Final answer:
The first trimester is the most critical for fetal development as all organ systems are structured in rudimentary form during this phase, and disturbances can have significant impacts.
Step-by-step explanation:
The nurse would explain to a client that the most critical period of pregnancy for fetal development is the first trimester. During this time frame, the stages of prenatal development include the germinal, embryonic, and early fetal stages. Early pregnancy is characterized by cell division, migration, and differentiation. By the end of the embryonic period, which is around the 8th week, all organ systems are structured in rudimentary form. Critical periods of development happen in these initial stages, and any disturbances can have significant impacts on fetal development. The placenta forms and begins to take over the role of providing nutrients and disposing of waste, and the pregnant individual's immunoglobulins pass through the placenta to provide passive immunity to the developing fetus.