Due Date Calculator
About Due Date Calculator
How Due Dates Are Calculated
The estimated due date (EDD) is calculated using Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a 28-day menstrual cycle with ovulation occurring on day 14. If your cycle is longer or shorter than 28 days, the calculator adjusts accordingly. Despite its simplicity, this method remains the standard first-line approach used by obstetricians worldwide. Only about 5% of babies arrive on their exact due date, but 80% are born within two weeks before or after.
The Calculation Methods
This calculator offers two approaches: From LMP: Due date = LMP + 280 days (adjusted for cycle length). From Conception/Ovulation Date: Due date = Conception date + 266 days. The LMP method is more commonly used because most women know their last period date with more certainty than their exact conception date.
How to Use This Calculator
Select your calculation method and enter the relevant date. If using LMP, optionally enter your average cycle length for a more accurate estimate. The calculator displays your estimated due date, current gestational age in weeks and days, trimester information, and upcoming pregnancy milestones (when you will hear the heartbeat, when the baby starts moving, viability date, and full-term date).
Frequently Asked Questions
How accurate is the due date calculation?
About 5% of babies arrive on the exact due date. The due date defines the middle of a normal delivery window (37-42 weeks). First-time mothers tend to deliver slightly past their due date on average.
Does cycle length affect my due date?
Yes. A longer cycle means you likely ovulated later, pushing the due date forward. A 35-day cycle adds about one week compared to the standard 28-day calculation.
What if my ultrasound gives a different due date?
First-trimester ultrasound is considered the most accurate dating method. If it differs from the LMP calculation by more than 7 days, your healthcare provider will typically use the ultrasound date.
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