Final answer:
Viability best describes the pregnancy at 20 weeks, indicating the fetus's potential capacity to survive outside the womb. This period is during the second trimester, known for fetal growth and development, and marks the midpoint of a full-term pregnancy.
Step-by-step explanation:
The term describing pregnancy reaching 20 weeks is viability. This is the point at which the fetus has the potential capacity to survive outside the womb, with or without medical assistance. A full-term pregnancy usually lasts about 40 weeks, and it's divided into three trimesters. The second trimester, which occurs from weeks 13 to 28, is a significant period where the fetus undergoes substantial growth and development, and the expectant mother may start to feel the fetus's movements, a phenomenon known as quickening. Around week 20, the pregnancy is at the midpoint, often referred to as mid-pregnancy.