Pregnancy Due Date Calculator
Estimate your due date and how far along you are from the first day of your last period.
How to use
- Enter the first day of your last menstrual period.
- See your estimated due date.
- Check roughly how many weeks along you are.
How the Pregnancy Due Date Calculator works
Estimates a pregnancy due date from the first day of your last menstrual period, and shows how far along you currently are.
It applies Naegele's rule, which adds 280 days — 40 weeks — to the first day of the last period. Current gestational age is the number of days elapsed since that date, expressed in weeks and days.
due date = last menstrual period + 280 days
Worked example
A last period starting 1 March gives a due date of about 6 December, and eight weeks later you would be recorded as 8 weeks 0 days.
Things worth knowing
- Gestational age is counted from the last period, not conception, so you are considered about two weeks pregnant at the moment of conception. This is the convention clinicians use.
- Naegele's rule assumes a regular 28-day cycle. Longer or irregular cycles shift the estimate.
- Only about 4% of babies arrive on the estimated date; most births fall within two weeks either side. An early ultrasound dates a pregnancy more accurately.
Frequently asked questions
How is the due date estimated?
It adds 280 days (40 weeks) to the first day of your last period — the standard estimate.
Is this a substitute for medical advice?
No — it's an estimate. Your healthcare provider gives the most accurate due date, often via ultrasound.
How accurate is the estimate?
It is a reasonable central estimate, but a first-trimester ultrasound is more accurate and is what clinicians use to confirm or revise dating.
What if my cycle is not 28 days?
Add the difference. A regular 32-day cycle typically means ovulation is about four days later, pushing the due date back by roughly that much.