FIGURE 2.16 Sensitive periods of prenatal development The most sensitive or critical period of prenatal development is the embryonic period. During the first 2 weeks, before implantation in the uterus, the zygote is generally not susceptible to environmental factors. Every major organ system of the body undergoes all or a major part of its development between the 3rd and the 9th week. The dark green portions of the bars in the figure denote the times of most rapid development when major defects originate. The light green portions indicate periods of continued but less rapid development when minor defects may occur. (Adapted from Moore & Persaud, 1993)