How Pregnancy Due Dates Are Calculated (And Why They're Estimates)
Your due date isn't a deadline — it's a statistical estimate based on a standard formula. Here's exactly how it's calculated, and why your baby's actual arrival could vary by weeks.
Due dates are calculated using Naegele's Rule — adding 280 days (40 weeks) to the first day of your last menstrual period, assuming a 28-day cycle. Only about 4-5% of babies are born exactly on their due date; most arrive within a 5-week window between 38-42 weeks gestation.
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The Standard Method — Naegele's Rule
Named after the German obstetrician who developed it in the 1850s, this method adds 280 days (40 weeks) to the first day of your last menstrual period (LMP), assuming a typical 28-day cycle with ovulation around day 14.
Alternative Calculation Methods
- Conception date: Due date = conception date + 266 days
- Ultrasound dating: Often more accurate, especially in early pregnancy, since it directly measures fetal development rather than relying on cycle assumptions
- IVF transfer date: Most precise method since the exact embryo age and transfer date are known
Why Due Dates Are Estimates, Not Predictions
Clinical research shows only about 4-5% of babies are born exactly on their calculated due date. A normal, healthy pregnancy can last anywhere from 38 to 42 weeks. Think of your due date as the center point of a 5-week likely window, not a deadline.
Gestational Age vs Fetal Age
Doctors typically use gestational age, counted from your LMP. Fetal (or conceptional) age counts from actual conception and is usually about 2 weeks less than gestational age, since ovulation typically occurs around day 14 of the cycle.
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