To prevent teenagers from smoking e-cigarettes, many states have put higher age restrictions on the purchasing of them in an effort to deter teenagers from starting a bad habit too early.

But for pregnant ones, a lack of access to e-cigarettes has caused an uptick in them smoking traditional cigarettes, according to a new study from Princeton and Cornell Universities.

Surprisingly, though, the research, published as a working paper in the National Bureau of Economic Research, showed the uptick didn’t result in negative birth outcomes.

Researchers analyzed more than 500,000 births among teens in the U.S. and found a 19.2 percent increase in cigarette smoking among pregnant teenagers and a 13.8 percent increase among underage pregnant teens following age restriction laws on e-cigarettes.

Prior research shows that 2014 was the first year that teenagers shifted away from using traditional tobacco products and began using electronic nicotine systems like e-cigarettes, vape pens and e-hookahs. State legislatures began tackling that problem by making the legal sale age 18.

"Traditional cigarette use typically declines during pregnancy, but our results show that laws limiting access to e-cigarettes actually slows down this decline, presumably because women are prevented from switching to e-cigarettes," said Janet M. Currie, an author on the study and the chair of the Department of Economics at Princeton, in a statement. "The figures suggest that pregnant women have an especially high demand for smoking cessation products early in their pregnancies. Hopefully, in future, we will be able to offer something that is better for their babies than e-cigarettes."

Currie and her team analyzed more than 547,000 birth records from the National Center for Health Statistics and compared women who gave birth before their 18th birthday, who had limits on their ability to buy e-cigarettes, with women giving birth at 19, who would have been able to buy the devices during the entire time they were pregnant. Both groups became pregnant between 2010 and 2014.

Results showed that 14 percent of 19-year-old mother smoked cigarettes during their pregnancy compared to 7.3 percent of the 17-year-old mothers, a finding that shows laws prohibiting selling cigarettes to underage individuals was helping to curb teens’ usage.

"The fact that pregnant teens use e-cigarettes where available may reflect the difficulty many pregnant teens face when trying to quit smoking, and the fact that few substitute products are available,” Currie said. “Pregnancy provides a unique window when women are open to guidance about resources and products available to help them quit smoking. This is the time to encourage pregnant women's use of tobacco cessation pharmacotherapy and interventions.”