Final answer:
Gestational age is calculated from the first day of the mother's last menstrual period, with a full-term pregnancy considered to be 40 weeks from the LMP.
Step-by-step explanation:
Gestational age, or the age of the baby, is calculated from the first day of the mother's last menses. A full-term pregnancy typically lasts around 270 days from conception to birth, but due dates are set as 284 days (40.5 weeks) from the last menstrual period (LMP). This calculation assumes conception occurred on day 14 of the woman's menstrual cycle. The average duration of a human pregnancy is 40 weeks, and it is divided into three trimesters, with a variety of developmental milestones occurring in each trimester.