Gestational Age Calculator
Work out gestational age in weeks and days on any date — from your last period or from a dated ultrasound measurement.
How it works
gestational age = as-of date − effective start · from a scan: effective start = scan date − (weeks × 7 + days) · full term = 40 weeks (280 days).
Gestational age is the length of a pregnancy counted from the first day of the last menstrual period, reported as completed weeks plus days (for example 14w3d). When a dated ultrasound has assigned a gestational age, you can carry it forward by adding the days elapsed since the scan, which is what the ultrasound method does here. An early scan is usually the most accurate dating, so if it differs from the last-period date, clinicians follow the scan.
Worked example
A scan on August 1, 2026 reading 8 weeks 3 days sets the effective start 59 days earlier. By September 15, 2026, 45 more days have passed, giving a gestational age of about 14 weeks 6 days.
Frequently asked questions
What is the difference between gestational and fetal age?
Gestational age counts from the last menstrual period; fetal (conceptional) age counts from conception, about two weeks later. Maternity care, ultrasounds and due dates all use gestational age.
Why does my scan age differ from my last-period age?
Ovulation timing and cycle length vary, so the last-period date can be off by days. An early ultrasound measures the embryo directly and is generally more accurate, so providers usually re-date to the scan.
What counts as full term?
39–40 weeks is full term, 37–38 weeks is early term, and 41 weeks onward is late or post term. Your provider monitors any pregnancy that goes past the due date.
Related calculators
Educational estimates from standard formulas and population averages — every pregnancy and baby is different. This is not medical advice; your doctor, midwife or pediatrician, with an examination or ultrasound, gives the accurate guidance.