Pregnancy · HC-011

Pregnancy Due Date by Last Menstrual Period

Pregnancy is conventionally dated from the first day of the last menstrual period (LMP), even though conception generally occurs later. This calculator adds 40 weeks to that date and optionally adjusts for cycle length.

Enter your values

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Result

Enter your values and select Calculate to see the result and an explanation.

Educational support only. Results are not medical advice and cannot replace professional assessment or your institution's protocols.

How the calculation works

Estimated due date = LMP + 280 days. As a simple approximation for regular cycles of different lengths, add the difference between your cycle length and 28 days.

Worked example

With LMP on January 1, an uncomplicated 28-day-cycle estimate gives October 8 as the due date in a non-leap year.

Understanding the result

The due date is an estimate, not a prediction of the exact birth date. Actual delivery dates vary, even in healthy pregnancies.

A clinician may use early ultrasound, assisted reproduction dates, and other information to establish the best obstetric estimate. Do not change an established clinical due date based only on this web tool.

Limitations

Assumes a known LMP and a relatively regular cycle. Irregular periods, uncertain LMP, and fertility treatment need individualized dating.