The embryonic period (from the third to the eight week) is the most sensitive time for causing structural birth defects. Organs have different periods of sensitivity to birth defects: Some are vulnerable for a long time (e.g., the brain), others for a shorter time (e.g., the arms). The embryonic period (weeks 3 to 8) is the most sensitive time for structural birth defects. During the fetal period, the brain is the most at risk for defects. Sometimes teratogens can affect multiple organs at the same time. Individual differences in susceptibility to teratogens may be caused by a fetus’s genetic makeup or peculiarities of the mother, including the effectiveness of her placenta or her overall health. The dose and timing of the exposure are both important.