How Naegele's rule estimates your due date
Naegele's rule is the standard convention for dating a pregnancy. It takes the first day of your last menstrual period (LMP) and adds 280 days — 40 weeks — to arrive at an estimated date of delivery. The shortcut version is to count back three months from the LMP and add seven days.
Pregnancy is dated from the LMP rather than from conception because the period is an easy, observable landmark, while the exact day an egg is fertilised is rarely known. The 280-day figure builds in roughly two weeks before conception actually occurs, which is why this is a gestational rather than a fetal age.
What the trimester dates mean
Alongside the due date, the tool marks when the second and third trimesters begin. The first trimester runs from the LMP through week 12, the second from week 13 through week 27, and the third from week 28 until birth. Each phase carries its own developmental milestones and check-ups.
These boundaries are counted in completed weeks of gestation from the LMP, so they line up with how clinicians describe pregnancy progress.
Why the date is only an estimate
A due date is a target, not a deadline. Only a small minority of babies arrive on the exact day, and a delivery anywhere from about 37 to 42 weeks is considered full term.
- Irregular or uncertain LMP dates shift the estimate.
- Cycle lengths that differ from 28 days can move the true conception date.
- An early ultrasound dating scan is usually more accurate and may revise the estimate.
- First pregnancies more often run slightly past the due date.
Medical disclaimer
This estimate is for general information only and does not replace professional prenatal care. Your healthcare provider may set a different due date based on ultrasound measurements or your medical history. Always follow the guidance of a qualified clinician regarding your pregnancy.
Formula
dueDate = LMP + 280 daysFrequently asked questions
- What is Naegele's rule?
- A standard method that adds 280 days to the first day of the last menstrual period to estimate the due date.