How accurate is the due date?

It's an estimate — only about 5% of babies arrive on the exact date. Most births happen within two weeks either side. An early ultrasound (before ~13 weeks) dates the pregnancy more precisely than the last period, especially with irregular cycles.

What are the pregnancy trimesters?

First trimester: weeks 1–13. Second: weeks 14–27. Third: weeks 28–40. Pregnancy is counted from the last menstrual period, so 'week 1' precedes conception by about two weeks.

Pregnancy Due Date Calculator

Q: How is a due date calculated?

Naegele's rule adds 280 days (40 weeks) to the first day of your last menstrual period, which assumes a 28-day cycle and ovulation on day 14. From a known conception date, the due date is about 266 days later.

Estimate your due date and how far along you are, from your last period or a known conception date.

Estimated due date

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How far along

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Trimester

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Conception (est.)

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Days remaining

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Naegele's rule

Due date = First day of last period + 280 days (40 weeks)
From conception: + 266 days (38 weeks)

How it works

Pregnancy is counted from the first day of the last period — about two weeks before conception — so "40 weeks" reflects roughly 38 weeks of actual gestation. If your cycle isn't 28 days, the estimate shifts by the difference from 28.

What the estimate means

A due date is a helpful midpoint, not a deadline: only about 1 in 20 babies arrive on the exact day, and a "full-term" birth is anywhere from 37 to 42 weeks. An early dating ultrasound (before roughly 13 weeks) is more accurate than period-based dating, particularly with irregular cycles — your provider's date takes precedence over any calculator. Track healthy weight gain over the pregnancy with the pregnancy weight gain calculator.

Frequently asked questions

How is a due date calculated?

Naegele's rule: last period + 280 days (or conception + 266 days).

How accurate is it?

An estimate — ~5% arrive on the date; most within two weeks. Ultrasound dating is more precise.

What are the trimesters?

First 1–13, second 14–27, third 28–40 weeks, counted from the last period.

Related calculators

Medical disclaimer: This provides an estimate only and is not medical advice or a substitute for prenatal care. Your healthcare provider's due date, based on examination and ultrasound, is authoritative. Formula: Naegele's rule. Last reviewed: July 2026.