Estimates only - not medical advice.
Pregnancy is dated from the last menstrual period (LMP), about 280 days to the due date. Weeks 1-12 are the first trimester, 13-26 the second, 27+ the third.
Find more pregnancy and fertility calculators on the home page.
From the first day of your last period (LMP).
1st: wk 1-12; 2nd: 13-26; 3rd: 27 to birth.
Either - we derive the other.
No - confirm with your provider.