Drugs That Can Affect Pregnancy (Teratogenic Substances)

These days many women are aware of the side effects and contraindications associated with drugs, either prescription or non-prescription medicines, and how it can affect their pregnancy adversely. However, with the list of teratogenic substances growing, it is important to have a greater understanding of how the fetus may be affected rather than just avoiding specific drugs and related substances.

What are Teratogenic Substances?

Any substance which can cause developmental malformations in the embryo is known as a teratogen. These may include prescription and over-the counter (OTC) drugs, herbal medicines and supplements, tobacco, alcohol, as well as illegal drugs.

Perhaps the most publicized teratogenic drug known is Thalidomide, which was prescribed and used extensively in the late 50’s and early 60’s for morning sickness in pregnancy. It resulted in the birth of babies with phocomelia (seal-like flippers in place of limbs) and other internal abnormalities and had to be withdrawn from the market.

How Drugs Affect the Fetus?

Drugs reach the fetus through the placenta, in the same way that oxygen and nutrients are delivered to the baby in the mother’s uterus. Depending upon the drug taken, the amount, duration, and stage of pregnancy, it can produce varying effects. Drugs can damage the fetus (resulting in abortion), cause developmental abnormalities (producing birth defects), and result in stillbirth.

A baby is most likely to be born with birth defects if exposed to the harmful effects of drugs during the time when the organs are developing, usually between the third and eighth week of fertilization. This means that sometimes, in very early pregnancy, the fetus may be exposed to harmful substances without a woman being aware that she is pregnant. Drugs taken after this period, although less likely to cause severe birth defects, may contribute to problems with growth and function of normally developed body parts.

Pregnancy Drug Categories

According to the degree of risk to the fetus when used in pregnancy, drugs are classified into categories by the Food and Drug Administration (FDA).

  • Category A
  • These drugs are considered absolutely safe for the fetus, as proved by well-designed studies and reputable clinical trials. This category includes prenatal vitamins and folic acid.
  • Category B
  • Animal studies show no harmful effects but results of human studies are inconclusive or animal studies show harmful effects but not so in human studies. This includes amoxicillin (antibiotic), ondansetron (for nausea), insulin (for diabetes).
  • Category C
  • Human and animal studies are inconclusive or animal studies show harm to the animal fetus. No data on the effect on the human fetus. This includes fluconazole (for yeast infection), sertraline and fluoxetine (for depression).
  • Category D
  • These drugs may cause risk to human fetus but may sometimes be considered for the treatment of life-threatening or severe conditions where the risks are outweighed by the beneficial effects. This includes phenytoin (for epilepsy) and some cancer drugs (chemotherapy).
  • Category X
  • Risks to the fetus cannot be outweighed by any possible benefit. This includes isotretinoin (for acne), thalidomide (for leprosy and certain types of cancer drugs.

Vaccines in Pregnancy

Vaccines made of live virus, such as rubella (German measles) and varicella (chicken pox) cannot be given to pregnant women because of their teratogenic effects. However, vaccines for cholera, typhoid, hepatitis A and B, tetanus, diphtheria, plague, and rabies may be given if considered necessary.

Tobacco, Alcohol and Narcotics

The harmful effects of tobacco and alcohol on the growing fetus are many, as are illegal drugs when used during pregnancy. This is discussed further under the dangers of cigarette smoking in pregnancy and dangers of alcohol consumption during pregnancy.

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