Nearly all pregnant women take prescription and/or over-the-counter (OTC) medication to treat a condition that may affect the health of the mother or the baby. Medications taken by the mother commonly include those used to treat pain, depression, diabetes, asthma and morning sickness. Drugs used to treat conditions such as abnormal heartbeats (arrhythmias) may be taken for the baby.

However, when the unborn child is not growing as quickly as expected—a condition known as intrauterine growth restriction (IUGR)—a typical medication dose taken by the mother may be too much for the fetus to break down (metabolize).

Because there are no professional guidelines for safe medication dosing in pregnant women with IUGR, researchers at the University of South Australia set out to investigate how drugs are metabolized during these pregnancies.

The review, which was published in Placenta, found that hormonal changes that occur during pregnancy may affect drug-metabolizing enzymes (DMEs), which play a key role in the baby’s ability to break down medication. These enzymes are less effective in IUGR babies, which can lead to drug toxicity at normal doses.

“If the fetus is smaller and a mother takes 20 mg of a drug, it may effectively be a higher dose than in a normal-sized baby,” said study author Janna L.  Morrison, PhD, head of the University of South Australia’s Early Origins of Adult Health Research Group. “The actual drug doesn’t make the fetus smaller, but if it is already smaller, the fetus may be less able to metabolize the drug and get rid of it.”

Worldwide, IUGR occurs in about one in seven pregnancies. During these pregnancies, the placenta fails to deliver the necessary nutrients and oxygen to the fetus. IUGR is more common in women with high blood pressure and in those who smoke, abuse drugs or drink alcohol during pregnancy.

While most babies weigh about six to eight pounds at birth, IUGR babies weigh less than 5.5 pounds. IUGR babies are at increased risk for diabetes, heart disease and impaired immunity throughout their lives.

Based on their findings, the researchers would like for additional studies to be conducted to better understand the effects of medication dosing during pregnancy. Depending on how the fetus metabolizes the drug, the optimal dose may need to be lower or higher.

“It doesn’t automatically correlate that a lower dose would be better if the fetus metabolizes it faster,” explained Dr. Morrison. “It may mean that with some complicated pregnancies, a higher dose is needed with some drugs. It’s about making sure that the right dose is given to help the mother, without harming the baby.”

Takeway: Until more is known, pregnant women are advised to check with their health-care provider before taking any prescription or OTC medication. In IUGR pregnancies, the mother should also ask whether the baby could be at higher risk from any drugs she may take. Any pregnant woman also should consult her doctor before taking a dietary or herbal supplement.

Source: Study titled “The Impact of Intrauterine Growth Restriction on Cytochrome P450 Enzyme Expression and Activity,” led by researchers at University of South Australia, published in Placenta.

Related Articles