What is Teratology?
Teratology is the study of birth defects caused by exposures during pregnancy.
What is a Teratogen?
A teratogen is any medication, chemical, infectious disease, or environmental agent that may interfere with the normal development of a fetus and result in the loss of a pregnancy, a birth defect, or a pregnancy complication.
There are a variety of teratogens that are relatively common. Some examples are listed below. Please be aware that there are many other teratogens that are not included on this list.
- Social Drugs: alcohol, cocaine
- Medications: Seizure medications (such as Dilantin, Valproic Acid, and Tegretol), Accutane, thalidomide, chemotherapy, lithium
- Environmental Agents: organic solvents, lead, organic mercury
- Infectious Diseases: rubella, cytomegalovirus, toxoplasmosis, Fifth disease (parvovirus), chicken pox (varicella)
- Other: diabetes, radiation, hyperthermia (high fever)
Why Have Teratogen Counseling?
When assessing the risk of any exposure during pregnancy, we always keep in mind that any time a couple conceives a pregnancy, there is about a 3-5% chance that the pregnancy will have a major physical birth defect and then try to determine whether a medication or other exposure increases that risk. It is frequently difficult to prove that a medicine or other agent is totally safe in pregnancy, or conversely, responsible for causing birth defects.
The timing of exposure is also important to assess which systems of the unborn baby could potentially be affected. The first trimester (with the exception of the first two weeks following conception) is when the organs are developing and concern is greatest for a physical birth defect. The second and third trimesters involve the growth of the baby and are unlikely to result in a physical birth defect. The brain and the accompanying central nervous system develop throughout the pregnancy and after delivery.
Risks versus Benefits
When deciding about using any medicine in pregnancy, it is also important to keep in mind the benefit to the woman’s health, and in turn the pregnancy, that a medication offers. For example, an untreated seizure disorder during pregnancy may lead to the mother and pregnancy being deprived of oxygen. Additionally, some types of seizures could also cause the mother to fall or injure herself and/or the pregnancy. Therefore avoiding all medicine is not always the "no risk" decision.
Environmental or Occupational Exposures
It is difficult to provide a specific risk assessment in an occupational setting where the amount of exposure is important, but cannot be quantitatively measured by a description of activities. Occupational exposures are also difficult to assess because human studies have not been specifically done to evaluate each exposure. This means that the only information we have is from studies done where very high exposures are given to animals, and these studies are difficult to interpret for humans.
The letter categories of medicines (A,B,C,D,X) are not very helpful in assessing medicine risk because they largely rely on experimental animal data and may falsely imply an increased risk with increasing letter. Reviewing the outcomes of human pregnancy studies are the best way to assess effects of medications on fetal development rather than relying on the letter category.