Human Growth and Development / Unit 2 HGD 0192 worked example, 60 to 90 seconds
How Teratogens Work
The idea
Whether a teratogen causes harm depends on several principles. Timing matters most: a structure is most vulnerable while it is forming, so the embryonic period is the riskiest, and the nervous system stays susceptible throughout pregnancy. Dose and duration matter: more exposure, for longer, raises the likelihood of harm. Genetic susceptibility and the overall quality of the prenatal environment matter. Fathers' exposures count too, because toxins, drugs and smoking affect sperm. Finally, the environment after birth moderates long-term effects.
In practice
The same exposure in week five and in week thirty, one striking while organs form and one arriving after.
Where people slip
The misconception is that a teratogen either harms or it does not. Outcomes are probabilities that shift with timing, amount, genes and later care, which is why two exposed pregnancies can end very differently. It also means the postnatal environment is part of the story: support after birth can soften effects that began before it.
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