What a pregnancy due-date estimate means
Understand the assumptions behind a last-period estimate and why clinical dating matters.
Put it into practice
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A date estimate uses assumptions
A conventional last-period calculation counts 280 days from the first day of the last menstrual period. It assumes a regular 28-day cycle. ToolMallow allows a regular-cycle adjustment, but the output remains an estimate.
Put this guide into practice with the Pregnancy Due Date. Review the tool’s inputs, example and limitations alongside your result.
Use the correct starting day
Enter the first day of the period, not the final day. If you are unsure of that date or have irregular cycles, a calendar calculation can be less reliable.
Clinical dating can be more accurate
ACOG describes early ultrasound as the most accurate way to establish or confirm gestational age. The NHS also explains that an antenatal dating scan can estimate how many weeks pregnant you are more accurately.
Keep your maternity clinician's date
This calculator cannot establish a diagnosis or replace your clinician's dating assessment. It should not be used to decide when to seek care or to revise a due date already established in clinical care. For assisted conception, ask your maternity team which dating method applies.
Sources: ACOG dating guidance and NHS due-date calculator.
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