The assumption behind LMP dating

The 280-day LMP method assumes a predictable cycle and ovulation near day 14. The calculator can shift the estimate for a longer or shorter average cycle, but “average” does not describe when ovulation occurred in one specific cycle.

Why irregular cycles create uncertainty

Cycle length can vary because of many factors, and bleeding may not always represent a typical menstrual period. If ovulation occurred later than the formula assumes, the pregnancy may appear earlier by ultrasound than the LMP estimate suggests.

Information that can help

  • The first day of the last clearly remembered normal period
  • Usual cycle range rather than only one average
  • Ovulation-test or fertility-tracking information
  • Dates of pregnancy testing
  • Early ultrasound measurements
  • Documented fertility-treatment records

Bring the information to prenatal care rather than trying to choose the “best” date online.

How ultrasound adds context

Early ultrasound measurements can be used with clinical dating guidelines when LMP is uncertain or differs enough. The timing and size of the difference matter, so a clinician should interpret the result.

Using the due-date calculator

If LMP is known, enter the date and a reasonable average cycle length in the due-date calculator. Treat the output as a discussion aid. Do not use repeated recalculation to replace an official date already established in care.

Do not delay care because dates are uncertain

Uncertainty about dates is a reason to contact prenatal care, not a reason to wait. An online calculator cannot assess symptoms, viability, or pregnancy location.

Frequently asked questions

Can cycle length adjustment make LMP exact?

No. It improves the arithmetic but cannot identify the actual ovulation day.

What if I do not remember my last period?

Tell the prenatal clinician what information is available. Ultrasound and history can contribute to dating.

Can PCOS affect due-date calculations?

Conditions associated with irregular ovulation can make simple LMP assumptions less reliable. Clinical dating is important.

Should I change my due date after every ultrasound?

No. Clinicians use formal criteria and normally establish one documented estimated due date.

Sources

References and further reading

Last reviewed: July 10, 2026