Pregnancy Due Date Calculator
Due date and likely conception from the first day of the last period.
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Formula
How the calculation works
The due date is 280 days from the first day of the last period, which is 40 weeks. It is counted from the period rather than from conception because the period is a date the mother knows and conception is one nobody does. A cycle longer than 28 days shifts ovulation later, so the extra days are added; a shorter cycle takes them off.
The likely conception date is shown as well, at 14 days plus the cycle adjustment, because that is the other date people ask for. Both are estimates from a rule, and the page says so: only about one baby in twenty arrives on the due date, and the doctor's ultrasound dating is the figure that counts.
Common mistakes
- Counting from conception instead of the last period, which puts the date about two weeks early.
- Ignoring the cycle length. A 35-day cycle moves the date a week later, and the date is used to book scans and leave.
- Treating the date as fixed. It is a 40-week estimate; the scan dating can move it.
When to use it
- Use it to plan leave and appointments, and to know roughly when the pregnancy reaches each mark.
- It is not medical advice. Book the scan and follow the clinician's dating.
Worked example
5 January 2026 with a 28-day cycle: a due date of 12 October 2026.
Common questions
Why 280 days?
That is the clinical convention, Naegele's rule: 40 weeks from the first day of the last period. It is not counted from conception, because that date is not known for certain.
Is the date exact?
No. Only about one baby in twenty arrives on the date. A cycle longer or shorter than 28 days shifts it, which is why the cycle length is an input. The doctor's ultrasound dating is the one to go by.
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