About the Due Date Calculator
The standard method doctors use to estimate a due date is based on the first day of your last menstrual period, not the actual conception date - a common point of confusion for expecting parents. Our Pregnancy Due Date Calculator uses that same standard method.
How It Works
The calculator adds 280 days (40 weeks) to the first day of your last menstrual period, following Naegele's rule - the standard clinical method for estimating a due date, which assumes a typical 28-day cycle with ovulation around day 14.
Formula & Methodology
Naegele's rule, developed in the 19th century, is built on a specific assumption: a 28-day cycle with ovulation (and therefore conception) occurring around day 14. Pregnancy itself lasts roughly 266 days from actual conception, but since conception date is rarely known precisely while the last period's start date usually is, adding 280 days (266 plus the 14 days from period start to typical ovulation) to the last period start became the practical clinical standard.
Step-by-Step: Calculating It By Hand
- 1Note the first day of the last menstrual period.
- 2Add 280 days (40 weeks) to that date.
- 3The result is the estimated due date under Naegele's rule.
Examples
Standard estimate
A last period starting on a given date produces an estimated due date exactly 280 days later - the standard 40-week pregnancy estimate used clinically.
Current gestational age
The calculator also shows how many weeks pregnant you currently are, based on the same last-period starting point, useful for tracking pregnancy milestones.
Advantages
- Uses the same standard method (Naegele's rule) doctors use for initial due date estimates
- Also shows current gestational age based on the same starting point
- Simple single-input calculation from a known date
- Useful early estimate before a dating ultrasound provides a more precise figure
Common Mistakes
- Treating the estimated due date as a guaranteed delivery date - only about 5% of babies are born on their exact due date
- Not accounting for irregular or non-28-day cycles, which this standard method assumes
- Confusing the last menstrual period date with the actual conception date, which typically occurs about two weeks later
- Relying on this estimate over an ultrasound-confirmed due date once one is available, which is generally more accurate
Edge Cases to Watch For
- This assumes a standard 28-day cycle with ovulation on day 14 - cycles that are shorter, longer, or irregular shift the actual conception date relative to this assumption, making the estimate less reliable.
- A first-trimester dating ultrasound, which measures the embryo or fetus directly, is generally considered more accurate than a last-period-based estimate, especially for anyone with irregular cycles.
- Only a small percentage of babies are actually born on their exact calculated due date - it represents the middle of a normal delivery range, not a precise prediction.
- This method doesn't apply directly to pregnancies achieved through IVF or other assisted reproduction, where the actual conception or transfer date is known and used instead.
Common Use Cases
- Getting an early estimated due date based on last menstrual period
- Tracking current gestational age week by week
- Preparing for early prenatal appointments and planning
- Cross-checking against an ultrasound-based due date estimate