Ateratogen is a material that can causebirth defects. Teratogens include somechemicals,medications, andinfectious diseases. The name goes back to Ancient Greek "theras", which can be translated as "monster".

About 75% of all birth defects are caused by teratogens.^{[1][2]p. 23-3}

Alcohol is the most common cause of congenital disorders that can be prevented.^[3] Alcohol ispoisonous to afetus and can causebrain damage.^[4]Drinking alcohol whilepregnant can cause the fetus to getfetal alcohol syndrome.^[3]

Illegal drugs, likeheroin andcocaine, are also poisonous to the fetus and can cause many different congenital problems. For example, cocaine use during pregnancy can causemicrocephaly (a smaller head size than usual) and problems with the way the fetus'surinary system andgenitals grow.^[1]

Some medications can also hurt the fetus if its mother takes them while she is pregnant. For example:^{[1][2]p. 23-6}

• Lithium carbonate (used to treatbipolar disorder) can cause problems in the way the fetus'sheart grows
• Phenytoin andvalproic acid (anti-seizure medications) can causeintellectual disability, microcephaly, and many other problems in fetuses
• Warfarin (a blood-thinning medication) can cause problems with the fetus'scentral nervous system, including intellectual disability
• ACE inhibitors (a group of medications forhigh blood pressure) can cause problems with the fetus'skidneys

Before the 1960s, many countries did not have rules about testing medications for their effects on fetuses.^[5] This changed partly because ofthalidomide. This medication was given to pregnant women fornausea in the 1950s and 1960s. Between 1956 and 1962, more than 10,000 children in 46 different countries were born with birth defects, like arms and legs that had not grown.^[6] Thalidomide had not been tested well enough before it started beingprescribed.^[6] Now, many countries require more testing before a medication can be said to be safe during pregnancy.^[5]

If a woman gets an infection while she is pregnant, sometimes the infection can affect her fetus. Theplacenta protects the fetus from many differentviruses,bacteria, and otherpathogens that cause infections. However, some pathogens can get through the placenta and infect the fetus. This is calledvertical transmission. Some of these infections can cause birth defects.

Examples of infections that can cause birth defects include:

• Syphilis^[7]
• Toxoplasmosis^[8]
• Chickenpox^[9]
• Rubella (German measles)^[10]

Some chemicals can cause birth defects, if a pregnant woman isexposed to enough of them. For example:^[11]

• Lead: If a woman ever hadlead poisoning, she can pass lead on to her fetus, even if she is not exposed to lead while she is pregnant. This happens because most lead is stored in a person'sbones and can come out into thebloodstream many years later. Lead can causemiscarriage andstillbirth as well as birth defects.
• Mercury: High levels of mercury can causebrain damage in fetuses.

High levels ofionizing radiation can cause birth defects.^[2]p. 23-8

Examples of things that do not cause birth defects include:^[11]

• Spermicides (these were suspected of causing birth defects in the 1970s and 1980s, but they have been proven to be safe)
• Acetaminophen (paracetamol)
• Prenatal vitamins
• Non-ionizing radiation. This is the type of radiation that comes fromsunlight andmicrowave ovens. It does not cause birth defects.

• Health problems

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