Using e-cigarettes -- touted as a safer alternative to regular smoking -- may impair fertility and pregnancy outcomes, according to a study carried out in mice.

Many young and pregnant women are using e-cigarettes, but little is known about their effects on fertility and pregnancy outcomes, researchers said.

According to the US Centers for Disease Control and Prevention, the number of middle and high school students using e-cigarettes rose from 2.1 million in 2017 to 3.6 million in 2018 -- a difference of about 1.5 million youth.

"We found that e-cigarette usage prior to conception significantly delayed implantation of a fertilised embryo to the uterus, thus delaying and reducing fertility (in mice)," said Kathleen Caron, of University of North Carolina in the US.

"We also discovered that e-cigarette usage throughout pregnancy changed the long-term health and metabolism of female offspring -- imparting lifelong, second-generation effects on the growing foetus," Caron said.

The study, published in the Journal of the Endocrine Society, used a mouse model to examine whether e-cigarette exposure impairs fertility and offspring health.

After exposure to e-cigarette vapour, female mice showed decreased embryo implantation and a significant delay in the onset of pregnancy with the first litter.

Female offspring exposed to e-cigarettes in utero also failed to gain as much weight as control mice by the 8.5 month mark.

"These findings are important because they change our views on the perceived safety of e-cigarettes as alternatives to traditional cigarettes before and during pregnancy," Caron said.