Appendix 3. Critical Periods of Human Development (sensitivity to Teratogens)

Major morphologic abnormalities occur during weeks 3 to 7. However, physiologic defects and minor morphologic abnormalities do occur from week 8 to term

1. Weeks 1 and 2: period of dividing zygote, implantation, and bilaminar embryo
1. THE EMBRYO IS NOT NORMALLY SUSCEPTIBLE to teratogens during this period
1. A substance will damage either all or most of the cells at this time, resulting in prenatal death, or the embryo will survive with few defects, if any
2. Week 3
1. THIS IS A HIGHLY SENSITIVE PERIOD for the developing heart and central nervous system
1. Highly sensitive for the heart from the middle of week 3 to week 6
2. Highly sensitive period for the central nervous system from the beginning of week 3 through early in week 6
3. Week 4
1. DURING THIS PERIOD, the eyes, ears, arms, and legs begin to develop
1. Sensitive period for the eyes is the middle of week 4 to the middle of week 8
2. Sensitive period for the ears is the middle of week 4 to the middle of week 9
3. Sensitive period for the arms is the middle of week 4 to the end of week 7
4. Sensitive period for the legs is the middle of week 4 to the end of week 7
4. Week 6
1. THE TEETH are most sensitive between the end of week 6 until the end of week 8
2. THE PALATE is most sensitive between the end of week 6 until early in week 9
5. Week 7
1. THE EXTERNAL GENITALIA are most sensitive from the middle of week 7 until the end of week 9
6. Periods of lesser sensitivity to teratogens
1. THE CENTRAL NERVOUS SYSTEM: from early in week 6 to term
2. THE HEART: from late in week 6 to the end of week 8
3. THE ARMS: during week 8
4. THE EYES: middle of week 8 to term
5. THE LEGS: during week 8
6. THE TEETH: during weeks 9 and 10
7. THE PALATE: during week 9
8. THE EXTERNAL GENITALIA: late in week 9 to term
9. THE EAR: middle of week 9 to the end of week 16