Britannica
Sources used in this answer
- congenital disorder - Blastogenesis
- congenital disorder - Minor anomalies: abnormalities of the fetal stage
- congenital disorder - Malformations: abnormalities of the embryonic stage
- prenatal development - Teratology
- poison - Teratogenesis
- organogenesis
- congenital disorder - Organogenesis
- prenatal development - Fetal deviations
During which stages of development do major malformations usually arise?
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Major malformations typically arise during embryogenesis, which encompasses the first eight weeks of human development. This period is conventionally divided into two distinct stages:12
- Blastogenesis (Days 1–28): During this initial stage, the basic body plan is established and the developmental fate of all embryonic parts is determined. Because development is highly integrated and interdependent at this time, malformations occurring during blastogenesis are usually severe, frequently lethal, and often affect multiple parts of the body.34
- Organogenesis (Days 29–56): This stage involves the development of specific organ systems. Defects acquired during organogenesis tend to be milder than those occurring during blastogenesis, often affecting single parts of the body and generally allowing for the survival of the developing organism.4
While malformations originate during these embryonic stages, minor anomalies—which are subtle structural or appearance-based defects—typically arise during phenogenesis (the fetal stage), occurring between days 57 and 266 of development.5