Final answer:
A pre-term pregnancy occurs when the baby is born before 37 weeks of gestation, with the typical full-term pregnancy lasting from 38 to 40 weeks. Therefore, the correct answer is '20 to 36 weeks' for the duration of a pre-term pregnancy.
Step-by-step explanation:
A pre-term pregnancy is one in which the baby is born before the completion of 37 weeks of gestation. A full-term pregnancy typically lasts for about 38 to 40 weeks, measured from the first day of the last menstrual period (LMP). Given the options provided, the correct answer for the duration of a pre-term pregnancy would be from 20 to 36 weeks (Option A). This time frame is considered pre-term because the fetus is born before it has fully developed and is ready for birth. The fetal stage extends from week 9 after fertilization and continues until about 38 weeks, which is the average time of birth. Births before 37 weeks are considered preterm or premature, and the infants may face various health challenges due to underdevelopment of their organ systems.