During which stage of gestation are embryologic effects least likely to occur?

The third trimester is characterized by significant growth and maturation of the fetus, with the majority of critical organogenesis activities already completed during the first trimester. By this stage, the tissues and organs are largely developed and are less susceptible to embryologic effects from teratogens or other harmful influences. Most major structures are formed, and the risk of major structural abnormalities due to environmental factors is significantly reduced.

In contrast, the first trimester is the most critical period for embryologic development. This is when the foundations of major organ systems are established, making any exposure to harmful substances more likely to result in serious developmental issues. During the second trimester, while the risk is lower than the first, there remains the potential for issues to arise as the organs continue to grow and mature. The postnatal period focuses on development after birth, which does not fall under the typical definition of gestation and is not relevant to embryologic effects that occur during the stages of prenatal development.