Gestation period is the time between conception and birth, typically lasting around nine months. The term 'trimester' is used to describe the three-month periods that make up the gestation period. Each trimester is marked by distinct changes in the development of the fetus. During the first trimester, the fetus develops its major organs and systems. During the second trimester, the fetus grows rapidly and begins to move. During the third trimester, the fetus continues to grow and mature, preparing for birth.