Count the days from the first day of the last menstrual period to today. Divide by 7. The whole number is the weeks. The remainder is the extra days.
That count is gestational age. It is the number clinicians usually mean by “how many weeks.” It starts about two weeks before conception in a 28-day cycle, so it is ahead of the time since fertilization.
The result is an estimate from a date you enter. It does not confirm a pregnancy.
Example
Last period on 1 June, and today is 20 August.
1 June to 20 August is 80 days.
80 ÷ 7 = 11 weeks and 3 days.
The pregnancy week calculator does this from the period date and also estimates a due date by adding 280 days, shifted if the cycle is not 28 days. A future period date does not produce a pregnancy week.
What the number is not
- It is not months. Week 11 is month 3 on the weeks-to-months chart, not “month 2.5” from dividing by 4.
- It is not fetal age. Fetal age is about two weeks less when ovulation was near day 14.
- A longer cycle does not change the raw count from the period date. It does change the estimated due date. If ovulation was known and was not day 14, the ovulation due date is the better dating start.
Trimester from the same count
On this site, weeks through 12 are the first trimester, weeks 13–27 are the second, and week 28 onward is the third. Eleven weeks and 3 days is still the first trimester. The boundaries, with calendar dates for one example period, are in trimester dates.
An ultrasound date replaces this count when the two disagree. How the due date is calculated is the 280-day step that sits next to the week count.