Vaping during pregnancy is becoming more common, but its impact on early human development is not well understood. A new study by scientists at the University of California, Riverside, now reports that the flavor chemical menthol used in electronic cigarettes could pose risks to a developing baby.

The study, published in Stem Cells Translational Medicine, used human embryonic stem cells (hESCs) to characterize early stages of embryonic development and examined how low concentrations of menthol affect important cellular processes.

The researchers found the concentration of menthol in the blood of pregnant women who vape is enough to activate stress-related channels called transient receptor potential channels in the hESCs.

“This activation led to inhibition of cell growth, increased cell death, and abnormal cell movement in the hESCs,” said Shabnam Etemadi, first author of the research paper and a bioengineering graduate student who works with Prue Talbot, a professor of the graduate division. “These changes could interfere with a critical stage of development called gastrulation, potentially increasing the risk of birth defects.”

Transient receptor potential channels are a large family of ion channels located in the membranes of many different cell types, including in reproductive tissues and embryos. They are involved in a wide range of sensory and physiological processes, including detecting heat, cold, pain, pressure, taste, and smell.

The researchers found that one of the channels, the TRPA1 channel, was activated by nanomolar concentrations of menthol, which their exposure model predicts would be in the blood of pregnant women who vape and would reach the embryo.

Etemadi explained that gastrulation involves movement of epiblast cells—cells that can differentiate into any cell type in the body—to form the definitive endoderm and the mesoderm. Gastrulation is considered a critical stage of development because it is the process where the three primary germ layers—the ectoderm, mesoderm, and endoderm—are formed during the third or fourth week of development, she said.

“These germ layers serve as the building blocks for all future organs and tissues in the embryo; any disruption during this stage can lead to significant structural birth defects due to the misallocation of cells destined for specific tissues and organs,” she added.

Talbot, the senior author of the research paper, said the findings highlight the need for more research on how vaping during pregnancy may harm embryonic and fetal development and point to possible dangers of using mentholated electronic cigarettes.

“The use of e-cigarettes by pregnant women should be discouraged until the effects on their embryos of flavor chemicals, such as menthol, are fully understood,” she said.