Pregnancy due date calculator

How the estimated due date is calculated

For a 28-day cycle, this calculator adds 280 days, or 40 weeks, to the first day of the last menstrual period. That is the conventional method described by the American College of Obstetricians and Gynecologists (ACOG). When the entered average cycle differs from 28 days, the estimate moves by the same number of days, matching the cycle-length adjustment offered by the NHS calculator.

Gestational age starts before estimated conception

Pregnancy dating convention counts from the last menstrual period, not from conception. For a 28-day cycle, estimated conception is shown about 14 days after that period began. The chosen reference date is converted into completed weeks and remaining days using the same adjusted due-date timeline.

A due date is a guide, not an appointment

The output also shows a 37-to-42-week window because the NHS explains that pregnancy commonly lasts within that span when counted from the last period. Ovulation timing, implantation, cycle variation, and uncertain recall can all shift a period-based estimate.

Clinical dating can replace this estimate

ACOG states that first-trimester ultrasound is the most accurate way to establish or confirm gestational age. Fertility treatment dates and an early ultrasound may provide better evidence than a last-period calculation. Once a clinician establishes the best obstetric estimate, use that date for care planning rather than repeatedly recalculating it here.

This calculator is educational and does not confirm pregnancy, fetal development, viability, labor timing, or clinical gestational age. Contact a qualified maternity-care professional for dating and care decisions, especially if dates are uncertain, cycles are irregular, or assisted reproduction was used.