Final answer:
The highest risk of birth defects due to teratogen exposure is during the embryonic stage of pregnancy, when the developing organs are most vulnerable. Key factors influencing the risk include the duration and timing of exposure, and the potency and dose of the teratogen.
Step-by-step explanation:
The risk of birth defects due to teratogen exposure is highest during the embryonic stage of pregnancy. This is because each organ of the fetus develops during a specific period in the pregnancy, known as the critical or sensitive period. During this time, the developing organs are most vulnerable to disruptions in their development, leading to potential birth defects. Factors such as the duration of teratogen exposure, the time of exposure, the degree of transfer from maternal to fetal circulation, the potency of the drug as a mutagen, the susceptibility of the fetus, and the dose of the teratogen all play a role in the potential severity of defects.
For example, exposure to alcohol during the embryonic stage can result in fetal alcohol syndrome, characterized by distinctive facial abnormalities and brain damage. Substances such as heroin, cocaine, methamphetamine, prescription and over-the counter medications, as well as environmental factors like radiation and certain viruses, are considered teratogens. It is crucial for pregnant women to avoid these harmful substances to minimize the risk of birth defects.