Estimate your due date and full pregnancy schedule from your due date, last period, or conception date.
Confusingly, "week 1" of pregnancy starts on the first day of your last menstrual period — about two weeks before conception actually happens. This is called gestational age, and it's the standard doctors use because the exact date of conception usually isn't known precisely, while the start of the last period usually is.
Not very, in terms of the exact day — fewer than 4% of babies are born exactly on their due date. About 60% arrive within a week of it, and nearly 90% within two weeks. Treat the due date as the center of a several-week window, not a precise prediction.
Last period is the standard default and works well for regular cycles. If you know your conception date precisely (for example from fertility tracking), or already have a due date from your doctor, either of those will give an equally valid schedule.
This calculator uses the common convention of weeks 1-12 for the first trimester, 13-27 for the second, and 28-40+ for the third — though exact boundaries are sometimes defined slightly differently between sources.
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