Gestational age - Wikipedia
https://en.wikipedia.org/wiki/Gestational_ageAccording to American College of Obstetricians and Gynecologists, the main methods to calculate gestational age are: • Directly calculating the days since the beginning of the last menstrual period• Early obstetric ultrasound, comparing the size of an embryo or fetus to that of a reference group of pregnancies of known gestational age (such as calculated f…