Due Date Calculator

How the due date is calculated: Naegele's rule

A due date is not a guess pulled out of thin air. It comes from a rule doctors have used since the 1800s: count 280 days, which is 40 weeks, forward from the first day of your last period. That rule carries the name of the German professor who wrote it down, and it is still where most providers start.

The piece people trip over is where the count begins. Pregnancy is dated from the first day of your last menstrual period, not from the day you conceived. Ovulation usually falls about 14 days after a period starts, so at the moment of conception you are already counted as two weeks pregnant. When someone tells you pregnancy runs nine months, the clock has been ticking for roughly two weeks before the egg was even released.

Older textbooks phrase the same rule a different way: add seven days to the first day of your last period, count back three months, then add a year. For a last period of January 1, 2026 that gives January 8, back three months to October 8, 2025, then a year forward to October 8, 2026, which matches the 280 day count exactly. It does not always match. With a last period of March 14, 2026, the three month version lands on December 21, 2026 while 280 days lands on December 19, 2026. The two day gap is nothing but the uneven length of calendar months, and it is why most calculators add 280 days directly instead of shuffling month names around. This calculator does the 280 day version.

Why your cycle length changes the date

The 280 day figure assumes a 28 day cycle with ovulation on day 14. Real cycles are not that tidy. Anything from 21 to 35 days counts as typical for an adult, and the extra days are mostly added to the first half of the cycle, the part before ovulation.

The fix is simple. Work out how far your own cycle sits from 28 days and move the date by that many days. A 35 day cycle runs seven days longer than 28, so the date moves seven days later. A 24 day cycle moves it four days earlier. The shift sounds small and it is not: on a 35 day cycle, the plain 280 day rule would put your date a full week too early, and a week matters when a c-section or an induction is being scheduled.

If your cycles are irregular, or you were on hormonal birth control in the months before you conceived, the last period is a weak anchor. Someone who ovulated late in a long cycle will look further along than they are on paper. That is exactly the situation where an early scan earns its keep.

Early ultrasound dating, and when it wins over your period

A scan in the first trimester measures crown rump length, the distance from the top of the head to the bottom of the rump. At that stage a fetus grows at a predictable rate, so one measurement dates a pregnancy to within about five to seven days. That is far tighter than a calendar guess built on a period someone may have tracked loosely or not at all.

Later in pregnancy the growth spread widens. By the third trimester two healthy babies of the same age can differ by three weeks in size, because genetics, placenta function and a dozen other things pull the numbers apart. A scan at 34 weeks can no longer tell you the date with any real confidence, which is why providers prefer to set the date early and then leave it alone.

Practical upshot, in the order matters are usually decided:

• Before 14 weeks. A scan accurate to 5 to 7 days. If it disagrees with your periods by more than 5 to 7 days, the scan date is normally the one kept.
• 14 to 21 weeks. Accurate to about 7 to 10 days, so it only overrides the period when the gap is larger than 10 days.
• 22 to 27 weeks. Accurate to about 10 to 14 days, and it rarely changes the date.
• 28 weeks and later. Accurate to three weeks at best. Most providers will not redate this late at all.

The professional guidance behind those cutoffs is set out in the ACOG committee opinion on methods for estimating the due date. Once a date has been set from a first trimester scan it is normally frozen for the rest of the pregnancy, and later scans that put the baby a week ahead or behind are read as growth, not as a new date.

Only about 1 in 20 babies arrive on the due date

Here is the part that surprises people. The due date is a midpoint estimate, not an appointment. Studies of large groups of births find that only about 5% of babies are born on the estimated date itself. Put the other way, roughly 19 out of 20 parents are either early or late, and both outcomes are normal.

The realistic window is wider than most people expect. About half of all babies arrive within a week of the date in either direction, and most of the rest land within two weeks. The numbers below come from national birth data held by the National Center for Health Statistics at the CDC, rounded for readability.

The CDC page on preterm birth puts the preterm rate at about 1 in every 10 births, and notes that delivery should not be scheduled before 39 weeks unless there is a medical reason. That is worth knowing if you are ever offered an early date for convenience rather than a health reason. Use the date to size up leave from work, to decide when the nursery needs finishing, and to know when to have the hospital bag packed. Do not treat the morning of the due date as a deadline that something has gone wrong about.

Early term, full term, late term and postterm

The words your provider uses for the weeks around the date have specific meanings, and they changed in 2013. Before that, anything from 37 to 42 weeks was lumped together as term. Now the range is split, because a baby born at 37 weeks and one born at 39 weeks do not have the same average outcomes.

• Preterm: before 37 weeks 0 days. Babies this early may need help breathing or feeding.
• Early term: 37 weeks 0 days through 38 weeks 6 days. Born healthy, but with a slightly higher rate of breathing problems than a baby who stays in a little longer.
• Full term: 39 weeks 0 days through 40 weeks 6 days. The best average outcomes sit here.
• Late term: 41 weeks 0 days through 41 weeks 6 days.
• Postterm: 42 weeks 0 days and beyond.

Worked through for a last period of January 1, 2026 on a 28 day cycle, the same week marks fall on these calendar dates. It is a useful exercise, because it shows how narrow the full term window is once you spread it across a wall calendar.

Notice that the due date, October 8, sits inside the full term window rather than at the end of it. A pregnancy that reaches its date has not run out of time. It has arrived at the middle of the range where most births happen.

Twins, IVF, inductions and scheduled C-sections

Some pregnancies get a date that does not come from a period at all. In vitro fertilization gives the most exact one, because the transfer day is known. For a day 5 blastocyst transfer you count 261 days forward; for a day 3 transfer it is 263 days. There is no last period to argue with and no cycle length to adjust for, so the date is as solid as dating gets.

Twins change the picture in a different way. The due date is still calculated at 40 weeks, but twins rarely get there. The average twin delivery happens around 36 weeks, and well over half of twin births land before 37 weeks. Many providers schedule a c-section or an induction between 37 and 38 weeks for twins, so the 40 week date is a bookkeeping figure rather than the day to expect.

Inductions and planned c-sections get scheduled around the term ranges rather than the date itself. A planned c-section without a medical reason is usually held until 39 weeks 0 days, and an induction for a pregnancy that has run long is commonly offered somewhere between 41 and 42 weeks. The NICHD pages on pregnancy and labor go through the reasons a delivery gets scheduled early, and why 39 weeks is the line most providers hold.

How to use the date without driving yourself mad

Once you have a number, the useful move is to turn it into a few concrete dates and then stop checking it.

• Pack the bag by 36 weeks. That is about four weeks before the date, and it covers most early term arrivals.
• Finish the nursery and the car seat by 37 weeks. Installing a car seat is worth doing well before you need it, because the fitting is fiddly and the instructions you read at 3am will not help.
• Plan work around a range, not a day. If you are telling an employer when you expect to be out, give them the date and then add that births commonly run two weeks either side of it.
• Do not announce the date as a promise. Family members who ask every morning from the due date on are being normal, but the date is an estimate and the calls will stop when the baby comes.
• Keep the two week spread in mind for travel. Most airlines have their own cutoffs for flying late in pregnancy, and most of them ask for a doctor's note from around 36 weeks.

The other thing worth doing early is writing down how your date was set, period or scan, because that answer matters at every appointment afterwards. A provider who knows the date came from a 9 week scan will not move it because a 32 week scan shows a baby measuring a week ahead.

How accurate is the estimate, and when to trust your provider instead

A first trimester scan date is accurate to about 5 to 7 days. A date built on a last period with a regular 28 day cycle is accurate to roughly two weeks in either direction, because ovulation and implantation both add real variation. A date built on a period from someone with irregular cycles or recent birth control use can be off by three weeks or more, which is enough to change the plan at the end.

When the two methods disagree, the order in the earlier section settles it: the scan wins in the first trimester, and after that the existing date stays put. If your own calculation and your provider's date differ by a few days, the provider's number is the one that will be used for scheduling, for growth checks and for deciding whether an induction is on the table.

There are also limits on what this page can do. It has no way to check fetal growth, position, placenta health or anything else a scan looks at, and it cannot account for a pregnancy where ovulation happened unusually early or late. Treat it as a way to understand the number you were given, or to get a working date before your first appointment.

This is an estimate for planning, not medical advice. Dates move for plenty of reasons, including a first trimester scan, and the only date that should guide a real decision is the one your own provider has confirmed. Ask them if you are unsure which date is the one being used.