Final answer:
Gestation is segmented into three trimesters with crucial developments such as placenta formation and myelination occurring in specific stages. During the fetal stage, the body undergoes significant physical changes to prepare for birth and life outside the womb, such as lung and immune system development.
Step-by-step explanation:
The question relates to the different stages of fetal development and the corresponding physiological changes that occur in both the fetus and the gestational parent's body. Gestation is divided into three trimesters, with the first covering the first two to four weeks where the placenta begins to form, taking over the nutritional and waste requirements of the embryo and fetus.
Myelination in the spinal cord begins during development weeks 21-30, which is necessary for efficient nervous system functioning, and surfactant production begins in the lungs to help with proper lung expansion after birth.
A fetus is defined as a prenatal human organism between the embryonic stage and birth, specifically weeks 8 through 38 after fertilization. Important developmental milestones occur throughout this period, like the bone marrow completely taking over erythrocyte synthesis and rapid weight gain to help maintain body temperature after birth. Additionally, certain physiological processes such as the descent of the testes into the scrotum and the closing of the anterior fontanel happen during specific developmental windows.