Calculates the number of pregnancy weeks on a given date, based on the last menstrual period or the due date.
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Calculation basis and official references
Reviewed on:
This calculator is not a diagnosis, treatment plan, or prescription. Please consult a qualified clinician about your medical decisions.
Formula basis
The number of days between the reference date (last menstrual period, or the due date minus 280 days) and the date to check is divided by 7 to get whole weeks and a remainder in days (0-6, or negative if before the reference date).
Calculation method
Calculates pregnancy weeks on a given date, using either the last menstrual period or the due date as the reference date.
Exclusions and checks required
Results can differ from the actual gestational age when the menstrual cycle is not 28 days or ovulation timing varies (see CQ009 of the JSOG/JAOG obstetric practice guideline).
Ultrasound-based dating should be prioritized clinically; this calculation is a simple reference based on the last menstrual period.
Example input:：Last menstrual period January 1, 2026, checking April 1, 2026
method
Last menstrual period
base
2026-01-01
target
2026-04-01
12 weeks 6 days is a theoretical value based on the last menstrual period and can shift with actual ovulation timing.
Who is this calculator for? Pregnancy Week Calculator
Calculates the number of pregnancy weeks on a given date, based on the last menstrual period or the due date.
What values should I enter?
Applies to: Pregnancy & growth
How should I interpret this estimate?
Calculates pregnancy weeks on a given date, using either the last menstrual period or the due date as the reference date.
How to use
Enter your input values and press Calculate.
Review the result and compare it with expected context.
The number of days between the reference date (last menstrual period, or the due date minus 280 days) and the date to check is divided by 7 to get whole weeks and a remainder in days (0-6, or negative if before the reference date).