Due Date Calculator
LMP Method with Cycle Adjustment
How It Works
The Math Behind It
We use Naegele's Rule as the foundation: add 280 days (40 weeks) to the first day of your last menstrual period. If your cycle is longer or shorter than 28 days, we adjust the calculation by the difference. For example, a 31-day cycle adds 3 extra days to the standard estimate. The ultrasound tab reverse-calculates from gestational age at scan, which is often more accurate in early pregnancy.
How to Use
Pick the tab that matches what you know: your last period date, a conception date, or an ultrasound measurement. Enter the values and click Calculate. Switch between tabs to compare methods. Save results to your history for tracking changes over time.
Pro Tips
- Only about 5% of babies arrive exactly on the estimated due date — the "due month" (37–42 weeks) is a safer expectation.
- If your cycles vary by more than 7 days, use the ultrasound method for better accuracy.
- Save multiple calculations to compare how different methods shift your timeline.
Related Articles
The 28-day cycle myth ruined my first due date
I have 31-day cycles. I've always had 31-day cycles. Ever since I started menstruating at 13, my cycles have been longer than the textbook average....
My due date changed three times and I almost lost my mind
I still remember the exact moment. December 15, I told my mom. She booked her flight. She bought non-refundable tickets.
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