Pregnancy Due Date Calculator
Estimate your pregnancy due date, current gestational week, and trimester from the first day of your last menstrual period using Naegele's Rule.
First day of your last menstrual period (LMP)
Estimated due date
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Current gestational age
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Trimester
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How the Pregnancy Due Date Calculator Works
This calculator estimates your due date using Naegele's Rule, the standard formula used by clinicians: due date = first day of your last menstrual period (LMP) + 280 days (40 weeks). It assumes a regular 28-day cycle with ovulation around day 14.
How to Use It
- Enter the first day of your last menstrual period.
- Your estimated due date, current gestational week, and trimester appear instantly.
Example
If your last period started on January 1st, adding 280 days gives an estimated due date of October 8th — about 40 weeks later.
Common Use Cases
- Getting a quick due date estimate before or during an early prenatal visit.
- Tracking which week and trimester of pregnancy you're currently in.
- Planning for milestones like the second-trimester anatomy scan or maternity leave.
FAQs
How accurate is Naegele's Rule?
Naegele's Rule is a widely used estimate, but only about 5% of babies are born exactly on their due date — most arrive within a couple of weeks before or after. It also assumes a regular 28-day cycle with ovulation on day 14, so it's less accurate for people with longer, shorter, or irregular cycles. This calculator provides an estimate only, is not a substitute for professional medical advice, and should not replace dating confirmed by a doctor or an early ultrasound.
What are the three trimesters?
The 1st trimester spans weeks 1–13, the 2nd trimester spans weeks 14–27, and the 3rd trimester spans week 28 until birth (around week 40).
Why does the calculator use my last period instead of the conception date?
Conception date is rarely known precisely, but the start of the last period is easier to remember and correlates predictably with typical ovulation timing, which is why Naegele's Rule and most pregnancy wheels are built around LMP rather than the estimated conception date.