Week-counting method checked against the same last-period convention ACOG uses.
Count the number of complete weeks between the first day of your last menstrual period and today; that number is your current gestational age. If your LMP was 63 days ago, divide by 7 to get 9 weeks. This is a calendar exercise, not a biological one, which is exactly why it needs explaining: the embryo itself is younger than the week count suggests.
Count forward from your last period, not conception
Clinicians settled on last-menstrual-period dating for a practical reason: most people can name the first day of their last period with reasonable confidence, while the exact day of conception is rarely known without deliberate fertility tracking. The Pregnancy Calculator on this site does the day-count automatically and shows your trimester alongside it, but the arithmetic behind it is simple enough to check yourself.
A worked example
Say your LMP was April 10, and today is June 12. That is 63 days apart. Divide by 7: 9 weeks exactly. If the remainder is not zero, say 66 days, that is 9 weeks and 3 days, usually written as "9w3d." Providers and calculators alike report weeks and days together rather than rounding, because a difference of a few days matters more early in pregnancy than it does later.
Weeks, not months
Forty weeks from LMP works out to roughly 9 months and a week on a calendar, or 10 lunar months of exactly 4 weeks each, which is where the "10 months" version of the pregnancy-length claim comes from. Clinicians use weeks specifically because a week is a fixed, precise unit and "7 months" could mean anywhere across a four-week range.
Gestational age vs fetal age
Gestational age, counted from LMP, is what appears in your chart and on this site. Fetal age, sometimes called embryonic age, is counted from conception and runs about two weeks behind in a typical cycle, because ovulation and fertilization happen around day 14 of a 28-day cycle while the week count already started on day 1. If your provider says you are 20 weeks pregnant, the embryo itself is closer to 18 weeks old. Unless a source specifies otherwise, any week number in a medical context is gestational age.
What happens if you do not know your LMP
An ultrasound can estimate gestational age directly by measuring the embryo or fetus, and a first-trimester scan is the most accurate method available for that, described in the due date guide. If your own dates are uncertain, that measurement, not a guess at your last period, is the one to rely on.
Find your current week
Track your week-by-week progress and current trimester from your own last period date.