Final answer:
The most critical period in fetal development is the first trimester. This is when the fetus's basic structures form and when it is most vulnerable to environmental factors that can cause developmental abnormalities.
Step-by-step explanation:
The most critical period in fetal development is A. First trimester. During this period, the basic structures of the body, including the limb buds, heart, eyes and liver are laid down. It is known as a critical period because it is the time when specific parts or organs develop and the fetus is most susceptible to environmental influences that might cause defects. The embryo is particularly vulnerable to the teratogenic effects of substances, such as alcohol which can cause significant anomalies if exposure occurs during this sensitive period.
Subsequent trimesters involve further development and growth. The second trimester continues the development of all of the organs and systems, while the third trimester is characterized by the greatest growth of the fetus, culminating in labor and delivery. However the foundational aspects of fetal structure and the establishment of organ systems are primarily determined within the first trimester, making it the most critical for fetal development.