Due date calculator
See an estimated due date, current pregnancy week and trimester in one step.
Direct answer: an LMP due date is conventionally estimated as 280 days, or 40 weeks, from the first day of your last period. This calculator also adjusts that estimate for cycle length.
This uses the standard 40-week estimate and adjusts for cycle length. It cannot account for irregular ovulation or replace ultrasound dating.
Choose the first day of your last period to begin.
What this estimate means
Pregnancy dating commonly begins with the first day of the last menstrual period, placing the estimated due date 40 weeks later. That convention is useful, but it assumes a predictable cycle and approximate ovulation timing. Babies also arrive on their own schedules, so think of this as a planning date—not an appointment.
ACOG's pregnancy-dating guidance identifies first-trimester ultrasound as the most accurate way to establish or confirm gestational age. The NHS due-date guidance also notes that the 12-week scan may estimate the pregnancy more accurately. Keep the date your care team records as your official estimate.
Common questions
How is a due date calculated from the last period?
ACOG describes the conventional estimate as 280 days, or 40 weeks, from the first day of the last menstrual period. Cycle length can affect the estimate.
Is an LMP due date exact?
No. It is an estimate and assumes reasonably predictable cycle and ovulation timing. ACOG identifies first-trimester ultrasound as the most accurate method to establish or confirm gestational age.
What if I do not know my last period date?
Tell your midwife, doctor or other qualified clinician. They can use your history and an ultrasound to establish the best clinical estimate.
Want the reasoning behind pregnancy dating? Read how doctors calculate and confirm your due date.