How Due Dates Are Calculated

Naegele's rule: last period + 280 days

The standard method is named for Franz Naegele, a German obstetrician who popularised it in the early 19th century. It takes the first day of the last menstrual period (LMP) and adds 280 days — 40 weeks. Equivalently, add a year, subtract three months, add seven days.

The rule embeds assumptions: a 28-day cycle with ovulation on day 14. If your cycle is regularly 32 days, ovulation likely falls around day 18, and the estimate should shift about four days later. This site's calculator applies exactly that adjustment: 280 + (your cycle length − 28). With irregular cycles, the LMP anchor weakens, which is one reason early ultrasound exists.

Conception + 266 days

Development starts at fertilisation, not at the period. Since conception typically occurs about 14 days after the LMP in the textbook cycle, counting from conception subtracts those 14 days: 280 − 14 = 266 days, or 38 weeks. This method suits people who tracked ovulation, used fertility treatment with known dates, or otherwise have a reliable conception estimate. Its weakness is that "conception date" is itself fuzzy — sperm can survive for several days, so fertilisation may lag intercourse.

Ultrasound dating

An ultrasound converts a measurement into an age. In the first trimester the crown–rump length grows so predictably that it can date a pregnancy within a few days; the due date is then the scan date plus the days remaining to reach 280. Because growth rates diverge later on, dating is rarely revised based on late scans — a large measurement late in pregnancy usually means a large baby, not a wrong date.

Why dates change — and why that's normal

If an early scan and the LMP method disagree by more than a few days, many providers adopt the scan date. Nothing has gone wrong: the LMP method's assumptions (28-day cycle, day-14 ovulation, perfectly recalled dates) are approximations, and the scan measures the pregnancy directly. Either way, the output is an estimate. Only around 5% of babies arrive on the estimated day itself; most arrive within about a week either side, and "term" spans several weeks.

Which method should you use?

• Regular cycles, confident dates: Naegele's rule with your cycle adjustment is a reasonable first estimate.
• Known ovulation or treatment dates: the conception method removes the day-14 assumption.
• An early scan on record: use the ultrasound date — and if your provider has set a date, theirs is the one to plan around.

Try each method on the due date calculator and compare. Small differences of a few days are expected and rarely matter.