This note defines a teratogen and explains the principles governing the capacity of a teratogen to produce teratogenic effect with emphasis on the effect of a teratogen on different developmental stages.

A teratogen is any agent/factor that can produce a congenital anomaly or raise the incidence of an anomaly in the population. The teratogens have potential to affect the normal development of human embryo following maternal exposure to them. 
These agents are usually environmental factors such as drugs, chemicals, radiation, maternal diseases and viruses.The effect varies from death of embryo to any structural, functional, metabolic and behavioral abnormalities. Developmental disorders present at birth are termed either as birth defects, congenital anomalies or congenital malformation.

Teratology is the branch of science that studies the cause, mechanisms, and patterns of abnormal development. A fundamental concept in teratology is that certain stages of embryonic development are more vulnerable to disruption than others.

Teratogenicity, the capacity of an agent to produce birth defects, is governed by certain factors described as Principles of Teratology.

1.         Developmental stage at the time of exposure
2.         Dose and duration of exposure
3.         Genotype of the embryo
4.     Mechanisms of action is specific and may involve inhibition of specific biochemical or  molecular process; cell death; decreased cell proliferation or other cellular phenomena.
5.     Manifestation of abnormal development are death, malformation, growth retardation and  functional disorders.

Stages of Developmental and Effects of Teratogens

1) Pre-embryonic period
This is the first two weeks after fertilization. During this period, the zygote divides; implantation occurs; amnion, chorionic sac and yolk sac are formed and the embryo becomes bilaminar.

Effect of teratogens
Teratogens acting during this period either kill the embryo (leading to spontaneous abortion which goes unnoticed as the woman may not realize that she is pregnant) or their disruptive effects are compensated for by powerful regulatory properties of early embryo (so unlikely to produce congenital anomalies).

2) Embryonic period or period of embryogenesis or organogenesis
This is the period from third to eighth weeks of gestation, when organ systems are being established.

Effect of teratogens: 
This is the most sensitive period for inducing major birth defects.

3) Fetal period
This is the period from 9th week till birth. This period is characterized by rapid growth and differentiation of the organ system.

Effect of teratogens: 
Fetal damage is unlikely to produce malformations but can cause death, growth retardation, disruptions or functional deficits.

Which stage of development is most vulnerable to teratogens?
The period of embryogenesis is considered to be the most critical stage of development in terms of teratogenic effect.

However, no stage of development is completely safe. Each part, tissue and organ of an embryo has a critical period for which its development may be disrupted. The type of congenital malformations produced depends on which parts, tissues, and organs are most susceptible at the time of exposure to teratogens.


Developing Human by Moore and Persaud
Langman's Medical Embryology by T W Sadler