Inherited or congenital abnormalities
As we have already seen, two segments must be distinguished within the embryonic period.
The preembryonic period, in which the embryo is subject to the "all or nothing" law and in which the embryonic cells still possess regulation and repair abilities. Before the 14th day an insult will not cause an abnormality because the embryo either repairs the damage or dies (spontaneous miscarriage).
On the other hand, in the course of the embryonic period the embryo is very vulnerable to exogenous noxae. Most congenital abnormalities appear in this period, in which the embryonic cells go through numerous mitotic divisions. One estimates that more than 90% of the 4500 structures in the human organism are laid down within the embryonic period.
Embryopathies are treated in another chapter.
- Embryonic period
- Fetal period
Segment A represents the embryonic period in which the embryo is especially sensitive with respect to developmental abnormalities. Within the first eight weeks, the incidence of deformities (blue curve), which lead to miscarrieages, decreases from more than 10% to 1%. The frequency of neural tube defects decreases from 2.5% to 0.1% (green curve) by the end of the embryonic period.