Final answer:
The first trimester is the most important time for the development of a baby's spinal cord, brain, and internal organs, as the foundational development occurs during this period.
Step-by-step explanation:
The first trimester is crucial for the physical development of a baby's brain, spinal cord, and all internal organs. The foundational development of all major body organs and structures takes place during this crucial time. The liver, heart, eyes, and limb buds are essentially formed by the fifth week of development. Although many organs, including the liver and lungs, are not yet functioning by the end of the eighth week, the fetus's basic structure is still in place. Toxin exposure during this stage of development is especially risky because developing organs are highly susceptible to environmental influences, and developmental disruptions can have serious long-term consequences.