Due Date Calculator

What The Estimate Means

The estimated due date is a dating reference, not a promise about the day labor will begin. This tool counts 280 days, or 40 weeks, from the first day of the last menstrual period (LMP).

Gestational age is conventionally counted from the LMP, about two weeks before conception in a textbook 28-day cycle. That convention explains why pregnancy dating can appear older than time since fertilization.

Formula And Assumptions

Naegele's rule assumes the LMP date is known, cycles are reasonably regular, and ovulation occurred near the expected time. Cycle length differences, uncertain bleeding dates, recent hormonal contraception, and assisted reproduction can require a different method.

Worked Example

If the first day of the LMP was January 1, adding 280 calendar days gives an estimated due date around October 8 in a non-leap year. At 70 elapsed days, gestational age is 10 weeks 0 days.

How Clinical Dating Can Change

ACOG recommends establishing and documenting gestational age using the LMP, the first accurate ultrasound, or assisted-reproduction dating as appropriate. Early ultrasound may revise an LMP estimate when the dates disagree beyond clinical thresholds.

Use the date confirmed by the prenatal clinician for medical decisions, screening windows, and delivery planning.

When To Seek Care

This calculator cannot confirm pregnancy location, viability, or fetal health. Contact a qualified clinician for prenatal care and for pain, heavy bleeding, fainting, severe symptoms, or uncertainty about dates. Urgent symptoms require urgent medical assessment.