Every calculator in this section starts from the same reference point: the first day of your last menstrual period, not the date of conception. That convention is what makes a "40-week" pregnancy run to a due date that is actually about 38 weeks after conception, and it is the reason two people can compare notes and get confused about whose dates are "ahead."
Calculators in this section
From that single starting date, the calculators below answer the questions that change week to week: how many weeks and days pregnant you are, which trimester that falls in, and how far off the due date sits. Clinical milestones — viability, full term, the end of the first trimester — are anchored to these same week counts, which is why knowing the number precisely matters more in pregnancy than in most other contexts.