To calculate your due date, add 280 days (40 weeks) to the first day of your last menstrual period. If you know when you conceived, add 266 days instead. After IVF, add 261 days to a day 5 embryo transfer date. An early ultrasound can then confirm or adjust that date, and it usually becomes the official one.
The quickest way to do this is with the due date calculator, which handles all four methods and adjusts for cycle length. This guide explains what is behind each method, works through real dates, and covers why your due date is better thought of as the middle of a window than as a deadline.
Why pregnancy is counted from your last period
Doctors and midwives count pregnancy from the first day of your last menstrual period (LMP), not from conception. That seems odd, since you are not pregnant for the first two weeks of “pregnancy”, but there is a practical reason: most people know when their period started, while very few know the exact day they ovulated or conceived.
In a typical 28-day cycle, ovulation happens around day 14. So by the time you conceive, you are already counted as two weeks pregnant. A full-term pregnancy is 40 weeks from LMP, or about 38 weeks from conception. This age counted from LMP is called gestational age, and it is what every scan, test and milestone refers to.
How to calculate your due date from your last period
Naegele’s rule
The traditional shortcut, named after an 1800s German obstetrician, is: take the first day of your LMP, add one year, subtract three months and add seven days. For an LMP of March 2, 2026, that gives December 9, 2026.
The 280-day method
Counting exactly 280 days from March 2, 2026 gives Monday, December 7, 2026. Naegele’s rule is two days later here because months have different lengths, and the shortcut does not account for that. Calculators and pregnancy wheels use the 280-day count, so expect a day or two of difference if you work it out by hand.
Adjusting for cycle length
Both methods assume a 28-day cycle with ovulation on day 14. If your cycles are longer, you probably ovulated later, so your due date moves later by the same number of days. With the same March 2 LMP and a 32-day cycle, ovulation was likely around day 18, and the due date becomes 280 + (32 − 28) = 284 days later: Friday, December 11, 2026. A 25-day cycle moves it three days earlier.
This works only if your cycles are fairly regular. The second half of the cycle, from ovulation to the next period, is usually close to 14 days, and most of the variation is in the first half. If you tracked ovulation with tests or temperatures, the ovulation calculator and your own records will be more precise than any average.
Calculating from conception or IVF dates
From a known conception date
If you know the date of conception, for example from ovulation tracking or a single possible date, add 266 days (38 weeks). For a March 16, 2026 conception, the due date is again December 7, 2026. Working the other way, the conception date calculator estimates when you most likely conceived from your LMP or due date and shows the fertile window.
Keep in mind that sperm can survive up to five days, so the day of intercourse is not always the day of conception.
From an IVF transfer
IVF gives the most precise dating there is, because the age of the embryo is known exactly. The formula is transfer date + 266 days − embryo age in days:
- Day 5 blastocyst transfer: add 261 days.
- Day 3 embryo transfer: add 263 days.
A day 5 transfer on June 10, 2026 gives a due date of Friday, February 26, 2027, and a day 3 transfer on the same date gives Sunday, February 28, 2027. For dating purposes, the equivalent LMP for that day 5 transfer is May 22, 2026, 19 days before the transfer. The IVF due date calculator does this for you and also shows typical dates for the first blood test and scan.
How ultrasound dating changes your due date
A first-trimester ultrasound measures the crown-rump length (CRL), the length of the embryo from head to bottom. In early pregnancy embryos grow at a very consistent rate, so CRL predicts gestational age to within about 5 to 7 days. That is usually more accurate than a remembered LMP.
The American College of Obstetricians and Gynecologists recommends changing the LMP due date only when the ultrasound differs by more than a set margin:
| Gestational age at scan (by LMP) | Change the due date if the difference is more than |
|---|---|
| Before 9 weeks 0 days | 5 days |
| 9w0d to 13w6d | 7 days |
| 14w0d to 15w6d | 7 days |
| 16w0d to 21w6d | 10 days |
| 22w0d to 27w6d | 14 days |
| 28 weeks and later | 21 days |
A worked example
Say your LMP was March 2, 2026, and you have a scan on May 5, 2026. By LMP you are 9 weeks 1 day (64 days). The sonographer measures a CRL of 16 mm. Using the Robinson formula, gestational age = 8.052 × √16 + 23.73 = 55.9, so about 56 days, or 8 weeks 0 days.
The difference is 8 days. At 9w1d the threshold is 7 days, so the due date is changed to the ultrasound date: May 5 − 56 days + 280 days = Tuesday, December 15, 2026. If the scan had shown 8 weeks 3 days instead (5 days’ difference), the LMP date of December 7 would stand. You can run your own scan numbers through the crown-rump length calculator.
Once your due date has been set from an early scan, it should not be changed by later ultrasounds. Later in pregnancy, babies’ sizes vary much more, so a big or small measurement at 30 weeks usually means a big or small baby, not a wrong date.
Trimesters and key milestones
Here is how the timeline looks for the March 2, 2026 LMP, with a December 7, 2026 due date:
| Milestone | Gestational age | Date |
|---|---|---|
| Conception (approx.) | 2w0d | March 16, 2026 |
| Second trimester begins | 14w0d | June 8, 2026 |
| Third trimester begins | 28w0d | September 14, 2026 |
| Early term | 37w0d | November 16, 2026 |
| Full term | 39w0d | November 30, 2026 |
| Due date | 40w0d | December 7, 2026 |
| Late term | 41w0d | December 14, 2026 |
| Post-term | 42w0d | December 21, 2026 |
The pregnancy calculator builds this full timeline for your own dates, including the usual windows for screening tests and scans. If you want to know how far along you are on any specific date, the gestational age calculator gives it in weeks and days.
Why only about 4% of babies arrive on their due date
Your due date is the midpoint of a range, not a prediction of the day. Only about 4 to 5% of babies are born on their exact due date. Most arrive in the weeks either side of it, and natural pregnancy length varies by several weeks even when dating is precise. One study that pinned down ovulation with daily hormone tests found that natural pregnancy lengths still varied by as much as 37 days.
Several things shift the odds:
- First pregnancies tend to run a few days longer than later ones.
- Your own history matters. If your first baby came at 41 weeks, the next one often does too.
- Twins usually arrive around 36 to 37 weeks.
- Induction policies mean few pregnancies now go past 41 to 42 weeks.
About 1 in 10 babies in the US is born preterm, before 37 weeks. The National Institute of Child Health and Human Development has good background on preterm birth and what full term means. A useful mindset: think of “due month” rather than “due date”, and have your bag packed by 37 weeks.
This guide is general information, not personal medical advice. Your midwife or obstetrician will confirm your dates, usually at your first scan, and their date is the one to plan around.
Common mistakes when working out a due date
- Using the last day of your period instead of the first. That can push your date about five days too late.
- Using the date of a positive test as the start. Tests turn positive around 4 weeks of gestational age, so this adds roughly a month.
- Ignoring cycle length. With 35-day cycles, an unadjusted LMP date can be a week early.
- Counting from intercourse as if it were conception, then adding 280 days. From conception you add 266, not 280.
- Re-dating from a late scan. Once a first-trimester scan has set the date, later scans measure growth, not age.
- Treating the date as a deadline. Going a week past it is normal, and so is arriving two weeks early.
Frequently asked questions
How is my due date calculated?
It is 280 days after the first day of your last period, adjusted for cycle length, or 266 days after conception. An early ultrasound may then change it if the measurement differs by more than a set margin.
Is the due date 40 weeks from conception?
No. It is 40 weeks from your last period, which is about 38 weeks from conception, because conception usually happens around two weeks after your period starts.
Which is more accurate, LMP or ultrasound?
A first-trimester ultrasound is usually more accurate, within about 5 to 7 days. IVF dates are the most accurate of all because the embryo’s age is known exactly.
Can my due date change?
It can change once, after your first dating scan, if the scan differs enough from your LMP estimate. After that it normally stays fixed for the rest of the pregnancy.
How many weeks is full term?
Full term is 39w0d to 40w6d. From 37w0d to 38w6d is early term, 41 weeks is late term and 42 weeks or more is post-term.
Can I work out my due date if I don’t remember my last period?
Yes. An early ultrasound gives a reliable date on its own, and if you already have a due date you can work backwards with the reverse due date calculator to find your equivalent LMP and likely conception date.
Your due date gives everyone a shared reference point for scans, tests and planning, and the method behind it is straightforward: count 280 days from your last period, adjust for your cycle, and let an early scan confirm it. Use the date to plan, hold it loosely, and get ready from 37 weeks. If you are also thinking about nutrition and hydration in pregnancy, the guides to daily water intake and calorie needs are good next reads.