About the Pregnancy Due Date Calculator
A due date is an estimate, not a prediction — only a small percentage of babies are actually born on their calculated due date. Still, having an estimated date is essential for scheduling prenatal appointments, screenings, and tracking fetal development milestones by week.
This calculator applies Naegele's rule, the standard method used by clinicians: adding 280 days (40 weeks) to the first day of the last menstrual period. Because that assumes a textbook 28-day cycle, the tool also adjusts for a user-entered average cycle length, shifting the estimate earlier or later accordingly.
Frequently Asked Questions
How accurate is a due date calculated this way?
Naegele's rule is the standard first estimate, but early ultrasound dating (typically in the first trimester) is generally considered more precise and is what most providers use to confirm or adjust the date.
What if my cycle isn't a regular length?
Irregular cycles make LMP-based dating less reliable. In these cases, an ultrasound-based estimate from a healthcare provider is usually more accurate than any LMP calculation.
What's the difference between conception date and due date?
Conception typically occurs around two weeks after the start of the last period (near ovulation), while the due date is calculated as 40 weeks from that same starting point.