Cigarette use typically declines during pregnancy, researchers say but as of 2010, there was a 14% increase in pregnant teens smoking tobacco Experts believe this is because the new laws, making e-cigarettes illegal for under-18s, have been far more strictly enforced than the identical age limit on tobacco products. It means, without an alternative, teenage smokers who fall pregnant seem to simply continue with their habit.

Previous research has found that when these laws are implemented, traditional cigarette smoking rises among adolescents.

However, this is the first study to suggest that age-specific policies limiting access to e-cigarettes may have influenced smoking of traditional cigarettes among pregnant teens.

According to national surveys, 2014 was the first year that teens made a dramatic shift away from traditional tobacco products, They instead turned to electronic nicotine delivery systems, including e-cigarettes, personal vaporizers, vape pens, e-cigars, and e-hookahs.

In response, a number of states enacted various laws to reduce access to these devices. The most common policy was a minimum legal sale age of 18, enacted by 40 states between 2010 and 2014.

The study, conducted by Princeton University and Cornell University, looked at almost 550,000 birth records provided by the National Centre for Health Statistics. Matched with this data were the laws placing age limits on access to e-cigarettes, obtained through the Centres for Disease Control and Prevention.

The researchers 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. Scientists found a 19 percent increase in cigarette smoking among pregnant teenagers - both of age and underage - and an almost 14 percent increase among underage pregnant teens following the legal sale ages placed on e-cigarettes.

Co-author Janet Currie, professor of economics and public affairs at Princeton, said: '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. 'The figures suggest that pregnant women have an especially high demand for smoking cessation products early in their pregnancies. Hopefully, in the future, we will be able to offer something that is better for their babies than e-cigarettes.'

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 more 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. Scientists found that 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,' Professor Currie said. 'Pregnancy provides a unique window when women are open to guidance about resources and products available to help them quit smoking.'

Researchers say the results of this study highlight the need for more data to properly determine how age-limit laws on electronic delivery systems influence maternal and infant health.

Co-author Dr Michael Pesko, assistant professor of health care policy and research at Cornell, said: 'At present, birth records record traditional cigarette use but ignore electronic nicotine delivery systems. We believe states should collect data on both. 'States may also wish to consider adding questions on other sources of nicotine exposure, such as through the use of nicotine replacement therapy.'