Gestational Age Calculator
Enter the first day of the last menstrual period, or a due date, to get gestational age in weeks and days, and the trimester it falls in.
Frequently Asked Questions
How is gestational age calculated?
As the number of days since the first day of the last menstrual period (LMP), shown in completed weeks and remaining days — "19 weeks, 2 days" rather than a decimal. If you only know the due date, this page derives the LMP first (due date minus 280 days, Naegele's rule) and works from there.
Why does gestational age start counting before conception?
By medical convention, gestational age is measured from the LMP, roughly two weeks before ovulation and conception in a typical cycle — so "4 weeks pregnant" is about 2 weeks after conception, not 4. It's a dating convention, not a claim about when conception actually happened.
When does each trimester start?
This page uses the common convention: 1st trimester through week 13, 2nd trimester weeks 14–27, 3rd trimester from week 28 on. Some sources draw the lines slightly differently by a week either way.
Can I calculate gestational age on a past or future date?
Yes — change "as of" to any date to see what the gestational age was, or will be, on that day instead of today.