1 weeks pregnant: what is happening this week

Nobody is pregnant in week one. That sounds like a technicality and it is, but it is the technicality the whole calendar is built on. Gestational age is counted from the first day of the last period, not from conception, because that date is one a person can usually name and the date of conception almost never is. So week one is the week of a period. The count has started; the pregnancy has not. Two consequences follow, and both catch people out. The first is that by the time a test reads positive, the count is usually already at four or five weeks, which is why the number can feel like it has skipped ahead. The second is that everything downstream inherits the same starting point: the estimated due date, the term window, the timing of every scan. If the date at the top of this page is a guess, everything computed from it is a guess with the same error in it. This is the week to pin the date down while it is still recent - on a calendar, in a note, anywhere it will survive the next eight months. A first-trimester scan will refine it later, and ACOG treats that scan as the better measurement, but the arithmetic has to start somewhere and this is where.