Due Date Calculator
Estimate the due date and current week from your last period or ovulation
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About this tool
This calculator uses Naegele's rule - the first day of your last period plus 280 days - to estimate a due date. If your cycle is not 28 days it shifts the date by the difference, and if you know your ovulation or conception date you can switch to that basis, which adds 266 days instead. Alongside the due date you get how many weeks and days along you are, which trimester that falls in, and the typical size and development for that week.
How to use
- Choose your basis. Last period is the common one; ovulation date is more precise if you know it.
- Enter the date. On the last-period basis, also enter your usual cycle length.
- Read the due date, the current week and the trimester.
- Look below for this week size and development, and expand the list to browse weeks 1 to 40.
Frequently asked questions
- Why 280 days from the last period?
- Pregnancy actually runs about 266 days from conception, but few people know the exact date of conception. The first day of the last period is easy to remember, so the convention adds the roughly 14 days to ovulation, giving 280. That is why weeks 1 and 2 are not yet pregnancy at all.
- What if my cycle is not 28 days?
- A longer cycle means later ovulation, so the due date moves back by the same amount. With a 35-day cycle ovulation is a week later and so is the due date. Without that adjustment the estimate comes out earlier than it should.
- My clinic gave me a different due date.
- Clinics re-date the pregnancy from ultrasound measurements of the fetus. Early scans are especially accurate because individual variation is small then, so when the last period is uncertain or cycles are irregular the ultrasound date is used. That one takes precedence.
- Why does the size jump at week 21?
- Because the measurement changes. Up to week 20 the legs are curled, so length is measured from crown to rump; from week 21 the full crown-to-heel length is used. The baby has not suddenly grown.
- Do babies actually arrive on the due date?
- Fewer than 5% do. Most arrive within two weeks either side, and first babies are somewhat more likely to be late than early.
This is an estimate, not a medical diagnosis. Accurate dating comes from ultrasound at your clinic and takes precedence over this calculation. Week-by-week notes and sizes describe a general pattern, and individual variation is wide. Talk to your healthcare provider about anything that worries you, or if you pass your due date. The dates you enter stay in your browser and are never sent to a server.