Pregnancy Week Calculator
Estimate completed pregnancy weeks and days from the first day of the last menstrual period (LMP). The tool also shows a standard 40-week due-date estimate for a 28-day cycle. It is a date-based estimate, not confirmation of pregnancy or a replacement for antenatal care.
Use the Pregnancy Week Calculator
How the calculation works
Gestational age is counted from the first day of the LMP. Divide the elapsed days by seven to obtain completed weeks and remaining days. The displayed standard due date adds 280 days to the LMP and does not adjust for irregular cycles, ovulation timing or ultrasound findings.
Worked example
An LMP of 1 September 2026 checked on 5 October 2026 gives 34 elapsed days: 4 completed weeks and 6 days. The standard 280-day due-date estimate is 8 June 2027. A clinician may assign a different gestational age or due date.
Completed weeks versus the week you are in
At 4 weeks and 6 days, four full weeks have been completed and the fifth week is in progress. Recording both weeks and days is clearer than rounding the result upward. The calculator does not infer a trimester diagnosis or fetal development from the date.
When to use another dating method
IVF dates, scan-based dates and clinical dating can differ from a simple LMP estimate. Keep the source of the date clear in your records. The Pregnancy Due Date article explains due-date context, while the Baby Age article concerns calendar age after birth.
Important limitations
Use the full date for the first day of the period, not its final day. If it is uncertain, cycles are irregular or fertility treatment was used, the estimate may be unsuitable. NHS guidance notes that a dating scan can more accurately estimate pregnancy weeks. Follow the date supplied by your maternity team.
Frequently asked questions
Does this confirm pregnancy?
No. It only calculates an interval from an entered date.
Can the due date change?
Yes. Clinical information and dating scans can lead your maternity team to revise it.
Reference
NHS pregnancy due-date guidance. The method and limitations above identify what this page does and does not infer from that reference.
