2 weeks pregnant: what is happening this week

Week two is the week the convention assumes ovulation happens - specifically, around day 14 of a 28-day cycle. Read that sentence again, because two assumptions are doing a great deal of work in it. Cycles are not all 28 days, and ovulation does not reliably land on day 14 even in cycles that are. ACOG says this plainly when it describes the 280-day rule: the rule assumes a regular cycle with ovulation on the fourteenth day, and it does not account for irregular cycles, variation in ovulation timing, or the ordinary difficulty of remembering a date. A longer cycle usually ovulates later, which pushes everything downstream later by roughly the same number of days. That is why our due date calculator takes a cycle length at all, and why the shift it applies is labelled as arithmetic on ACOG's assumption rather than as an ACOG recommendation. None of this is a reason to distrust the estimate. It is a reason to hold it loosely. The estimate is the midpoint of a distribution five weeks wide, and the width is the honest part. If you are tracking, this is the week the tracking is about; if you are not, nothing observable happens and there is nothing to miss.