By the end of month 2, approximately what percent of adult structures are formed?

• A. 90 percent
• B. 50 percent
• C. 10 percent
• D. 100 percent

Answer: (A)

The main idea here is how early prenatal development sets up the body’s basic structures. By the end of the second month, many of the major body plans and organ systems are in place—the embryo has a recognizable head, trunk, limb buds, and developing heart with chambers, and the nervous system is forming rapidly. Because these foundational structures are already established by this point, most of the basic adult body forms are in place, with further growth mostly adding size, refinement, and maturation rather than creating new body plans. That’s why a rough estimate often cited is about 90 percent of adult structures formed by the end of month two. After this stage, development shifts more toward growth and fine-tuning of those structures rather than building new ones from scratch.