How a due date is estimated
Pregnancy is counted from the first day of the last menstrual period, not from conception. A typical pregnancy is 280 days (40 weeks) from that day. This is the method most doctors use, often called Naegele's rule.
From conception: due date = conception + 266 days
IVF: day-5 transfer + 261 days, day-3 transfer + 263 days
If your cycles are longer or shorter than 28 days, ovulation usually happens later or earlier, so the calculator moves the date by the difference.
How accurate is it?
Only a small share of babies arrive exactly on the due date; most come in the weeks around it. An early ultrasound measurement is usually more accurate than a date from the last period, and your doctor or midwife may change your due date after the first scan.
This calculator gives an estimate for information only and is not medical advice. Please follow the due date and care plan from your doctor or midwife.
Frequently asked questions
How many weeks is a full-term pregnancy?
Full term is from 39 weeks 0 days to 40 weeks 6 days. Babies born from 37 weeks are called early term.
Why is pregnancy counted from my last period?
The day of conception is rarely known, while the first day of the last period usually is. Counting from it gives a shared starting point.
My ultrasound gave a different date. Which is right?
Follow the date from your doctor or midwife. Early ultrasound measurements are generally more precise than the last-period method.
Sources
ACOG: Methods for Estimating the Due Date
Last reviewed: November 2026 · By Muhammad Shujah