How this calculator works
Gestational age is counted from an equivalent menstrual dating start, about two weeks before fertilisation in the standard convention. Use a clinician-established estimated due date where available. The reverse mode accepts completed weeks plus zero to six days. This calculator uses 40 weeks at the due date and rejects dates before the dating start. An equivalent start is not proof of an actual menstrual period. The result is an estimate for information only and is not medical advice.
Formula
Gestational days = 280 + assessment date - due date. Due date = assessment date + 280 - gestational days.
Worked example
An established due date of 2026-10-08 corresponds to 12 weeks 6 days on 2026-04-01. On the due date the age is 40 weeks 0 days.
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Frequently asked questions
Will the date be exact?
No. This is calendar arithmetic under the stated assumptions. Clinical dating and biological variation can change the interpretation.
Does a calendar confirm pregnancy?
No. A date estimate cannot confirm pregnancy, ovulation or fetal wellbeing.
Why use calendar days?
Whole days keep the calculation stable across daylight-saving changes. A day here is a date interval, not a time-of-day measurement.