Due Date Calculator
Naegele's rule adds 280 days (40 weeks) to the first day of the last menstrual period, so a last period that started on 1 January 2024 gives an estimated due date of Mon, Oct 7, 2024. After IVF the due date is 261 days after a day-5 embryo transfer or 263 days after a day-3 transfer.
How to use
- Choose how you want to date the pregnancy: last period, conception date, IVF transfer (day-3 or day-5 embryo) or ultrasound.
- Enter the date. For a last period you can set your usual cycle length (leave it at 28 if unsure). For an ultrasound, also enter the weeks and days measured at that scan.
- Read the estimated due date, the estimated conception date, how far along you are today and the dates when each trimester ends.
How it works
Last period (Naegele's rule): estimated due date = first day of the last menstrual period + 280 days (40 weeks). It assumes a 28-day cycle with ovulation about day 14. With a longer or shorter cycle the date moves by the difference: a 32-day cycle gives a due date 4 days later, a 24-day cycle 4 days earlier.
Conception date: due date = conception + 266 days (38 weeks), the same as counting back 14 days to an equivalent last period and adding 280 days.
IVF: the due date comes from the embryo age and the transfer date. For a day-5 embryo it is 261 days after transfer, for a day-3 embryo 263 days after transfer.
Ultrasound: the gestational age measured at the scan is subtracted from the scan date to get an equivalent last period start, then 280 days are added. A first-trimester ultrasound is the most accurate way to date a pregnancy, and ACOG advises using it when it disagrees with the last period date by more than set limits.
Only about 4 to 5 percent of babies arrive on the estimated date, and a normal birth can fall anywhere from about 37 to 42 weeks. Your midwife or doctor may change the date. This is an estimate, not medical advice.
Due date by method
| Method | You enter | Due date |
|---|---|---|
| Last period | First day of last period | + 280 days (cycle-adjusted) |
| Conception | Conception date | + 266 days |
| IVF day-3 embryo | Transfer date | + 263 days |
| IVF day-5 embryo | Transfer date | + 261 days |
| Ultrasound | Scan date and measured age | Scan date − measured age + 280 days |
Method and sources
Method: Naegele's rule with the cycle-length adjustment, IVF dating from embryo age and transfer date, and ultrasound dating, as described by the American College of Obstetricians and Gynecologists (Committee Opinion 700, "Methods for Estimating the Due Date"). Pregnancy is counted as 280 days or 40 weeks from the first day of the last menstrual period.
This tool is not medical advice. See a doctor or midwife for prenatal care and for confirming your due date.
Sources: ACOG CO 700Last reviewed 2026-10-13
FAQ
How accurate is the due date?
It is an estimate. Few babies are born on the exact day. Most arrive within two weeks either side. If an early ultrasound disagrees with the date from your last period, your care provider will usually use the ultrasound.
How is the due date worked out after IVF?
From the transfer date and the embryo age. A day-5 blastocyst transfer gives a due date 261 days later, a day-3 transfer 263 days later. This matches dating from an egg retrieval or fertilization date 266 days before birth.
What if I do not remember my last period?
Use a conception date if you know it, or ask for a dating ultrasound and enter the scan date with the weeks and days measured. Guessing a last period date will give a guessed result.
Why does cycle length change the due date?
The 280-day rule assumes ovulation on day 14 of a 28-day cycle. If you ovulate later or earlier, conception and birth move by the same number of days.
How many weeks pregnant am I and what happens each week?
This page shows today's weeks and days as a side result. For a week-by-week timeline of scans, screening tests and term dates, use the Pregnancy Calculator.
Is my data sent anywhere?
No. The calculation runs in your browser and nothing you enter is sent to a server.