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Learn what a teratogen in pregnancy is, relevant time lines during gestation, the most common examples, their effects on developing fetuses, and ways to avoid them.

Definition of Teratogen

Basically, a teratogen is an agent that, once in contact with a pregnant mother, will produce a defect in the developing fetus.

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A teratogen can be either a physical substance or a condition in the mother. The resulting defect can be either a physical abnormality or a functional defect. With this general definition in mind, it’s obvious that types of teratogens and their resulting issues can run the gamut.

Exposure to Teratogens

After an egg is fertilized by a sperm, the resulting zygote travels to the uterus and develops into an embryo. Cells begin to divide, increasing the embryo’s size, and it migrates from the fallopian tubes towards the uterus, where it eventually settles, implanting in the uterine wall. This does not occur until nearly a week after conception. After implantation, growth of supporting structures such as shared blood vessels begins.

Finally, about 2-3 weeks after fertilization, the embryo shares a blood supply with the mother. It’s around this time that the embryo can be affected by teratogens.

Examples of Teratogens

The following are examples of common teratogens and the defects they can cause:Alcohol is one of the most commonly recognized teratogens. Alcohol damages brain cells, which can lead to mental retardation in many children who were exposed during gestation.

It can also lead to abnormalities in face and head size, as well as generally slow growth and cause abnormalities in the rest of the body.Nicotine is another widely recognized teratogen, and pregnant women are strongly encouraged not to smoke. Nicotine exerts its harmful effect on the fetus by constricting the blood vessels, which leads to less blood flow to the baby. Because blood circulating from the mother is the only way the fetus can get oxygen, this leads to hypoxia, or low flow of oxygen, and the fetus effectively suffocates. The result can be low birth weight and even premature birth. These conditions can cause a variety of health problems that may last for the baby’s lifetime.

Many illegal drugs, especially the stimulants cocaine and amphetamines, are also teratogenic. These drugs disrupt development of neurons and brain tissue, and babies whose mothers abuse them can be found to have cysts, or hollow spaces, in the brain. If the child survives, learning and behavioral problems often result.

There are also a number of legal prescription drugs that are not safe for pregnant women to use. All drugs approved by the FDA have been studied and assigned a score from A to X. Category A drugs have no effect on a pregnancy. Category D drugs have enough possible effect on a pregnancy that they are avoided. Category X drugs are the most harmful to a developing fetus. Thankfully, most of those aren’t on the market, but it’s worth being aware of them.

Some bacteria can even affect pregnancy. Most notably, the bacteria causing the sexually transmitted disease syphilis are considered teratogens. In the case of an untreated syphilis infection in a pregnant mother, bacteria cells infect the fetus from the blood supply and are often fatal. If the baby is born alive, it may suffer malformations and damage to the nose, eyes, ears, and bones as a result of a heightened inflammatory response to the early infection.While most viruses are successfully combated by a pregnant mother’s immune system, a few specific strains are known to be teratogens during pregnancy. The virus causing chicken pox can have catastrophic effects on a fetus, including underdeveloped limbs, lesions in the brain, and scarring on the skin.

Thankfully, most women have already had the chicken pox by the time they reach child-bearing age, which protects them from contracting the virus again. Other examples of viral teratogens include herpes and rubella.In addition to bacteria and viruses, single-celled organisms called protozoa can be teratogenic during pregnancy, such as the toxoplasmosa protozoa.

Cats are notorious carriers of this one, and women are advised not to empty kitty litter boxes while pregnant. Toxoplasma can also be found in meats, but it’s killed by cooking. This is why pregnant women are also advised to avoid undercooked or raw meats and fish. If a pregnant woman is exposed to toxoplasma, the fetus may be killed or suffer damage to nerve tissue, leading to brain malformation, seizures, or blindness.

Other examples of teratogens found in the environment and in extraordinary circumstances can include metals, chemicals, radiation, and even heat. Examples of these teratogens can include mercury, potassium iodide, nuclear fallout radiation, and even high-temperature hot tubs!

Lesson Summary

A teratogen is an agent that causes a malformation or defect in a developing fetus. Teratogens can affect the fetus up to two to three weeks after fertilization of the egg, or the zygote. Common teratogens range from alcohol; nicotine, which can lead to hypoxia (or lowered flow of oxygen); and both illegal drugs and prescription drugs categorized from A to X by the FDA; to infectious agents such as bacteria, like syphilis; viruses, like chicken pox; and protozoa, like the cat-spread toxoplasmosis; as well as intangible elements, like radiation and heat.

The effects of exposure on a fetus range widely and up to death, but the nervous system defects and low birth weight are the most common.


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