Naegele's rule: the 280-day estimate
Most due dates in the world are first estimated with a rule from the early 1800s, named after the German obstetrician Franz Naegele. Take the first day of the last menstrual period (LMP), and add 280 days — 40 weeks. A period that began on January 1 points to a due date of October 8. The rule quietly assumes a textbook 28-day cycle with ovulation on day 14, and that assumption is exactly why the answer is an estimate: real cycles run shorter and longer, ovulation drifts, and no two pregnancies share a calendar. What the rule gives you is a reliable centre for the window in which the birth is most likely, which is how midwives and doctors actually use it.
Counting from conception instead
If the conception date is known — common with fertility treatment, ovulation tracking, or simply a memorable date — add 266 days, or 38 weeks. The 14-day difference between the two methods is the assumed gap between the first day of the last period and conception in that textbook cycle. When both dates are known and the two methods disagree, the conception-based figure is generally the closer one, because it skips the guess about when ovulation happened — though it still cannot know exactly when implantation followed.
How many weeks along?
When the LMP date you enter is in the past, the calculator also reports how far along the pregnancy would be today, in completed weeks and days, and how many days remain until the estimate. Pregnancy is counted from the LMP, which is why a person is "four weeks pregnant" at roughly the moment a test first turns positive, even though conception was about two weeks earlier. Trimesters divide the 40 weeks roughly into thirds: the first trimester runs to about week 13, the second to about week 27, and the third from week 28 to the birth. A baby is considered full term between 39 and 40 completed weeks, and "early term" from 37.
Why the date moves at the first scan
An early ultrasound — ideally in the first trimester — measures the embryo directly, and in those weeks embryos grow at a remarkably consistent rate. When the scan's implied due date differs from the LMP calculation by more than about five to seven days, most practices adopt the ultrasound date. Later scans are less useful for dating, because babies' growth rates diverge. None of this makes a calculator like this one pointless: it gives the estimate that starts the conversation, plans the first appointments, and tells a family which month to circle. Just circle it in pencil.
Frequently asked questions
How is a due date calculated from the last period?
By Naegele's rule: add 280 days (40 weeks) to the first day of the last menstrual period. A last period starting January 1 gives a due date of October 8 of the same year. The rule assumes a 28-day cycle with ovulation on day 14.
How accurate is a calculated due date?
It is an estimate, not a prediction. Only about 1 in 20 babies arrive on their exact due date; most are born within a week or two either side of it, and a first-trimester ultrasound often adjusts the date by several days. Treat it as the centre of a likely window.
How is the due date calculated from conception?
Add 266 days (38 weeks) to the conception date. The figure is 14 days shorter than the LMP method because conception typically happens about two weeks after the first day of the last period in a textbook cycle.
Why might my doctor's due date differ?
Cycles longer or shorter than 28 days shift ovulation, and early ultrasound measures the baby directly, which is more accurate than calendar rules in the first trimester. Doctors usually revise the date when an early scan disagrees with the LMP calculation by more than a few days.
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