Due Date Calculator
Estimate your baby's due date from your last period or conception date.
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How the Due Date Is Calculated
The standard method for estimating a due date is Naegele's rule, which adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a regular 28-day cycle with ovulation on day 14.
Due date = LMP + 280 daysIf you know your conception date instead, the estimate is conception + 266 days (38 weeks), since pregnancy is dated from the last period, about two weeks before conception.
Only about 1 in 20 babies arrives exactly on the estimated date. A normal, full-term birth occurs anywhere between 37 and 42 weeks.
Worked Example
If your last period began on 1 March 2026, adding 280 days gives an estimated due date around 6 December 2026. The calculator also shows how many weeks and days along the pregnancy currently is.
The Three Trimesters
| Trimester | Weeks | Highlights |
|---|---|---|
| First | 1 – 13 | Major organ development |
| Second | 14 – 27 | Growth, movement felt |
| Third | 28 – 40 | Rapid weight gain, positioning |
Frequently Asked Questions
How accurate is this estimate?
It is an estimate based on a 28-day cycle. An early ultrasound dating scan is more accurate, especially if your cycle is irregular. Always confirm with your healthcare provider.
What if my cycle isn't 28 days?
Naegele's rule assumes a 28-day cycle. If yours is longer or shorter, the actual due date shifts by the difference. Ultrasound dating accounts for this.
Why is pregnancy dated from the last period?
The LMP is an easily identified, consistent reference point, whereas the exact moment of conception is rarely known. This is why "40 weeks pregnant" actually means about 38 weeks since conception.
Is it normal to give birth before or after the due date?
Yes. A full-term delivery is anywhere from 37 to 42 weeks. The due date marks the middle of that range, not a deadline.
Note: This calculator provides general estimates for informational purposes only and is not medical advice. Consult a qualified healthcare provider for pregnancy care and accurate dating.