During which weeks are major fetal organs most susceptible to damage?

• A. Week 2-3
• B. Week 4-8
• C. Week 9-12
• D. Week 13-16

Answer: (B)

During organ formation, roughly four to eight weeks after fertilization, the major fetal organs and systems are developing. This is when tissues are differentiating and shaping the heart, limbs, brain, spine, eyes, and other structures, so disruptions from harmful exposures (teratogens) can create lasting, structural defects. Because of this intense formation activity, this window carries the highest risk for major congenital anomalies. Earlier weeks, like two to three, are before most organs are formed, and later weeks—nine to twelve or thirteen to sixteen—enter a period of growth and maturation rather than initial formation, so the risk shifts toward functional issues or milder anomalies rather than widespread structural defects. That’s why weeks four through eight are the most susceptible for major organ damage.