calendar date, the calculator adds a fixed number of days, and the answer is only ever as good as the date and the assumption behind it.
- The arithmeticLast period plus 280 days, shifted by your cycle length minus 28. Conception plus 266 days.
- What it assumesThat you ovulated on a predictable day of the cycle. Dating by the last period varies more than dating by ovulation, with a coefficient of variation of 4.9 percent against 3.7 percent in the Jukic study.
- Where it breaksIrregular cycles, a period you are unsure about, bleeding in early pregnancy mistaken for a period, or a conception date that was really a guess.
- Still worth havingIt is available from the first day, months before any scan, and it is the number your first appointment will be booked from.
First trimester scan, IVF transfer
Nothing has to be remembered. A scan measures the embryo, and an IVF cycle knows the day fertilisation happened because it happened in a laboratory.
- The arithmeticScan date plus 280 minus the age the scan reported. Transfer date plus 266 minus the embryo stage in days.
- What it assumesOnly that early growth is consistent between pregnancies, which before 14 weeks it largely is. Crown rump length dating carries a margin of roughly 5 to 7 days.
- Where it breaksLate. Third trimester dating is accurate only to about 21 to 30 days, and above a crown rump length of 84 mm the scan has to switch measurements.
- Why it usually winsA first trimester scan is the method guidelines treat as the reference. If your dates and an early scan disagree by more than the thresholds in the table below, the scan date is the one that should stand.
Both columns produce a date. Neither produces a delivery date, because no method can. What they produce is the centre of a range, and the range is wide enough that the table above prints it as dates rather than leaving it as a caveat.
When your dates and the scan disagree
This is the question that sends most people to a pregnancy calculator twice, and almost no calculator answers it. You worked out a due date from your last period, a scan gave you a different one, and nobody explained which to believe. There is a published answer. The American College of Obstetricians and Gynecologists sets a threshold for each stage of pregnancy: if the scan and the menstrual dates differ by more than that many days, the estimated due date should be changed to match the scan. Inside the threshold, the original date stands.
The thresholds widen as pregnancy goes on, because ultrasound dating gets steadily less precise while menstrual dating does not change at all. That is the whole logic of the table.
| Gestational age at the scan | What the scan measures | Change the date if they differ by more than | How firm |
|---|---|---|---|
| Up to 8 weeks 6 days | Crown rump length | 5 days | Most precise |
| 9 weeks 0 days to 13 weeks 6 days | Crown rump length | 7 days | Most precise |
| 14 weeks 0 days to 15 weeks 6 days | Head, abdomen and femur | 7 days | Still reliable |
| 16 weeks 0 days to 21 weeks 6 days | Head, abdomen and femur | 10 days | Still reliable |
| 22 weeks 0 days to 27 weeks 6 days | Head, abdomen and femur | 14 days | Loosening |
| 28 weeks 0 days and beyond | Head, abdomen and femur | 21 days | Least reliable |
Read the last row carefully. In the third trimester a scan dates a pregnancy only to about 21 to 30 days, so a difference there is at least as likely to be telling you something about how the baby is growing as about when the pregnancy started. That is a conversation with a clinician, not a calculation.
The spread nobody prints
An estimated due date is a single date drawn from a distribution, and the distribution is wide. Fewer than 4 percent of births land on the exact date, about 60 percent land within a week of it and almost 90 percent within two weeks. A second consumer source puts the exact date figure at about 1 in 20. The two agree closely enough that the honest statement is somewhere between 1 in 20 and 1 in 25.
The cleanest measurement of the underlying variation comes from a study that dated conception hormonally rather than by memory. Across 125 naturally conceived singleton pregnancies, the median time from ovulation to birth was 268 days, which is 38 weeks 2 days. After the six preterm births were removed, the remaining pregnancies still varied across a 37 day span. Not 37 days of error in the calculation, 37 days of genuine variation in how long a pregnancy runs.
National data says the same thing from the other direction. Of US singleton births in 2023, 8.71 percent were preterm, 29.64 percent were early term at 37 or 38 weeks, 56.68 percent were full term at 39 or 40 weeks, and 4.97 percent arrived at 41 weeks or later. The single most common outcome is not the due date. It is somewhere in the fortnight around it.
Why the answer here is a table
A due date is useful for about one thing, which is knowing roughly when to stop working. Everything else you need from pregnancy arithmetic is a different date: when the dating scan window opens, when the anomaly scan window closes, when the second trimester starts, when a birth stops being early term and becomes full term. Those dates are all fixed offsets from the same starting point, so a calculator that prints one date and hides the other fifteen is throwing away almost all of its own output.
The table above prints all of them, marks which have already passed, and puts the estimated due date in its proper place: one row among sixteen, sitting inside the full term band rather than standing above it.
Due dates, dating methods and what the estimate can honestly tell you
Where the 280 days comes from, why four methods give four answers, and how wide the range around the date really is.
A pregnancy calculator does one piece of arithmetic and then dresses it up. It takes a date you supply, adds a fixed number of days, and prints the result. That is genuinely useful, and it is also far less certain than the confident single date most tools display. This page shows the date, shows the spread around it, and shows the fifteen other dates that fall out of the same calculation, because those are the ones you will actually put in a diary.
Where 280 days comes from
Pregnancy is counted in gestational age, measured from the first day of your last menstrual period rather than from conception. The convention exists for a practical reason: almost nobody knows the day they conceived, and almost everybody knows roughly when their last period started. Forty weeks, or 280 days, from that first day gives the estimated due date. The World Health Organization treats 37 to 42 weeks as the normal span for a term pregnancy, so the due date is the middle of a five week window rather than the end of a countdown.
Conception itself happens roughly two weeks into that count, which is why the second rule uses 266 days rather than 280. Subtract a 14 day luteal phase from 280 and you get 266, the days from ovulation to birth. That single subtraction is the hinge the whole page turns on, and it explains every other number here.
Four methods, and what each one is really doing
The calculator above offers four starting points. They are not equivalent, and the difference between them matters more than any rounding.
| Start from | The calculation | What it needs you to know |
|---|---|---|
| First day of last period | Add 280 days, then shift by your cycle length minus 28 | A date you remember, and a cycle you can describe |
| Conception or ovulation | Add 266 days | A tracked ovulation date, not a guess |
| First trimester scan | Add 280 minus the age the scan reported | The scan date and the gestational age printed on the report |
| IVF day 3 transfer | Add 263 days | The transfer date and the embryo stage |
| IVF day 5 blastocyst | Add 261 days | The transfer date and the embryo stage |
| IVF day 6 blastocyst | Add 260 days | The transfer date and the embryo stage |
The IVF rows look arbitrary until you see the subtraction behind them. An embryo transferred on day 5 has already lived five of the 266 days, so the calculator adds 266 minus 5, which is 261. Day 3 gives 263 and day 6 gives 260. Nothing else about the cycle changes the figure, which is why IVF dating is the only method where the day of conception is genuinely known rather than inferred.
Naegele's rule is really two rules, and they disagree
The classic version of Naegele's rule is a mnemonic: take the first day of the last period, subtract three months, add seven days, add a year. The modern version is simply to add 280 days. They are usually described as the same rule. They are not. Because calendar months differ in length, the mnemonic can land up to three days away from the 280 day count. A last period on 1 May 2026 gives 5 February 2027 by the day count and 8 February 2027 by the mnemonic. A last period on 1 January 2026 gives the same answer either way. This calculator uses the 280 day count throughout, and the difference is proven case by case in the page's test script rather than assumed.
The cycle length adjustment is the other half of the rule. Naegele assumed a 28 day cycle with ovulation on day 14. If your cycles run 35 days you probably ovulate about a week later, so the estimate moves a week later too. The calculator shifts the date by your cycle length minus 28. It is a reasonable correction rather than a precise one, and in the one study that dated conception hormonally rather than by recall, pregnancy length measured from the last period varied more than pregnancy length measured from ovulation: a coefficient of variation of 4.9 percent against 3.7 percent.
Gestational age, fetal age, and the two weeks in between
Because the count starts at the last period, you are recorded as two weeks pregnant at the moment of conception. That is not an error, it is the convention, and every clinician you meet will use it. Fetal age, sometimes called conceptional age, runs about 14 days behind gestational age and is almost never used in care. If a scan report and an app disagree by exactly two weeks, this is usually why.
Gestational age is always written as weeks and days, for example 24 weeks 3 days, never as a decimal. The calculator above prints it the same way, because that is the form that has to be read out at an appointment.
How wide is the range, really
Fewer than 4 percent of births happen on the exact estimated date, about 60 percent happen within a week of it and almost 90 percent within two weeks. A second widely cited figure puts births on the exact date at about 1 in 20. Either way, the date is a centre rather than a target.
The best measurement of the underlying variation comes from a study of 125 naturally conceived singleton pregnancies in which ovulation was dated by hormone measurement. The median time from ovulation to birth was 268 days, or 38 weeks 2 days. Once the six preterm births were excluded, the remaining pregnancies still spanned 37 days. That is not calculation error, it is how much genuine variation there is in how long a pregnancy lasts.
The national picture agrees. Of United States singleton births in 2023, 8.71 percent were preterm, 29.64 percent arrived early term at 37 or 38 weeks, 56.68 percent arrived full term at 39 or 40 weeks and 4.97 percent arrived at 41 weeks or later. The early term share has been climbing, up 22 percent between 2014 and 2023.
Reading the milestone table
Every row in the result table is a fixed offset from the same day zero, which is why one date generates all of them. The dating scan window opens at 11 weeks and closes at 14. The anomaly scan window runs from 18 to 21 weeks and checks for 11 physical conditions. The first trimester ends at 13 weeks 6 days on the convention used here, the second at 27 weeks 6 days, and the third runs from 28 weeks to birth. Note that sources differ on where the first trimester ends, with some putting it at 12 weeks; the 13 weeks 6 days boundary is used here because it tiles the 40 weeks without leaving a gap.
The four term bands replaced the single word "term" in 2013. 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 begins at 42 weeks 0 days. The change was made because outcomes at 37 and 38 weeks are measurably worse than at 39 and 40, with a roughly 20 percent higher risk of complications reported for babies born before 39 weeks.
When the arithmetic runs out
A calculator cannot tell you whether the date you typed is the right one. If an early scan disagrees with your dates by more than the published threshold, the scan date is the one that should stand, and your clinician records the change. After the first trimester, dating loses precision quickly, and by the third trimester a scan places the pregnancy only to within about 21 to 30 days, so a discrepancy that late is often about growth rather than about dates. Past 41 weeks the conversation about induction may begin, and guidance in 2021 shifted from recommending it between 41 and 42 weeks to offering it from 41 weeks 0 days, which makes it a decision rather than a default.
None of that is something to settle with a spreadsheet. Take the dates from this page to your appointment, and take any bleeding, pain or worry straight to your midwife, GP or maternity unit instead.
Not pregnant yet? This page starts from a pregnancy that already exists and counts forward. If you are still working out your cycle, your fertile window and when conception is most likely, the ovulation calculator is the one you want.
Frequently Asked Questions
Why does the 40 week count start about two weeks before I was pregnant?
Because the count is anchored to something you can actually date. Almost nobody knows the day conception happened, but most people know roughly when their last period began, so pregnancy is measured from the first day of that period. Conception then falls about two weeks into the count, which means you are recorded as two weeks pregnant at the moment it happens. It is a convention, not a mistake, and every clinician you meet will use it. It is also why the conception rule adds 266 days while the period rule adds 280: the difference is those first two weeks.
My scan gave a different due date from my dates. Which one counts?
There is a published answer, and it depends on how far along the scan was. Up to 8 weeks 6 days, the due date should be changed if the scan and your dates differ by more than 5 days. From 9 weeks 0 days to 15 weeks 6 days the trigger is more than 7 days, from 16 weeks 0 days to 21 weeks 6 days it is more than 10 days, from 22 weeks 0 days to 27 weeks 6 days more than 14 days, and from 28 weeks more than 21 days. Inside the threshold your original date stands. Outside it, the scan date should replace it. The decision and the record belong to your clinician, not to a calculator.
Can my due date be changed again later in pregnancy?
It can, but it rarely should. The estimated date is normally fixed once, as soon as the last menstrual period, the first accurate scan or both are available, and later changes are reserved for unusual circumstances and documented when they happen. The reason is precision: a first trimester scan dates a pregnancy to within roughly 5 to 7 days, while a third trimester scan places it only to within about 21 to 30 days. A late disagreement is therefore more likely to be telling you something about how the baby is growing than about when the pregnancy started.
Why do the 280 day rule and the "minus three months, plus seven days" shortcut give different answers?
Because calendar months are not the same length. The shortcut is the original form of Naegele's rule and it is close, but it can land up to three days away from a straight 280 day count. A last period on 1 May 2026 gives 5 February 2027 by the day count and 8 February 2027 by the shortcut. A last period on 1 January 2026 gives 8 October 2026 either way. This calculator uses the 280 day count, and so do the clinical calculators that apply guideline dating rules.
How do I get a due date if I do not know when my last period started?
Ask for a dating scan rather than guessing. A scan before 14 weeks measures crown rump length and is the most accurate way to establish gestational age, and it is the method used as standard whenever menstrual dates are unknown or unreliable. Guessing a period date, or guessing a conception date, produces an answer that looks just as precise on screen and is not. If you have already had a scan, use the scan option above and copy the gestational age exactly as the report prints it.
What are the odds I give birth on the estimated date itself?
Low. Fewer than 4 percent of births land on the exact date, and a second commonly quoted figure puts it at about 1 in 20. Widen the window and the picture improves quickly: roughly 60 percent of births fall within a week of the estimate and almost 90 percent within two weeks. The national distribution says the same thing from another angle. Of United States singleton births in 2023, 56.68 percent arrived full term at 39 or 40 weeks, 29.64 percent early term at 37 or 38 weeks, 8.71 percent preterm and 4.97 percent at 41 weeks or later.
Why does a day 5 blastocyst transfer add 261 days rather than 266?
Because the embryo has already lived five of those days before it is transferred. The 266 day figure is the interval from conception to birth, so the calculator subtracts the embryo's age at transfer from it. A day 3 transfer therefore adds 263 days, a day 5 blastocyst adds 261 and a day 6 adds 260. This is the one situation where the day of fertilisation is known exactly rather than estimated, which is why an IVF due date does not need a cycle length and does not benefit from being redated by an early scan in the way a menstrual date does.
What does full term mean now, and why is 37 weeks no longer called it?
In 2013 the single label "term" was replaced by four bands, because outcomes across those five weeks are not the same. Early term runs 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, and post term from 42 weeks 0 days. The split was driven by evidence that babies born before 39 weeks carry a higher risk of breathing, feeding and temperature problems, with a roughly 20 percent greater risk of complications reported overall. The milestone table above dates all four bands for your own pregnancy.
What happens if I reach 41 or 42 weeks?
You are in the late term band from 41 weeks 0 days and the post term band from 42 weeks 0 days, and a conversation about induction usually begins somewhere in there. United Kingdom guidance changed in 2021, moving from a recommendation to induce between 41 and 42 weeks to an offer of induction from 41 weeks 0 days, alongside an explanation of the risks either way. That wording change matters: it makes the timing a shared decision rather than a fixed point. In practice very few pregnancies get that far. Only 4.97 percent of United States singleton births in 2023 arrived at 41 weeks or later.
Why does the calculator ask for my cycle length as well as my period date?
Because Naegele's rule quietly assumes a 28 day cycle with ovulation around day 14, and most cycles are not exactly that. If your cycles run 35 days you probably ovulate about a week later than the rule expects, so the estimate is shifted a week later to match, and a 21 day cycle shifts it a week earlier. The correction is sensible rather than exact. Dating from the last period is measurably more variable than dating from ovulation, with a coefficient of variation of 4.9 percent against 3.7 percent, and the cycle field narrows that gap without closing it.