Estimate your baby’s due date from your last menstrual period, conception date, or IVF transfer date — and see how far along you are right now.
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Quick answer: your estimated due date is 280 days (40 weeks) after the first day of your last period. If you know your conception date, add 266 days instead. Only about 1 in 20 babies arrive on the exact day, so treat the date as a guide, not a deadline.
Due date examples (280-day count, 28-day cycle)
| First day of last period | Estimated due date |
|---|---|
| January 1, 2026 | October 8, 2026 |
| April 1, 2026 | January 6, 2027 |
| July 1, 2026 | April 7, 2027 |
| October 1, 2026 | July 8, 2027 |
Naegele’s rule can differ from an exact 280-day count by a day or two, depending on the month. Last updated: October 2026.
How your due date is estimated
Most due dates are estimated using Naegele’s rule: take the first day of your last menstrual period (LMP), add one year, subtract three months, and add seven days — which works out to about 280 days, or 40 weeks, from your LMP. This calculator applies the same 280-day estimate, adjusted for your average cycle length if it isn’t exactly 28 days.
If you know your conception date instead (for example, from ovulation tracking), the estimate is 266 days from that date, since conception typically happens around two weeks after the LMP. For IVF pregnancies, the due date is calculated from the embryo transfer date, since that date is precisely known.
Why the method matters
Every method here is an estimate, not a guarantee — only about 1 in 20 babies actually arrive on their exact due date. Ultrasound dating in the first trimester is generally considered the most accurate method overall, so if you’ve had an early ultrasound, your care provider’s estimate may differ slightly from this calculator.
Trimester breakdown
- 1st trimester: from the start of pregnancy dating through 13 weeks 6 days
- 2nd trimester: 14 weeks to 27 weeks 6 days
- 3rd trimester: 28 weeks until birth
Frequently asked questions
How accurate is a due date calculator?
It’s an estimate based on averages, typically accurate to within a week or two for LMP and conception-based methods. IVF transfer dates tend to be more precise since the exact embryo age is known. Only your doctor’s dating, especially from an early ultrasound, should be treated as authoritative.
What if my cycle isn’t 28 days?
Enter your average cycle length in the calculator — longer cycles push the estimated due date later, and shorter cycles pull it earlier, since ovulation timing shifts with cycle length.
Can I use this if I don’t remember my last period?
Yes — switch to the conception date method if you know roughly when conception occurred, or use an ultrasound-based due date from your healthcare provider if you have one, which is generally more reliable than a date-based estimate.
Does this replace a doctor’s estimate?
No. This tool gives a general estimate for planning purposes only. Always follow the due date and guidance given by your doctor or midwife, who can factor in ultrasound measurements and your specific health history.
Common reasons your due date may change
- An early ultrasound that measures the baby and shows a different gestational age than your last period suggests.
- Irregular cycles, where ovulation did not happen around day 14.
- Twins or multiples, which usually arrive earlier than 40 weeks. This calculator is designed for single pregnancies.
Medical note: This calculator provides a general estimate only and is not medical advice. Contact your doctor or midwife about dating, scans, and any concerns about your pregnancy.
Sources and further reading
- American College of Obstetricians and Gynecologists (ACOG) — guidance on estimating the due date
- NHS: Pregnancy — due dates and what to expect