Calculate your estimated due date and pregnancy milestones based on the first day of your last menstrual period (LMP).
How the Due Date Is Calculated
This calculator applies Naegele's rule: it adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). The rule assumes ovulation on day 14 of a 28-day cycle, so entering your actual average cycle length adjusts the estimate — a longer cycle shifts ovulation and the due date later, a shorter cycle shifts both earlier.
What Happens Each Trimester
Pregnancy is split into three roughly 13-week trimesters. The first trimester (weeks 1–13) covers early organ development, the second (weeks 14–27) is when most anatomy scans happen, and the third (weeks 28–40) covers final growth before delivery. The calculator shows your current week and trimester alongside the due date.
Why the Date Is an Estimate, Not a Guarantee
Only about 4–5% of babies arrive on their exact calculated due date. Full-term delivery spans a window of roughly two weeks before to one week after the estimate, since actual ovulation timing, cycle length, and individual gestation length all vary. An early-pregnancy ultrasound dating scan is more accurate than LMP-based estimates for cycles that are irregular or not close to 28 days.
How is the due date calculated?
The due date is calculated using Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period. This assumes a 28-day cycle, adjustable to your own average cycle length.
How accurate is a due date calculated from LMP?
Only about 4–5% of babies are born on their exact calculated due date. Full-term delivery normally falls within a window of roughly two weeks before to one week after the estimate.