How far along you are, your trimester and estimated due date, from your last menstrual period.
It counts the days between the first day of your last menstrual period (LMP) and the "as of" date, then converts that to weeks and days. This is the standard way pregnancy dating works clinically, even though conception itself happens about two weeks after the LMP.
This calculator uses the standard 280-day (40-week) convention measured from the LMP, which assumes a textbook 28-day cycle. It's an estimate — your doctor's dating ultrasound may adjust it based on actual fetal measurements.
It's simply LMP + 14 days, assuming ovulation around day 14 of a 28-day cycle. If your cycles are longer or shorter, your actual conception date could fall earlier or later than this estimate.