Professor Jarvis' team analysed levels of a by-product of nicotine called cotinine in the saliva of 2, 031 adult smokers.

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P450s, and one of its jobs is to convert nicotine into a less harmful chemical called cotinine that can then be excreted.

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It revealed that only 10% of all non-smokers had no measurable cotinine.

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In the maternal samples, they measured testosterone, a hormone, and cotinine, a breakdown product of nicotine that shows a person's level of smoking.

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