Final answer:
The fetus is most vulnerable to outside agents during the first trimester when the major organs are forming. Exposure to toxins at this stage can result in significant developmental issues or conditions such as fetal alcohol syndrome.
Step-by-step explanation:
The fetus is most susceptible to outside agents during the first trimester of pregnancy. During this critical period, all the major organs and body structures of the fetus begin their development. By five weeks, parts such as limb buds, eyes, heart, and liver have been formed. By the eighth week, the body of the fetus is essentially fully formed, even if organs like the lungs and liver are not yet functioning. Exposure to toxins at this stage is particularly hazardous, as it can severely affect the ongoing development and potentially lead to permanent damage or conditions like fetal alcohol syndrome if the teratogenic effects of substances like alcohol interfere with the fetus's growth.
The fetus is most susceptible to outside agents during the first trimester of pregnancy. This is the period when the organs and body structures of the fetus start to develop. By five weeks, the basic formation of limbs, eyes, heart, and liver has occurred. By eight weeks, the body is essentially formed, although many organs are not yet functioning. Exposure to toxins during this critical period can have a severe effect on the fetus' survival.