The 40-week rule
EDD = first day of LMP + 280 days + (cycle length − 28)Estimated conception is about 14 days after the LMP, with the same cycle-length adjustment. This is an initial convention, not a confirmed birth date.
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FAMILY, CHILDREN AND HEALTH
Pregnancy due date calculator
Last menstrual period and pregnancy week
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HOW IT WORKS
A result to understand, not just read.
Initial pregnancy dating starts from the first day of the last menstrual period, not the date of conception. The standard method adds 280 days and adjusts for a cycle that is longer or shorter than 28 days.
FREQUENTLY ASKED QUESTIONS
Is the estimated due date the exact birth date?+
No. It is a dating reference, not a prediction of the exact day labour will begin.
What if my cycles are irregular?+
The estimate becomes less reliable. Early ultrasound dating and the date confirmed by a healthcare professional should take priority.
RESULT GUIDE
How the estimated due date is calculated
The calculation uses the conventional 40-week pregnancy count from the last menstrual period and adjusts the initial estimate for average cycle length.
Example with a 28-day cycle
- first day of last menstrual period: 1 January 2026
- average cycle: 28 days
A 31-day cycle would move the initial estimate three days later.
How to interpret the result
- Gestational weeks are conventionally counted from the last menstrual period.
- Early ultrasound may date the pregnancy more reliably when the cycle or LMP is uncertain.
What the estimate does not include
- Assisted-conception pregnancies require the relevant clinical dates.
- The date confirmed in the pregnancy record by the healthcare team takes precedence.
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