Final answer:
Gestation period is the time from conception to birth, typically 266 days (40 weeks) in humans, divided into three trimesters with specific developmental milestones. It refers to the length of pregnancy (option 1) and not to the post-birth period, one trimester, or labor and birth.
Step-by-step explanation:
The term gestation period refers to the length of time of development, from conception to birth, of the young of a viviparous animal, particularly humans. In humans, the gestation period, also known as the length of pregnancy, is 266 days (or approximately 40 weeks when calculated from the first day of the last menstrual period) and is divided into three trimesters. Each trimester lasts about 13 weeks and is associated with different developmental milestones and physiological changes in both the fetus and the gestational parent.
The first trimester involves laying down the basic structures of the body, including the heart, limb buds, and liver. During the second trimester, the organs continue their development. The third trimester exhibits the most significant growth of the fetus and ends with labor and birth. Labor consists of three stages: dilation of the cervix, delivery of the baby, and expulsion of the placenta.