Estimate Delivery Date — How Due Date Calculators Work

An estimated delivery date is the date a baby is expected to be born. It is calculated from the mother's last menstrual period, conception date, or early ultrasound measurements.

Naegele's rule

The most common due date method is Naegele's rule: add 280 days to the first day of the last menstrual period. For example, an LMP of 1 March 2026 becomes a due date of 8 December 2026. This assumes a 28-day cycle with ovulation on day 14.

Conception-based dating

If you know the approximate conception date, add 266 days (38 weeks). This is often shorter than LMP-based estimates by about two weeks.

Ultrasound dating

First-trimester ultrasounds are the most accurate way to estimate gestational age, with an error margin of about 5-7 days. Later ultrasounds are less precise.

Related calculators

Due Date Calculator

Estimate your delivery date from your last period or conception date.

Pregnancy Calculator

Track weeks, trimesters, and expected milestones.

Sources and further reading

These independent, authoritative sources inform the methods used on this page.

Frequently asked questions

How do you estimate delivery date?

Delivery date is commonly estimated by Naegele's rule: add 280 days to the start of the last menstrual period. See our <a href='/due-date-calculator/'>due date calculator</a>.

What is Naegele's rule?

Naegele's rule adds 280 days to the first day of the last menstrual period. It assumes a 28-day cycle with ovulation on day 14.

How accurate are pregnancy due date calculators?

Only about 4% of babies arrive on their exact due date. Most births fall within two weeks before or after.

Can I calculate my due date from conception date?

Yes. Add 266 days to the conception date. Use our <a href='/pregnancy-conception-calculator/'>conception calculator</a>.

What happens if my period is irregular?

Irregular cycles make LMP-based dating less reliable. Early ultrasound dating is preferred.

What is the difference between gestational age and fetal age?

Gestational age counts from the first day of the last menstrual period. Fetal age counts from actual conception.

How do pregnancy calculators work?

Pregnancy calculators use LMP, conception date, or ultrasound data. Our <a href='/due-date-calculator/'>due date calculator</a> and <a href='/pregnancy-calculator/'>pregnancy calculator</a> cover both methods.