Gestational age calculator
An estimate from a standard formula. A dating scan measures the embryo directly and takes precedence. Follow the advice of your midwife or doctor.
Gestational age is counted from the first day of the last menstrual period and written as completed weeks plus days — "24w 3d". It runs about two weeks ahead of the actual embryonic age, because the count starts before conception.
How to work out gestational age
The convention exists because the last period is a knowable date and conception is not. It means a woman described as "six weeks pregnant" has an embryo roughly four weeks old, which is a source of genuine confusion when comparing against embryology diagrams that count from fertilisation. Clinical care, viability thresholds and every milestone in maternity notes all use gestational age, so it is the number worth tracking.
Questions
From the first day of the last menstrual period, in completed weeks plus days.