Pregnancy Due Date Calculator
Estimate a due date from a last menstrual period or a conception date, using the standard 280-day convention. Only a clinician can confirm dating.
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About Pregnancy Due Date Calculator
An estimated due date from a last menstrual period or a conception date, using the 280-day convention. It is a date produced by arithmetic on an assumption, and the assumption does not hold for everyone. Free, with no account and nothing kept. A date this private is derived here in the browser and nowhere else.
How to use Pregnancy Due Date Calculator
- Enter the date you are working fromThe first day of the last menstrual period, or a known conception date.
- Adjust the cycle length if you know itThe standard calculation assumes 28 days, and a longer or shorter cycle shifts the estimate.
- Treat it as a rangeVery few births happen on the estimated date. A window of a couple of weeks either side is normal.
Where 280 days comes from
Naegele’s rule adds 280 days to the first day of the last period, which assumes a 28-day cycle with ovulation on day 14. The convention is a century and a half old and remains in use because it is simple and roughly right on average. For anyone whose cycle is not 28 days, the estimate is shifted by the difference, and an early ultrasound measurement dates a pregnancy considerably more accurately.
What it does not do
- An estimate from a convention, not an observation. Ultrasound dating is more accurate and is what a clinician will use.
- Irregular cycles make the calculation substantially less reliable.
- Not medical advice, and no substitute for antenatal care. Nothing entered here is stored or sent anywhere.
Pregnancy Due Date Calculator: questions people ask
- How accurate is a due date?
- Only a small proportion of births occur on the estimated date, and most fall within a couple of weeks either side. It is best understood as the centre of a range rather than an appointment.
- Which is more reliable, this or a scan?
- A scan, clearly — particularly a first-trimester one, which measures the pregnancy directly rather than inferring from a cycle assumption. Where the two disagree, clinicians use the scan.
- How is a due date calculated from the conception date?
- The page adds 266 days to the conception date, which is the usual 280 days from a last period minus the two weeks before ovulation. For IVF it adds 263 days to a day-3 embryo transfer and 261 days to a day-5 transfer. Any date it gives is an estimate for a clinician to confirm.
Questions about ToolsVerse itself — privacy, cost, what happens to your files — are answered in the frequently asked questions.
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