What Is Gestational Age?
Definition
Gestational Age: The time since the first day of the last period, in weeks and days. The estimated due date is at 40 weeks 0 days.
Gestational age is how long a pregnancy has lasted, in weeks and days from the first day of the last period, such as 21 weeks 4 days. Conception is about 2 weeks after that day, but the count starts at the last period. The estimated due date is at 40 weeks 0 days. This is general information, not medical advice.
Where it appears in the formulas
days pregnant = 280 − days to the due date and weeks pregnant = ⌊days pregnant / 7⌋. The due date calculator first finds the estimated due date, then counts back from it to the week of pregnancy on the day you choose. From a conception or IVF transfer date, it finds the same start day.
Trimesters
By ACOG, the 1st trimester runs to 13 weeks 6 days. The 2nd trimester starts at 14 weeks 0 days and the 3rd at 28 weeks 0 days. A first-trimester ultrasound gives the gestational age more accurately than the last-period date.