About this pregnancy due date calculator
Estimate a pregnancy due date from the first day of your last menstrual period, adjusted for your average cycle length, using Naegele's rule, the standard method taught in obstetric care. Enter the date and your typical cycle length to see the estimated due date, how many weeks and days into the pregnancy you are as of today or another date, and which trimester that falls in. This is a widely used population estimate; only around one in twenty births happen exactly on the calculated due date, so an ultrasound dating scan from a healthcare provider is more precise for any individual pregnancy.
How the pregnancy due date calculator works
Naegele's rule adds 280 days (40 weeks) to the first day of the last menstrual period, then adjusts by the difference between your cycle length and the textbook 28-day cycle. Weeks and days pregnant are counted from that same start date to the comparison date.
Pregnancy Due Date Calculator examples
Last period 2026-01-01, 28-day cycle
Estimated due date 2026-10-08
Last period 2026-01-01, 30-day cycle
Estimated due date 2026-10-10
Limitations & privacy
Naegele's rule assumes regular cycles and is a population-level estimate, not a diagnosis. It does not account for irregular cycles, assisted conception dating, or ultrasound measurements. This tool is not a substitute for prenatal care; confirm dating with a healthcare provider.
Inputs and results stay in this browser. Only tool identifiers are stored for your recently used tools. You can clear that history from the directory.
Pregnancy Due Date Calculator: frequently asked questions
How accurate is this estimate?
It is a widely used starting estimate. Only a small share of babies arrive on the exact calculated date; a full-term birth any time from about three weeks before to two weeks after is considered normal.
What if my cycle isn't 28 days?
Enter your typical average cycle length and the calculation shifts the due date to account for it, since ovulation timing shifts with cycle length.
Which trimester does each week fall into?
This tool reports weeks under 13 as the first trimester, 13 through 26 as the second, and 27 onward as the third, a common convention.
Updated September 20, 2026