While pregnancy is ordinarily a joyful time for expectant parents, it can also be stressful. Pregnant women need to be mindful of the growing list of prescription medications that have been deemed unsafe during pregnancy.

All drugs and chemicals ingested by the mother during pregnancy have the potential to cross the placenta – which provides oxygen and nutrients to the unborn baby from the mother – and some medications that are normally completely safe may cause harm to the unborn baby.

The Federal Drug Administration (FDA) uses a ranking system that helps doctors and pregnant patients decide which medications are safe. Medications are ranked into categories according to the known risk to unborn babies, and this list is growing longer.

The FDA recently strengthened its warning that the prescription medication topiramate, sold under the brand name Topamax, increases the risk of pregnant women having babies with cleft lips or cleft palates. Topamax is taken for migraines and seizures.

The FDA also recently added antipsychotic drugs such as Haldol, Zyprexa, Seroquel, and Abilify, warning that these medications could cause babies to suffer withdrawal symptoms – such as agitation and difficulty breathing and feeding – for hours or days after birth when their mothers took the drug while pregnant. The FDA also warned pregnant women not to use Terbutaline, a drug used to treat asthma and halt preterm labor, because it can cause maternal heart problems or even death.

Also last month, the Centers for Disease Control and Prevention warned that pregnant women should not take pain relievers containing opioids, such as Vicodin, OxyContin and Tylenol with codeine, just before conception or in early pregnancy, because it increases the risk of congenital heart defects, glaucoma and other problems.

There is a lack of safety information as to many drugs, because of ethical problems associated with exposing pregnant women and fetuses to unknown risks in trials used to test the safety of drugs. Yet studies show that 64 percent of expectant mothers take a prescription medication while pregnant, and take three to five on average.

Doctors agree, however, that despite potential risks, there are some medical conditions that women cannot leave untreated during pregnancy, because doing so could cause harm to the pregnant woman as well as to the fetus. Doctors must carefully weigh the balance of the risk of harm to the mother and fetus without treatment against the risk of harm to the fetus caused by the medication.