Obstetrics and Gynecology - A. M. Hromova 2000

Medical and Genetic Counseling in Obstetrics
Hereditary Diseases and Congenital Malformations
Critical Periods of Embryonic Development

The embryo exhibits peak sensitivity to the Adverse effects of damaging agents during critical periods.

There are 4 main critical periods:

1. Pre-implantation period (from Fertilization to day 6).

2. Implantation (days 6–9 post-fertilization).

3. Organogenesis (particularly weeks 3–6).

4. Placentation (weeks 12–14 of gestation).

The same teratogens can cause different developmental anomalies depending on the stage of fetal development at which they act. For instance, the fetal Nervous system is most vulnerable from day 15 to week 5 of Pregnancy. The Heart and Lungs are most susceptible from weeks 3 to 8. Ear anomalies typically develop between weeks 3 and 11, and eye anomalies between weeks 4 and 9. The limbs are most commonly affected from weeks 4 to 7. The Urogenital System is among the last to begin development. The Urinary System develops during the third month of pregnancy, while The formation of the reproductive Organs begins at weeks 7–8 and continues past the 4th month of gestation.

Placentation marks the onset of the fetal period, during which certain non-lethal Malformations of the cardiovascular and reproductive systems may arise.

In addition, susceptibility to Congenital Malformations varies depending on the pregnant individual's genotype and its interaction with the environment, as well as the concentration of the teratogen and the route by which it reaches the fetus.