Teratogenicity is the ability to cause developmental anomalies in a fetus.Agents which cause teratogenicity is known as teratogens (The agent may be a chemical,a drug, an infection,radiations or a maternal condition such as diabetes).And the study of teratogenicity and teratogens are known as Teratology.
Teratogens may cause anything from abnormal development of a limb to malfunction of an organ,and the effects for the developing fetus can vary depending on the teratogen,the gestational age of the fetus,and other factors.A wide range of different chemicals and environmental factors are suspected or known to be teratogenic in humans and animals.A selected few includes tobacco,alcohol,androgenic hormones,X-rays,atomic radiations etc.
Pregnant women are encouraged to avoid exposure to such agents and advised to take care of substances of unknown teratogenicity.The US FDA has established five pregnancy categories (in1979) to indicate the potential of a drug to cause teratogenicity if it is used during pregnancy,which runs from 'category A' (safest) to 'category X' (known danger- do not use).
These drugs are frequently used during pregnancy and do not appear to cause major birth defects or other problems. Either animal-reproduction studies have not demonstrated a fetal risk but there are no controlled studies in pregnant women, or animal-reproduction studies have shown an adverse effect (other than a decrease in fertility) that was not confirmed in controlled studies in women in the first trimester (and there is no evidence of a risk in later trimesters).Ex:Paracetamol (acetaminophen),Ondansetron,Metformin etc.
Studies in animals have revealed adverse effects on the fetus (teratogenic or embryocidal or other) but the drugs have not been tested on women.Also includes drugs which have not been tested on animals or humans. Drugs should be given only if the potential benefit justifies the potential risk to the fetus.Ex:Fluconazole
There is positive evidence of human fetal risk, but the benefits from use in pregnant women may be acceptable despite the risk (e.g., if the drug is needed in a life-threatening situation or for a serious disease for which safer drugs cannot be used or are ineffective).Ex: Phenytoin.
Studies in animals or human beings have demonstrated fetal abnormalities, or there is evidence of fetal risk based on human experience or both, and the risk of the use of the drug in pregnant women clearly outweighs any possible benefit. The drug is contraindicated in women who are or may become pregnant.Ex: Statins, Warfarin derivatives etc.
Unfortunately this system over simplifies the issues relevant to prescribing a medication to a pregnant patient.For example there is a tendency to assume that Category B drugs are safer than category C drug when there may be no human studies available to support the assumption.