Last menstrual period dating

The traditional method counts 280 days—40 weeks—from the first day of the last menstrual period. It assumes a 28-day cycle with ovulation near day 14. The due-date calculator can adjust the arithmetic for a different average cycle length, but it cannot know the actual ovulation day.

Conception-date dating

Estimated conception dating adds about 266 days. Conception is typically counted about two weeks after gestational age begins, which explains why pregnancy may be described as two weeks along around conception.

Unless conception timing is documented through fertility treatment, the exact date can be uncertain.

How early ultrasound contributes

Early ultrasound measurements can provide dating information when menstrual dates are uncertain or the difference meets clinical criteria. A clinician interprets the measurement, timing, cycle history, and established guidelines together.

IVF embryo-transfer dating

IVF dating uses the known transfer date and embryo age. A day-5 embryo transfer has already completed five days of development, so 261 days are added; a day-3 transfer uses 263 days. Clinic records should determine the official date.

Why due dates may change

  • LMP is uncertain or cycles are irregular.
  • Ovulation occurred earlier or later than assumed.
  • Early ultrasound differs enough to meet redating criteria.
  • The original date or cycle length was entered incorrectly.

Once established, the official estimated due date should be documented and changed only using clinical guidance.

The due date is not a deadline

Few births occur on the exact estimated date. The date helps organize gestational age, testing, growth assessment, and care planning. It cannot guarantee when labor begins.

What a calculator cannot do

An online calculator cannot confirm pregnancy, determine viability, interpret bleeding or pain, or replace prenatal care. Seek prompt medical help for concerning symptoms.

Frequently asked questions

Why are pregnancy weeks counted before conception?

Gestational age traditionally begins on the first day of the LMP, about two weeks before conception in a typical cycle.

Which due-date method is most accurate?

It depends on the information available. Documented IVF timing and early ultrasound can be particularly useful, while clinicians apply formal dating guidance.

Can a due date change late in pregnancy?

Late measurements are less precise for dating. Official dates are generally established early and changed only under clinical criteria.

Does cycle length affect LMP dating?

Yes. A longer or shorter cycle can shift the arithmetic, but cycle length still does not prove the exact ovulation date.

Sources

References and further reading

Last reviewed: July 13, 2026