What Causes Birth Defects?

A birth defect or congenital anomaly is defined as a structural or functional abnormality present at birth. It is believed that there is a 3-5% risk for a birth defect in each pregnancy.

Birth defects may be caused by genetic factors such as specific gene mutations or chromosomal rearrangements. It is only the genetically caused birth defect that can be transmitted to future generations in a family. Birth defects may also result from multifactorial influences involving gene interaction with environmental factors. Maternal exposure to toxic substances is the cause for many of the birth defects in a developing fetus.

For purposes of definition, teratology is the study of environmentally induced birth defects and a teratogen is the specific agent which can cause an abnormality in the structure in an exposed fetus. The literal meaning of teratogen is “monster-causing”.

Teratogens usually fall into one of the following categories:

1) infectious agents, particularly viral infections, 2) physical agents, such as mechanical forces, 3) drug and chemical agents, 4) maternal metabolic or other health conditions

The timing of the teratogenic exposure and the amount of exposure determine the effect and nature of the birth defect. Throughout the pregnancy, there are critical periods of development. Exposure to teratogens during these critical periods will have differing effects depending on what parts of the fetus are developing at that time.

The first two weeks of development is referred to as the “all or none effect” period, meaning that teratogen exposure will severely damage the embryo leading to miscarriage or have no effect at all. Teratogen exposure during weeks 2-8 can cause structural defects in the fetus. Exposures after week 8 of pregnancy can affect fetal growth and development of the central nervous system, but do not cause birth defects.

Yes, it is best to avoid exposures to teratogens during pregnancy but some women on medication must remain on that medication for their personal health maintenance. Most drugs are not teratogenic but next time we will consider some exceptions as well as certain infections and other environmental agents that are known human teratogens.