Final answer:
A typical full-term pregnancy lasts about 40 weeks from the first day of the last menstrual period or approximately 38.5 weeks from conception. Obstetricians normally set the due date at about 40.5 weeks from the LMP, with pregnancy divided into three trimesters of 13 weeks each.
Step-by-step explanation:
A typical full-term pregnancy is about 40 weeks long when measured from the first day of the last menstrual period (LMP). However, when counting from the moment of conception, a pregnancy lasts roughly 38.5 weeks or 270 days. An important point to remember is that a due date set by obstetricians is usually 284 days (or about 40.5 weeks) from the LMP, considering conception is assumed to occur on day 14 of a woman's menstrual cycle. The pregnancy is divided into three trimesters, each lasting about 13 weeks. During the second and third trimesters, anatomical changes occur in the pregnant person to accommodate the growing fetus, which reaches a stage ready for birth typically by the 38th week, weighing between 2.7 and 4.6 kilograms and measuring 36 to 51 centimeters in length.