How do I calculate my due date?

Naegele's rule, step by step

The standard method, Naegele's rule, starts from the first day of your last menstrual period. Add 7 days, subtract 3 months, and add 1 year. For example, a last period that began on May 10, 2025 gives an estimated due date of February 17, 2026.

Pregnancy is dated from the first day of your last period, not from conception, which is why a full-term pregnancy is counted as 40 weeks (280 days) even though fertilization happens about two weeks into that count. The rule assumes a regular 28-day cycle with ovulation around day 14.

Why ultrasound dating is more precise

An ultrasound in the first trimester, up to and including 13 weeks 6 days, is the most accurate way to establish gestational age. It measures the crown-rump length of the embryo or fetus, and the earlier in the first trimester it is done, the more accurate it is, within about 5 to 7 days.

Your provider compares the ultrasound estimate with the date from your last period. If the two differ by more than a set amount, the ultrasound date is used instead. A simple version of this rule: in the first trimester, a difference greater than about 5 to 7 days means the ultrasound date takes over.

• Up to 8 weeks 6 days: redate if the difference is more than 5 days
• 9 to 13 weeks 6 days: redate if the difference is more than 7 days
• 14 to 15 weeks 6 days: redate if the difference is more than 7 days
• 16 to 21 weeks 6 days: redate if the difference is more than 10 days
• 22 to 27 weeks 6 days: redate if the difference is more than 14 days
• 28 weeks and later: redate if the difference is more than 21 days

What "due date" really means

A due date marks when the pregnancy reaches 40 weeks. It is an estimate, not a prediction of the delivery day. Only about 5 percent of babies arrive on their exact due date, and roughly 90 percent are born within about two weeks on either side.

Providers use term categories to describe timing. Early term is 37 weeks 0 days through 38 weeks 6 days, full term is 39 weeks 0 days through 40 weeks 6 days, late term is 41 weeks 0 days through 41 weeks 6 days, and postterm is 42 weeks 0 days and beyond.

If your cycles are irregular

Naegele's rule depends on a fairly regular cycle. If your cycles are irregular, you are unsure of your last period, or you were using hormonal birth control recently, the date from your last period may be off.

In those situations, an early ultrasound is the more reliable way to date the pregnancy, and your provider will rely on it rather than the calendar calculation.

Related questions

How is a due date calculated from the last period?

Use Naegele's rule: take the first day of your last menstrual period, add 7 days, then subtract 3 months (and add a year). This sets a 40-week pregnancy dated from the first day of your last period.

Is an ultrasound or my last period more accurate for dating?

A first-trimester ultrasound is more accurate. It measures crown-rump length to within about 5 to 7 days. If it differs from your last-period date by more than the threshold for that point in pregnancy, your provider uses the ultrasound date.

What counts as full term?

Full term is 39 weeks 0 days through 40 weeks 6 days. Early term is 37 to 38 weeks 6 days, late term is 41 weeks, and postterm is 42 weeks and beyond.

What if I don't know the date of my last period?

If you are unsure of your last period or your cycles are irregular, an early ultrasound is the most reliable way to estimate your due date, and your provider will date the pregnancy from it.

Sources & further reading

1. ACOG — Methods for Estimating the Due Date (Committee Opinion 700)
2. ACOG — Definition of Term Pregnancy (Committee Opinion 579)
3. StatPearls (NCBI Bookshelf) — Estimated Date of Delivery
4. Mayo Clinic — Prenatal care: First trimester visits

This article reflects current AAP, CDC, and other public-health guidance and is for educational purposes only. It does not constitute medical advice. ParentFlow is a wellness companion — not a substitute for your pediatrician. For any medical concern, contact your healthcare provider.