How Is a Pregnancy Due Date Calculated?
Updated August 2026
Learn how estimated pregnancy due dates are calculated from the last menstrual period or conception, why 40 weeks is an estimate and how ultrasound dating can differ.
What is an estimated due date?
A pregnancy due date is an estimate of when a baby may be born. It is not a deadline, and normal births can happen before or after the estimated date.
Pregnancy dating is commonly described in weeks and days.
Want to try the idea yourself? Use the Pregnancy Due Date & Ovulation Calculator.
Dating from the last menstrual period
A common method counts about 40 weeks, or 280 days, from the first day of the last menstrual period. This assumes a typical cycle and estimates conception at roughly two weeks after that date.
Because cycles and ovulation timing vary, this method is an estimate rather than proof of the conception date.
Dating from conception
If conception or ovulation timing is known more accurately, an estimate may be based on roughly 38 weeks from conception.
In practice, the exact day of fertilisation is often uncertain even when someone knows when intercourse occurred.
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Why ultrasound dates can differ
Early ultrasound measurements can provide another estimate of gestational age. A healthcare professional may use ultrasound dating when it is considered more reliable than menstrual dates.
Do not change medical dates or decisions based only on an online calculator.
Use the calculator as a guide
The Pregnancy Due Date & Ovulation Calculator can show an estimated due date from the dates you provide and help explain the timeline.
For pregnancy care, appointments or concerns about dates, use the due date confirmed by your maternity or healthcare team.
Try it yourself
Ready to put this into practice? Open the relevant Wid9et tools directly in your browser.