Weeks and days, counted precisely
How far along you are is your gestational age: the time since the first day of your last period, or the equivalent date worked out from a scan or IVF dates. If you already have a due date, your gestational age on any day is 280 days minus the days left until the due date.1
Clinicians write it as completed weeks plus days. 24+3 means 24 weeks and 3 days. The count goes 6+5, 6+6, then 7+0: there is never a “7 weeks 7 days”.2
Completed weeks versus the week you’re in
“I’m 12 weeks” means 12 completed weeks, so you are in your 13th week. Apps and books sometimes mix the two, which is why sources can seem a week apart. Our week-by-week pages always use completed weeks: the page for week 12 covers 12 weeks 0 days to 12 weeks 6 days.
Which trimester you’re in
We follow ACOG’s trimester boundaries.3 Some sources, including the NHS week-by-week guide, start the second trimester at week 13. It is a labeling difference that doesn’t change your due date. See trimester dates for more.
Checking a different date
After you calculate, use “See dates as of” in the result to see how far along you were, or will be, on another day, such as a scan, a trip or a family event. Every date in the result updates.