How does a doctor know how old a fetus is?

A doctor typically determines the age of a fetus using the gestational age, which is measured from the first day of the woman's last menstrual period (LMP). Ultrasound examinations can also be used to estimate fetal age by measuring the size of the fetus, particularly in the first trimester when development is more uniform. Additionally, doctors may consider other factors, such as the timing of conception and fetal growth patterns, to provide a more accurate estimate.