Smoking during pregnancy could leave damaging markers in your child's blood for at least five years after birth, experts have warned.

New research has shown blood taken from children up to the age of five contains molecular evidence about whether or not their mothers smoked during pregnancy.

The findings offer strong evidence that environmental exposures in the womb remain in the body and potentially affect someone's health for years after birth.

The study also suggests that it may prove possible to detect exposures to other potential toxins during pregnancy, which are less evident, including chemicals in plastics, undetected infections or contaminants in drinking water.

Smoking during pregnancy leaves a genetic signature imprinted in your child's blood up to five years after birth. Scientists believe the marker could reveal links between chronic diseases, such as autism

Ultimately, researchers said they hope to be able to link these exposures to chronic diseases, such as autism, obesity or heart disease to better understand how diseases develop and possibly help prevent them.

Dr Margaret Daniele Fallin, study leader at Johns Hopkins University Bloomberg School of Public Health, said: 'If you have a blood sample, you may be able to ask research questions that you could never ask before.

'Smoking is one thing. But, if it turns out to be possible for other kinds of exposures, this could be a paradigm shift.

'We have long known that the body is an accumulator of past exposures - evidence of lead exposure lives on in our bones, for example.

'But we did not know that something as easy to collect as blood could contain evidence of exposures not only during your life but prenatally.

'That's what makes this so compelling.'

We did not know that something as easy to collect as blood could contain evidence of exposures not only during your life but prenatally. That's what makes this so compelling Dr Margaret Daniele Fallin, Johns Hopkins University

To arrive at their findings, Dr Fallin and her colleagues anaylsed epigenetics molecules that are not part of the DNA sequence, but sit on top of it and regulate which genes are turned on and off, when and where in the body.

Two years ago, another group of researchers looked at newborn cord blood, and discovered that the amount of an epigenetic mark, known as DNA methylation, at 26 locations on the genome, was correlated with whether that baby's mother had smoked during pregnancy.

For this new study, Dr Fallin's team took the experiment one step further.

They tested the blood of 531 preschoolers from six different sites in the US, and also spoke to the children's mothers about whether or not they had smoked during pregnancy.

Researchers analysed methylation patterns at the same 26 locations in the genome, and found that 81 per cent of the time their test was able to accurately predict prenatal smoking exposure.

It was not previously known whether this epigenetic signature would still be around as many as five years later, but the blood still contained this molecular memory.

It is possible, the researchers said, that the signature is also related to exposure to secondhand smoke after birth.

Scientists said the next step is to ascertain whether in utero exposures to smoking and other toxins are related to disease later in life, including autism

But that would not account for all of it since at birth - before they could be exposed to secondhand smoke - those babies whose mothers smoked during pregnancy, already had the signature.

Dr Fallin said she hopes this area of research has broader reach.

With smoking, she said, it is relatively easy to determine whether someone was exposed to cigarette smoking in the womb - simply by asking the mother.

But, with other toxins, it is more difficult to tease out.

For many, the mother may not know whether she was exposed.

'If epigenetic signatures can be found for other environmental exposures, these could provide clues to how certain prenatal exposures affect health and potentially decades into life,' co-author Dr Christine Ladd-Acosta, said.

Dr Fallin said, in particular, she is hoping to determine whether in utero exposures are related to the development of autism.

She said it remains unclear whether these epigenetic biomarkers are a direct cause of chronic diseases later in life or what else they may indicate.