Estimate your due date, current week of pregnancy, and trimester based on the first day of your last menstrual period.
Enter the first day of your last menstrual period (LMP) and your average cycle length, then click "Calculate Due Date." The tool will estimate your due date, current week of pregnancy, and trimester.
This tool uses Naegele's Rule, the standard method used by clinicians: due date = LMP + 280 days (40 weeks), adjusted by adding or subtracting the difference between your cycle length and the average 28-day cycle. Conception is estimated at roughly LMP + (cycle length - 14 days).
For an LMP of January 1 with a 28-day cycle, the estimated due date falls around October 8 of the same year, placing conception near January 15 and the pregnancy in its second trimester by around week 14.
How accurate is this estimate? Naegele's Rule gives a helpful estimate, but only about 5% of babies arrive exactly on the calculated date; a normal full-term delivery can occur anywhere from 37 to 42 weeks.
Why does cycle length matter? A longer or shorter cycle shifts the likely date of ovulation and conception, so adjusting for it produces a more personalized estimate than assuming a fixed 28-day cycle for everyone.
Most commonly using Naegele's rule: adding 280 days (40 weeks) to the first day of your last menstrual period.
Conception date is often uncertain, while the last menstrual period date is more reliably known.
It's an estimate — only a small percentage of babies are born on their exact due date, with a normal range of about two weeks either side.