Pregnant women tend to heed the advice of doctors when it comes to giving up smoking traditional cigarettes, but not necessarily for vaping.

Researchers at the University of Iowa found that when compared to the general population, pregnant women in the U.S. were less likely to use cigarettes. But when it came to using e-cigarettes, the rate of vaping was about equal. The study appears Monday in JAMA Pediatrics.

Though e-cigarettes have been marketed as a quit-smoking strategy, they do contain nicotine, which can be harmful to both the mother and fetus. It’s also not clear whether the flavored fluid used in e-cigarettes might produce hazardous byproducts.

The researchers looked at nearly 28,000 women ages 18 to 44 — 1,071 of whom were pregnant — using data from the National Health Interview Survey. The study is the first estimate of e-cigarette use by U.S. women who are pregnant or of child-bearing age, authors said.

In addition, the study found that e-cigarette use was more common among pregnant women who also smoked combustible cigarettes than it was among women who were not pregnant and used both kinds of products.

Nationally, there has been a dramatic increase in vaping, especially among teens. In 2018, about 4.9 million middle and high school students used some type of tobacco product in the 30 days before they were surveyed, according to the U.S. Centers for Disease Control and Prevention.

“I am very happy that a very low percentage of our patients smoke to begin with,” said Holly Cummings, assistant professor of clinical obstetrics and gynecology at the University of Pennsylvania’s Perelman School of Medicine. Patients at her practice have not reported turning to e-cigarettes as an alternative to traditional cigarettes, she said.

Cummings said she counsels any patient who is using a nicotine product, like cigarettes, e-cigarettes, gum or patches, about the potential dangers of nicotine for both the mother and fetus.

“Probably the biggest one we think about is that babies are born with low birth weight,” she said.

According to Children’s Hospital of Philadelphia, babies who are born weighing less than 2,500 grams (5 pounds, 8 ounces) fall into this category, sometimes because they are born too soon. Low birth weight babies may have a harder time gaining weight and fighting infections and may face other complications.

At the Southern New Jersey Perinatal Cooperative, there haven’t been clients who report vaping or using both e-cigarettes and regular cigarettes, said Cathy Butler-Witt, the assistant director of public health programs.

The group offers free smoking cessation programs for pregnant women and others in the house who may smoke or care for the children. And they work with providers who care for pregnant women and mothers to provide educational materials, she said.

“When people come to the program, they are aware of the risk factors during pregnancy,” said Butler-Witt. But she said there is the perception that because e-cigarettes do not have the same chemicals as conventional cigarettes, they are somewhat safer.

Nicotine is known to be harmful to developing brains and lungs, Butler-Witt said.

“The brain development is altered,” said Butler-Witt. The infant can go through a nicotine withdrawal that looks like colic. They are also at risk for learning difficulties, behavior problems and attention deficit hyperactivity disorder, she said.

“The baby’s lung capacity actually changes,” she said. Using a product with nicotine can result in an early delivery and loss of a pregnancy.

A 2018 study found that a mother’s exposure to nicotine whether from combustible cigarettes, nicotine patches or e-cigarettes increases the risk of sudden infant death syndrome for her child.