Find the due date from the last period, how many weeks along you are, and the trimester dates. Adjusts for a cycle longer or shorter than 28 days.
Why is pregnancy dated from my last period, not conception?
Because the last period is a date you can actually name, and conception usually is not. It adds about two weeks to the count, which is why you are considered four weeks pregnant when a test first turns positive rather than two.
What if my cycle is not 28 days?
Enter your own length. A 32 day cycle means you ovulate around day 18 rather than day 14, so the due date moves four days later. A short cycle moves it earlier by the same logic. It makes a real difference for anyone whose cycle is consistently long.
How accurate is Naegele's rule?
Close, and usually within a day or two of the 280 day count. It differs because calendar months are not equal: subtracting three months across February loses fewer days than across a run of 31 day months. This page shows both so a date you were told elsewhere makes sense.
Will my baby arrive on the due date?
Almost certainly not. Roughly one in twenty babies arrives on the estimated date, and anything from 37 to 42 weeks counts as full term. Treat it as the middle of a window, not a deadline.
Is a dating scan better than this?
Yes, and by a distance in the first trimester. A scan before 13 weeks measures the baby directly and is accurate to within about five days, so your doctor will revise the date to match it if the two disagree. Use this for planning between appointments.
When does each trimester start?
Counting from the last period: the first trimester runs to the end of week 13, the second from week 14 to the end of week 27, and the third from week 28 to birth. The dates for your own pregnancy are in the table above.
What if I do not remember my last period?
Then this page cannot help and a dating scan is the answer. Guessing the date puts the whole schedule out, including tests that are only valid inside a narrow window.