Last updated 23 August 2026
How this is calculated
This uses Naegele's rule — the standard method of adding 280 days (40 weeks) to the first day of your last menstrual period. It assumes a typical 28-day cycle, which is why it's a starting estimate rather than a fixed date. An early ultrasound, if you have one, usually gives a more precise figure your doctor will use going forward.
Method, limitations and sources
Formula. This uses Naegele’s rule: the estimated due date is 280 days (40 weeks) after the first day of your last menstrual period. The current week of pregnancy is counted from that same date, which is why “weeks pregnant” includes roughly two weeks before conception.
Inputs it needs. The first day of your last menstrual period.
What it assumes. A regular 28-day cycle with ovulation around day 14. If your cycle is longer, shorter or irregular, or if you are unsure of the date, the estimate shifts accordingly and can be out by a week or more.
An ultrasound or clinician estimate takes priority. The American College of Obstetricians and Gynecologists is explicit that a first-trimester ultrasound is the most accurate way to establish gestational age, and that the “best obstetric estimate” — not a date calculated from the last period alone — is what should be used for care. If your midwife, obstetrician or scan has given you a due date, use theirs, not this one. Pregnancies conceived through IVF or other assisted reproduction should be dated from the embryo age and transfer date instead.
When not to use it. This is a rough planning date only. Very few births happen exactly on the estimated date. Do not use it to make decisions about medication, travel, tests or anything else clinical — that is a conversation with your midwife or doctor.
- ACOG Committee Opinion No. 700 — Methods for Estimating the Due Date (with the American Institute of Ultrasound in Medicine and the Society for Maternal–Fetal Medicine).
Common questions
How is the due date calculated?
By adding 280 days (40 weeks) to the first day of your last menstrual period — known as Naegele's rule. It assumes a fairly typical 28-day cycle, which is why a doctor may adjust the estimate after an early ultrasound.
Why might my doctor give a different date?
Ultrasound measurements, especially in the first trimester, are often more precise than a date based on the last period, particularly if your cycle isn't exactly 28 days. Your doctor's estimate, once you have one, is the more reliable figure.
How accurate is a due date, generally?
Only about 1 in 20 babies are born on the exact estimated date. It's a useful planning midpoint — most births happen within about two weeks either side of it.