4 ways to calculate your due date
Your estimated due date (EDD) is 280 days after the first day of your last period — or 266 days after conception.
| You know… | Formula | Best for |
|---|---|---|
| First day of last period (LMP) | LMP + 280 days (+/− cycle length − 28) | Regular cycles, known LMP |
| Conception / ovulation date | Conception + 266 days | Ovulation tracking, known date |
| IVF embryo transfer | Transfer + 266 − embryo age | IVF and frozen embryo transfers |
| Ultrasound measurement | Scan date + 280 − gestational age at scan | Irregular cycles, unsure LMP |
Already know your due date and want to see how far along you are? Use the pregnancy calculator. Working backwards from a due date to find when you conceived? Use the conception calculator.
Naegele’s rule: the due date formula
Doctors have estimated due dates the same way since the 1800s. Naegele’s rule assumes a 28-day cycle with ovulation on day 14:
Example: your last period started on March 10, 2026. Subtract 3 months (December 10, 2025), add 7 days (December 17, 2025) and add a year: your due date is December 17, 2026.
If your cycles are longer or shorter than 28 days, ovulation shifts too. Our calculator adjusts for this: a 35-day cycle moves your due date 7 days later; a 25-day cycle moves it 3 days earlier.
Due date chart by last period
For a 28-day cycle, here’s the due date for a last period that started on the 1st of each month:
| First day of last period | Estimated due date |
|---|---|
| January 1 | October 8 |
| February 1 | November 8 |
| March 1 | December 6 |
| April 1 | January 6 |
| May 1 | February 5 |
| June 1 | March 8 |
| July 1 | April 7 |
| August 1 | May 8 |
| September 1 | June 8 |
| October 1 | July 8 |
| November 1 | August 8 |
| December 1 | September 7 |
IVF due date calculator
IVF gives the most precise due date because the embryo’s age on transfer day is known. Choose “IVF transfer” in the calculator and pick your embryo’s age:
- Day-5 blastocyst: due date = transfer date + 261 days
- Day-3 embryo: due date = transfer date + 263 days
- Day-6 blastocyst: due date = transfer date + 260 days
The same rule applies to frozen embryo transfers (FET): what matters is the embryo’s age when it was transferred, not when it was frozen.
Due date by ultrasound
In the first trimester, the baby’s crown-rump length (CRL) predicts gestational age to within about 5–7 days, which makes an early scan the most accurate way to date a pregnancy. ACOG recommends changing a period-based due date to the ultrasound date when the two differ by:
- more than 5 days before 9 weeks,
- more than 7 days from 9w0d to 13w6d,
- more than 7 to 14 days in the second trimester, depending on the week,
- more than 21 days from 28 weeks on.
Once a due date is set from an early ultrasound, it should not be changed by later scans.
Your due date is the middle of a window, not a deadline. Babies born from 39w0d to 40w6d are considered full term; 37–38 weeks is early term and 41 weeks is late term.
Frequently asked questions
How is my due date calculated?
The standard method adds 280 days (40 weeks) to the first day of your last menstrual period. A shortcut is Naegele’s rule: take the first day of your last period, subtract 3 months, add 7 days and add 1 year. For a period that started on January 1, the due date is October 8.
Can my due date change?
Yes. If an early ultrasound measures your baby as noticeably older or younger than your period-based date, your provider may move your due date. ACOG recommends changing it when the difference is more than 5 days before 9 weeks or more than 7 days between 9 and 13⅞ weeks. After that, due dates are rarely changed.
How do I calculate my due date if I have irregular periods?
If your cycles vary a lot, a date based on your last period may be off. Enter your average cycle length if you know it; otherwise an ultrasound in the first trimester is the most reliable way to set your due date.
How is an IVF due date calculated?
With IVF the age of the embryo is known exactly. Add 266 days to the transfer date and subtract the embryo’s age: 261 days after a day-5 blastocyst transfer, 263 days after a day-3 transfer, or 260 days after a day-6 transfer.
What are the chances of giving birth on my due date?
Low — only around 4% of babies arrive on the exact day. Most babies are born within about two weeks either side of their due date, which is why your due date is best thought of as the middle of a window.
Is it 40 weeks from conception or from my period?
From your period. The 40 weeks include roughly two weeks before you ovulated and conceived, so pregnancy lasts about 38 weeks (266 days) from conception.