How the pregnancy week calculator works
Gestational age vs fetal age
Pregnancy is timed from the last period (gestational age), making week 1–2 "pre-pregnancy" by conception logic — the embryo is always ~2 weeks younger than the chart says. Every provider, scan report, and guideline uses gestational weeks, so this calculator counts the same way rather than being technically correct and clinically confusing.
The milestone map
A handful of dated events structure the pregnancy: heartbeat visibility (6–7 wks), end of first trimester (13), anatomy scan (18–22), viability (23–24), full term (37), due date (40). Tracking week-by-day against this map converts an abstract countdown into a sequence of concrete arrivals — which is also how appointment schedules are built.
Trimester boundaries, both conventions
The common split (13/27) is by far the most used; hospital conventions occasionally differ by a week. The calculator uses the standard 13/27 boundaries and shows the week ranges explicitly, so nobody is surprised by an app that counts slightly differently.