Due Date Calculator
Estimate a due date from the first day of your last period or from a known conception date, using the standard clinical methods.
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How the due date calculation works
Naegele's rule adds 280 days to the first day of your last period. It assumes a 28-day cycle with ovulation on day 14, which is why cycle length is adjustable — a 35-day cycle means ovulation around day 21 and a due date a week later.
If the conception date is known, the calculation is simpler: 266 days, or 38 weeks, because pregnancy is dated two weeks before conception.
Only around one in twenty-five babies arrives exactly on the due date. Most arrive within the two weeks either side.
Worked example
Last period started on 1 March with a 28-day cycle.
- →1 March + 280 days
- →= 6 December
Answer: An estimated due date of 6 December, about 40 weeks later.
Due Date FAQs
How accurate is a due date?
For a regular 28-day cycle it is a good estimate. An early dating scan, usually between 8 and 14 weeks, is more accurate because it measures the baby directly.
What does full term mean?
37 to 42 weeks. Births before 37 weeks are preterm; after 42 weeks are post-term and usually induce labour.
Can I use this with IVF?
Not directly. IVF dating depends on the transfer date and embryo stage, so use the conception mode with the embryo transfer date and ask your clinic for the official date.
Due Date Calculator is part of the health calculators collection on Calcora, alongside 7 other tools. Results are calculated in your browser and are for information only — see our disclaimer.
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