How it works
Gestational age is counted from the first day of your last menstrual period. "8 weeks, 3 days" means eight full weeks plus three days have passed, so you are in your ninth week.
Trimesters here follow the common clinical convention: first trimester through week 13, second from week 14 through 27, and third from week 28 until birth. If you enter a due date, the calculator counts back 280 days to find the starting point.
Your care provider may adjust dates after an ultrasound, and their dating takes priority. Results are estimates, not medical advice.
Formula
Gestational age = today − first day of last period (due date − 280 days if dating from the due date)
Questions people ask
Am I in week 8 or week 9 if I am 8 weeks pregnant?
At 8 weeks and some days, you have completed eight weeks and are in your ninth week. Apps and providers sometimes describe it either way.
When does each trimester start?
The second trimester is usually counted from week 14 and the third from week 28. Some sources start the second trimester at week 13, so dates can differ by a week.
How many months pregnant is 20 weeks?
About four and a half months. Because months vary in length, week counts are more precise, which is why providers use them.