Why are malformations that begin in the first 28 days of development often severe?

Malformations that occur during the first 28 days of development—a period known as blastogenesis—are often severe and frequently lethal due to the nature of early embryonic growth.1

Several key factors contribute to the severity of defects during this stage:

• Establishment of the Body Plan: During these first four weeks, the basic body plan and the domains of gene expression are established, and the developmental fate of all parts of the embryo is determined.1
• Structural Interdependence: The early embryo is very small, and its organ rudiments are in close proximity to one another. Because early development is highly integrated and interdependent, a disruption in one area can easily affect multiple parts of the body.1
• Critical Timing: This period is foundational for organogenesis, the process by which organ systems are formed. Because this is a tightly programmed sequence of events, any interference during this critical window is likely to result in significant structural anomalies, such as gross brain or heart defects, facial clefts, or the absence of limbs.231