While laws placing age limits on the purchasing of e-cigarettes are intended to reduce the use of tobacco products, a recent study shows an unintended consequence: a rise in traditional cigarette smoking among pregnant teens.

But the uptick in cigarette smoking did not produce negative birth outcomes, according to research by Princeton University and Cornell University, who analyzed more than 500,000 births among teenagers in the United States.

Published as a working paper in the National Bureau of Economic Research (NBER), the research shows a 19.2 percent increase in cigarette smoking among pregnant teenagers - both of age and underage - and a 13.8 percent increase among underage pregnant teens following the enactment of minimum legal sale ages for electronic nicotine delivery systems.

Every state already has age limits on traditional cigarette sales, and if these limits were perfectly enforced, there would be no possibility for substitution between e-cigarettes and the traditional variety. Therefore, the findings suggest that limits on e-cigarettes are more binding on minors and that teens often substitute one source of nicotine for the other.

"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 co-author Janet M. Currie, Henry Putnam Professor of Economics and Public Affairs and chair of the Department of Economics at Princeton. "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."

According to national surveys, 2014 was the first year that teens made a dramatic shift away from traditional tobacco products, turning instead to electronic nicotine delivery systems - like e-cigarettes, personal vaporizers, vape pens, e-cigars, e-hookahs and more. In response, a number of states enacted varying laws to reduce access to these devices. The most common policy was a minimum legal sale age of 18, which was enacted by 40 states between 2010 and 2014.

Two previously published studies have shown that when these laws are implemented, traditional cigarette smoking rises among teens. This finding motivated Currie and her co-author Michael Pesko, assistant professor of health care policy and research at Cornell University, to study how age-specific policies limiting access to e-cigarettes may have influenced smoking of traditional cigarettes among pregnant teens.

To study this question, the researchers evaluated 547,176 birth records provided by the National Center for Health Statistics (NCHS). They compared women who gave birth before their 18th birthday - who faced new limits on purchases of e-cigarettes - with women giving birth at age 19, who would have been able to purchase such devices legally throughout the full length of their pregnancy in most states.

Both age groups conceived between 2010 and 2014; during these years, information about traditional and e-cigarette use on birth records was more complete (following a revision to the data collection form by the NCHS). Of the women studied, 40.8 percent were pregnant women under age 17, and 59.2 percent were women at age 19. Medicaid paid for more than 70 percent of all births.

The researchers matched this data with the laws placing age limits on access to e-cigarettes, which they obtained through the Centers for Disease Control and Prevention. They controlled for cigarette tax rates and workplace smoking restrictions.

The researchers found that 14 percent of 19-year-old mothers smoked cigarettes during their pregnancy compared to 7.3 percent of 17-year-old mothers. This difference likely reflects the fact that most 19 year olds can legally purchase cigarettes in many states, the researchers said.

Overall, the policy had the greatest effects on the youngest pregnant women. Following the law change, the average underage pregnant woman smoked about a half a cigarette extra per day. In the locations that passed e-cigarette age-limit laws, 13.7 percent of these women smoked traditional cigarettes in the three months prior to their pregnancies, 12.4 percent smoked in the first trimester, 11.3 percent smoked in the second trimester and 11.1 percent smoked in the third trimester. These rates were higher in the states with the law than those states without.

"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. 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," Currie said.

The study highlights the need for more data to properly determine how age-limit laws on electronic delivery systems influence maternal and infant health. "At present, birth records record traditional cigarette use but ignore electronic nicotine delivery systems. We believe states should collect data on both," Pesko said. "States may also wish to consider adding questions on other sources of nicotine exposure, such as through the use of nicotine replacement therapy."

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More information: The paper, "The Effect of E-Cigarette Minimum Legal Sale Age Laws on Traditional Cigarette Use and Birth Outcomes Among Pregnant Teenagers," first appeared in NBER.