Final answer:
The first trimester is the most critical time for fetal development, as it involves the formation of the basic structures of organ systems and is a period when the embryo is most vulnerable to harmful influences. Subsequent trimesters involve further development and growth.
Step-by-step explanation:
The most critical time for fetal development is during the first trimester. This initial stage of pregnancy involves the critical periods where the foundations of the organs and bodily systems are laid out. During the embryonic stage, which occurs from weeks 3-8 after fertilization, all of the organ systems are structured in rudimentary form. Although the organs are either nonfunctional or only semi-functional at this point, the groundwork established during this time is crucial, as the embryo is particularly susceptible to external influences that can result in congenital anomalies. The placenta formation also begins in the first trimester, taking over nutrition and waste requirements from the endometrial lining, which initially handles these functions by diffusion.
By contrast, the fetal growth stages in the second and third trimesters involve further development and the growth of the fetus, but the most critical developmental period has already passed. The second trimester witnesses the placenta taking over more of the functions, and the fetus grows and becomes active, with the mother feeling the first movements. In the third trimester, there is rapid growth and final maturation of the fetal tissues and organs in preparation for postnatal life.