The word Teratogen, in Greek, means "monster forming". Teratogens in, modern medical vernacular, are agents that interfere with normal development of an embryo.
Many things can be listed as teratogens and obviously some are more serious than others. Exposure to some teratogens, even in trace amounts, can have devastating effects. Some teratogens are benign until they build to "threshold concentration" levels in the mother and then their effects are seen. Some teratogens are dangerous throughout a pregnancy and some are dangerous only at specific points of embryonic development.
The only safe course of action is to actively avoid known teratogens before and during pregnancy. The list below is not all inclusive but does list some well known and dangerous agents that are proven teratogens.
Varicella Virus (Chickenpox)
Exposure to chickenpox virus, obviously, is very common as it's a common airborne disease and close proximity to an infected person can cause exposure - exposure can also occur by coming into direct contact with the rash of an infected person. The danger to a developing fetus, whose mother is *newly* infected, can be severe, including: skin scarring, a small head, blindness, seizures, low birth weight, and mental retardation.
Exposure to tobacco is, of course, common through smoking cigarettes or otherwise ingesting tobacco or being the recipient of second hand smoke (10% of U.S. women (minimum) smoke while pregnant.). The risk to a developing fetus, whose mother is exposed to tobacco, is that the placenta may not develop normally causing conditions like a placenta previa that can cause severe complications, including death, during pregnancy and/or delivery (my wife had this condition and had to be hospitalized for weeks). The fetus is also at increased risk of developing heart defects and being born at a low birthrate. Babies that do survive are often difficult to comfort and are sometimes described as nervous or "jittery". Another tangential point is that babies that continue to be exposed to tobacco (second hand smoke) after birth have increased risk of SIDS, asthma, and other lung related problems.
When a pregnant woman drinks alcohol, so does her unborn baby. There is no known safe amount of alcohol to drink while pregnant and there also does not appear to be a safe time to drink during pregnancy either. Therefore, it is recommended that women abstain from drinking alcohol at any time during pregnancy. Women who are sexually active and do not use effective birth control should also refrain from drinking because they could become pregnant and not know for several weeks or more.
Prenatal exposure to alcohol can cause a range of disorders, known as fetal alcohol spectrum disorders (FASDs). One of the most severe effects of drinking during pregnancy is fetal alcohol syndrome (FAS). FAS is one of the leading known preventable causes of mental retardation and birth defects. If a woman drinks alcohol during her pregnancy, her baby can be born with FAS, a lifelong condition that causes physical and mental disabilities. FAS is characterized by abnormal facial features, growth deficiencies, and central nervous system (CNS) problems. People with FAS might have problems with learning, memory, attention span, communication, vision, hearing, or a combination of these. These problems often lead to difficulties in school and problems getting along with others. FAS is a permanent condition. It affects every aspect of an individual's life and the lives of his or her family.
A person suffering from severe acne may be exposed to Accutane during treatment. Accutane is a prescribed medication that controls and prevents some of the mechanisms in the glands that contribute to acne. A developing fetus, whose mother is exposed to Accutane, is extremely likely to develop either facial deformities, heart defects, a small head, a cleft lip and palate, a buildup of fluid in the brain, or mental retardation. This is an extraordinarily dangerous substance for a developing fetus.
HIV, of course, is the virus that causes AIDS. A person typically becomes infected with HIV by having vaginal or anal sex (especially unprotected sex) with a carrier or being exposed to infected blood (most often by sharing needles) - There are other, more atypical, methods of exposure like oral sex and blood transfusions. The health risk to a developing fetus, whose mother is infected, is that they will contract the virus themselves (congenital HIV) sometimes spontaneously aborting or surviving but being born infected with, the currently incurable, HIV. Being born infected with HIV is probably the most common birth defect in the world today. Medications to reduce the virus levels in the mother's blood and a short delivery by caesarian can greatly reduce the risk to the baby. An infected mother can pass the virus on to her baby during breastfeeding.
This is an especially sad teratogen as the exposure is due to a pregnant woman taking medication in an attempt to protect her fetus and prevent a miscarriage. DES was in medication used to prevent these miscarriages; it increases production of certain hormones created by the placenta. A female fetus exposed to DES is at risk for developing vaginal and cervical cancer as well as uterine defects. A male fetus exposed to DES is at risk of developing abnormal genitalia
Thalidomide is an infamous teratogen. Women can be exposed to this substance while trying to control nausea during pregnancy. Thalidomide was given (mostly in the late fifties) as a medication to control the nausea (it was removed from the market in 1961). Thalidomide causes severe malformation risks to a developing fetus. It can cause defective intestines, hearing defects or the fetus not developing ears at all, and anomalies with eyes and vision. However, the risk to the fetus that is most infamous is the severe limb malformations where the bones in the arms and legs don't develop to their proper length or don't develop at all.
There are a number of ways to be exposed to radiation: having a medical X-ray, working at a nuclear power plant, working in any industrial setting where X-rays or other radioactive substances are used, nuclear accidents (like Chernobyl) nuclear bomb detonations (testing or the Japanese wartime explosions) and countless other minor sources like TVs, smoke detectors, and airplane trips. Radiation exposure at high enough levels can cause developmental risks to a developing fetus like a reduction in height, severe mental retardation, small head size, impaired brain development, childhood leukemia, and cancer in later life.
Exposure to lead is common. Lead is used in batteries, metals, paint, ceramic glazes, cable covering, ammunition, and recently, toys from China. Pesticides, paint and gasoline had enormous amounts of lead until the dangers of lead were more fully realized. Old homes may have lead in the tap water as it seeps from leaded pipes. A fetus, whose mother is exposed to lead, runs the developmental risk of being spontaneous aborted, being born with a low birth weight, being born prematurely, and developing skin tags or other abnormal markings. A male baby is also at risk for undescended testicles.
Rubella is very contagious disease caused by a virus. Exposure can happen through normal interaction with an infected person where fluid from the nose or throat of the infected person gets passed (shaking hands, sharing drinks, etc). There is a vaccine for the disease but for a pregnant woman who has not been vaccinated, there are serious and sundry risks to her fetus. They include vision and hearing problems, heart abnormalities, mental retardation, even death of her fetus, and stillbirth. If the baby survives they are still at risk for diarrhea, pneumonia, meningitis and anemia. They are even at risk for diabetes and cerebral palsy. There are so many possible defects that the term ‘congenital rubella syndrome' is used as an umbrella term for all of the possibilities. Get vaccinated!!
Syphilis is a sexually transmitted disease caused by a bacterium. Exposure happens in the normal method of most STDs: vaginal, anal, or oral sex (especially unprotected). The risk to the fetus is that the fetus becomes infected with the bacteria. This can cause stillbirth or death shortly after birth. Infected babies that live through birth are still at risk for being developmentally delayed, having seizures, or even dying. Syphilis is relatively easy to cure in adults.
Toxoplasmosis is an extremely common parasitic infection. Pregnant women can be exposed to this parasite in many ways including: handling contaminated meat, cleaning a cat's litter box and inhaling the parasite (cats eat diseased animals and their feces become contaminated), contact with soil or sand that may have been used by cats, etc. The risk to the fetus in development, especially a fetus whose mother is *newly* infected with the parasite, is the increased risk for mental retardation, pneumonia, eye infections, jaundice, and hydrocephalus ("water on the brain"). A surviving baby may be prone to seizures and to developing cerebral palsy.
CMV is and extraordinarily common herpes virus. Exposure to this virus can occur through sexual contact, kissing, or contact with the saliva, urine, tears, or nearly any other bodily fluid of an infected person. The danger to a developing fetus, especially a fetus whose mother is *newly* infected, include hearing loss, mental retardation, vision loss, liver/lung/spleen problems and dental anomalies.
Phenytoin (or Dilantin)
Phenytoin is a medication used to treat epilepsy and prevent seizures. Exposure to this medication is common for mothers suffering from epilepsy and therefore who have been prescribed the medication. The danger to the fetus occurs, in part, because Phenytoin metabolizes differently for the fetus and the fetus receives a much higher dose than the mother. An exposed fetus has increased risk for developing a cleft palate, developing heart defects, having small heads, abnormalities in the fingers and nails, and some mild developmental disabilities. The combination of defects is called "Fetal Hydantoin Syndrome".
Mercury is a shiny silver metal that is in liquid form at room temperatures. Exposure to mercury might happen around broken thermometers, dental fillings, broken florescent light bulbs and eating fish that have high levels of mercury (especially swordfish, shark, king mackerel and tilefish). A fetus, whose mother is exposed to mercury, could be in serious danger. Mercury can stop the duplication process of cells and lead to the death of the fetus. It can also alter the cells as the divide and introduce a variety of unpredictable mutations in the developing fetus, especially to their nervous system.
Arsenic is a metal like mercury. Exposure can occur when drinking well water that is contaminated with arsenic, living near or working at an industrial incinerator that burns waste containing arsenic, or living near or working at industrial metal smelters. Building lumber was treated with arsenic until 2003, so even walking barefoot on a backyard deck may allow exposure to arsenic. A fetus, whose mother is exposed to high levels of arsenic, has an increased risk of being spontaneous aborted, being born with a low birth rate, and even stillbirth.