Pregnancy Due Date Calculator
Estimate your baby birth due date using Naegele’s clinical rule.
Pregnancy Timeline Summary
Medical disclaimer. This calculator is for general information and education only. It is not medical advice, not a diagnosis, and not a substitute for professional care. A due date from a last menstrual period assumes a regular 28-day cycle. Only about 4% of births happen on the estimated date, and an early ultrasound dates a pregnancy more accurately.Speak to a doctor or another qualified clinician about any health decision.Last reviewed .
Naegele’s rule and why the date moves
The standard estimate uses Naegele's rule: take the first day of the last menstrual period, add one year, subtract three months, add seven days. That is simply 280 days — forty weeks — from the start of the last period, and it carries a built-in assumption that ovulation happened on day 14 of a regular 28-day cycle.
That assumption is where the imprecision comes from. Cycles vary in length, and the variation sits almost entirely in the first half, before ovulation. Someone with a 35-day cycle typically ovulates around day 21, not day 14, which pushes the true date about a week later than the rule suggests.An early ultrasound dates a pregnancy more accurately because it measures the embryo directly rather than inferring from a period, and antenatal care will usually revise the date if the two disagree by more than a few days.
Either way the result is a midpoint, not an appointment. Only about 4% of births occur on the estimated date, and a full-term delivery is anything from 37 to 42 weeks. Gestational age is also counted from the last period rather than from conception, which means you are considered two weeks pregnant at the moment of conception — a convention that consistently surprises people and is worth knowing when reading week-by-week guidance.
Frequently Asked Questions
What is Naegele’s Rule?
Naegele’s rule is a standard medical method to estimate a due date by adding 1 year, subtracting 3 months, and adding 7 days to the first day of the last menstrual period.
How does menstrual cycle length affect the due date?
If your cycle is longer than 28 days, ovulation occurs later, shifting the due date forward. For shorter cycles, ovulation occurs earlier, shifting the due date backward.
What percentage of babies are born on their exact due date?
Only about 4–5% of babies are born on their exact due date. Most births occur within 2 weeks before or after.
Is my input data private?
Yes. All calculations occur locally in your web browser. No dates are uploaded or saved.