One of the first questions after a positive pregnancy test is “when is the baby due?” Here is how that date is estimated, and why it is an estimate. Find yours with the pregnancy due date calculator.
Naegele’s rule
The standard method counts 280 days (40 weeks) from the first day of your last menstrual period (LMP). A shortcut: take the first day of your LMP, subtract three months, add seven days and add one year.
Example: LMP on January 10 gives a due date around October 17.
Why count from the last period?
The exact day of conception is usually unknown, while the start of a period is easy to remember. Because ovulation typically happens about two weeks after the period starts, you are considered “2 weeks pregnant” at conception.
Adjusting for cycle length
Naegele’s rule assumes a 28-day cycle. If your cycles are usually 32 days, ovulation likely happened about four days later, so the due date moves about four days later too. Our calculator makes this adjustment.
Ultrasound dating
An early ultrasound, usually in the first trimester, measures the baby and is often more accurate than the LMP method, especially if periods are irregular. Your doctor or midwife may update your due date after this scan.
How many babies arrive on time?
Only a small percentage are born on the exact due date. Most are born within about two weeks of it. A pregnancy is considered full term from 39 weeks, and early term from 37 weeks.
Trimesters at a glance
- First trimester: weeks 1 to 13
- Second trimester: weeks 14 to 27
- Third trimester: week 28 to birth
This article is general information. Your prenatal care provider is the best source for dates and care decisions.