Why this matters
The estimated due date is the single most referenced number throughout a pregnancy, yet the methods used to calculate it vary significantly in accuracy depending on the information available. Naegele's rule — adding 280 days to the first day of the last menstrual period — is the most widely known approach, but it assumes a textbook 28-day cycle with ovulation on day 14. For the roughly 30% of women whose cycles deviate from this pattern, the standard formula can be off by a week or more. Adjusting for actual cycle length corrects this, which is why the calculator includes a cycle-length field.
IVF pregnancies offer the most precise dating because the conception date is known exactly. A day-5 blastocyst transfer adds 266 days (38 weeks) to reach the due date, while a day-3 embryo transfer adds 263 days. Ultrasound-based dating, particularly when performed between 8 and 13 weeks, is considered the most accurate method for natural conceptions because fetal crown-rump length measurements in the first trimester have a margin of error of only about five days.
Due date methods compared
| Method | Input | Formula | Best Accuracy |
|---|---|---|---|
| Naegele's Rule | LMP date | LMP + 280 days, adjusted for cycle | Moderate |
| IVF (Day 5) | Transfer date | Transfer + 266 days | High |
| IVF (Day 3) | Transfer date | Transfer + 263 days | High |
| Ultrasound | CRL measurement | Based on gestational age tables | High (first trimester) |
How to use it
Select a calculation method: last menstrual period, IVF transfer date, or ultrasound date.
Enter the relevant date for the method you chose.
If using the last menstrual period method, enter your average cycle length in days (default is 28).
Read the estimated due date, current gestational age in weeks and days, and which trimester you are in.
Remember that only about 5% of babies arrive on their exact due date.
Testing your result
Verify the Naegele calculation manually: if your last period was January 1 and your cycle is 28 days, add 280 days to reach October 8 of the same year. For a 32-day cycle, add four extra days (32 minus 28) for an adjusted due date of October 12. For IVF, add 266 days to a day-5 transfer date of March 1 to get November 22. Compare each result against an online due date calculator from a medical source to confirm the math aligns.
Common mistakes
Using an uncertain last menstrual period date, which is the largest single source of due date inaccuracy.
Not adjusting for cycle length when your cycles are consistently longer or shorter than 28 days.
Treating the due date as a delivery deadline rather than an estimate — full term spans 37 to 42 weeks.
Using a second-trimester ultrasound for dating, which is less accurate than first-trimester measurements.
Confusing gestational age (from LMP) with fetal age (from conception), which differ by about two weeks.
Edge cases and options
For women with irregular cycles, the LMP method becomes unreliable and the calculator's note about using early ultrasound is the medically appropriate guidance. The trimester breakdown follows the standard division: first trimester is weeks 1 through 13, second is weeks 14 through 27, and third is weeks 28 through 40 and beyond. The gestational age display updates to show weeks and days, giving you a precise picture of where you are in the pregnancy timeline. This calculator is informational and does not replace clinical assessment by an obstetrician.
Real-world use cases
Expecting parents calculating a preliminary due date after a positive pregnancy test before their first prenatal appointment.
IVF patients computing an accurate due date from their known transfer date with precise embryo staging.
Comparing due dates from different methods (LMP versus ultrasound) to understand the range and discuss discrepancies with a provider.
Friends and family members calculating a due date to plan a baby shower or prepare support schedules around the expected delivery window.
Frequently asked questions
Q: What is Naegele's rule?
A: Due date = LMP + 280 days. The calculator adjusts for cycle length by adding (cycle minus 28) days for longer or shorter cycles.
Q: How accurate is the due date?
A: Only about 5% of babies arrive on their due date. Most arrive within two weeks of it — full term is 37 to 42 weeks.
Q: Why does IVF use a different method?
A: IVF conception date is known precisely, so the tool uses transfer date + 266 days (or 263 for 3-day embryos) rather than relying on LMP.
Q: What if my cycles are irregular?
A: Naegele's rule assumes a 28-day cycle. For irregular cycles, an early ultrasound (8 to 13 weeks) gives the most accurate due date.
Q: Is this a medical diagnosis?
A: No — this is informational. Your obstetrician will confirm your due date based on clinical assessment and ultrasound.
Q: How are trimesters divided?
A: First trimester covers weeks 1 through 13, second trimester weeks 14 through 27, and third trimester weeks 28 through 40 and beyond.
Start using it now
Try the Pregnancy Due Date Calculator tool. See also Pregnancy Weight Gain Calculator, Menstrual Cycle Calculator, and Age in Days Calculator.
