Ask when a baby is due and you'll get a single confident date. But that date is a statistical midpoint, not a deadline — only about one in twenty babies actually arrives on it. Understanding how the number is produced makes the whole timeline less mysterious and a lot less stressful.
The 40-week starting point
The standard estimate uses Naegele's rule: take the first day of your last menstrual period (LMP), add one year, subtract three months, and add seven days. That works out to roughly 280 days — 40 weeks — from the LMP.
Why count from your last period?
Conception is hard to pin down, but the last period is a date most people can name. Dating from the LMP means the count starts about two weeks before conception actually happens — which is why pregnancy is described as 40 weeks even though the baby only exists for about 38 of them.
Why cycles bend the estimate
Naegele's rule quietly assumes a textbook 28-day cycle with ovulation on day 14. Real cycles vary. If yours runs longer, you likely ovulate later, so a fixed LMP-based date can land a few days early. Adjusting for cycle length gives a more personal estimate.
Due Date Calculator
Enter your last period and cycle length for an adjusted estimate, plus your current week.
When an ultrasound moves the date
An early ultrasound measures the embryo directly, and in the first trimester that measurement is a more reliable dating method than the LMP. If the scan disagrees with your period-based date by more than a few days, clinicians usually adopt the ultrasound date. A shifting due date isn't a red flag — it's the estimate getting more accurate.
An estimate, not a promise
A normal, full-term birth can happen anywhere from 37 to 42 weeks. Treat the due date as the centre of a range, not a countdown to a fixed day.
Knowing the method behind the date turns it from a mysterious verdict into what it is: a well-reasoned best guess, refined as better information arrives.