The standard estimate adds 280 days (40 weeks) to the first day of the last menstrual period, an approach known as Naegele rule. It assumes a regular 28 day cycle, so it is an estimate and a care provider may adjust it with an ultrasound. A free due date calculator does the date math in your browser.
Free pregnancy due date calculator. Estimate your due date from your last menstrual period using Naegele rule, plus current week and trimester.
Open Pregnancy Due Date Calculator → Free toolFree date difference calculator. Count the days, weeks, months and years between two dates, with or without including the end date, instantly.
Open Date Difference Calculator →The classic method, Naegele rule, adds 280 days to the first day of the last menstrual period, which is the same as counting back three months and adding seven days. It counts pregnancy from the last period rather than from conception, because the last period is a date most people can pin down, while the exact conception date usually is not.
The 280 day rule assumes a regular 28 day cycle with ovulation around day 14, so a longer or shorter cycle shifts the real date. Only about a small share of births actually land on the estimated day; most arrive in the weeks around it. A care provider often refines the estimate using an early ultrasound. This calculator does the date arithmetic; it is not medical advice.
The first day of the last menstrual period. The standard method adds 280 days, or 40 weeks, to that date.
Because the last period is a date most people know precisely, while the exact conception date usually is not. The 40 week count is measured from the last period.
It is an estimate. Most babies arrive in the weeks around the date rather than exactly on it, and a provider may adjust it with an ultrasound.