The study shows that an epigenetic memory of the mother's smoking during pregnancy can remain in the child's blood as long as 5 years after birth.
This was the finding of a study led by Johns Hopkins Bloomberg School of Public Health in Baltimore, MD, and published in the journal Environmental Research.
The researchers suggest further investigation may well reveal similar findings about prenatal exposure to other toxins - including less obvious ones such as chemicals in plastics or contaminants in drinking water.
If that is the case, then it opens up the possibility that such exposures are linked to chronic diseases such as obesity, heart disease and autism - a fact that should help us understand them better, say the researchers.
Senior author M. Daniele Fallin, a professor in the Bloomberg School's Department of Mental Health, explains the significance of such a finding:
"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 this turns out to be possible for other kinds of exposures, this could be a paradigm shift."
Previous research has already established that the DNA of cord blood from newborns is altered if the mother smokes during pregnancy. The difference is not in the genetic code itself but in the presence of "epigenetic" marks left on the DNA at 26 locations of the genome.
Epigenetic markers are molecules that attach to genes and influence their activity - such as switching them on or off.
Smoking in pregnancy leaves 'epigenetic signature' in child's blood
In the new study, the team took the investigation further. They tested the blood of 531 preschoolers from six different places in the US and interviewed their mothers to find out if they had smoked when they were pregnant with those children.
They looked at the 26 epigenetic markers in the children's blood samples and found 81% of the time they could use them to predict whether the mother had smoked during pregnancy.
The study is the first to show that an epigenetic memory, or signature, of the mother's smoking during pregnancy can remain in the child's blood as long as 5 years after birth.
The researchers acknowledge it is possible that in some cases the epigenetic memory could be related to secondhand smoke exposure after birth, but this would not account for all of it, bearing in mind the previous finding that the signature can already be present in cord blood.
They suggest the findings point to possibilities beyond smoking. It is relatively easy to establish, by asking the mother, if a child has been exposed to tobacco smoking before birth. It is not so easy to establish if they have been exposed to other toxins - the mother may not know.
Lead author Christine Ladd-Acosta, assistant professor of epidemiology at the Bloomberg School, concludes:
"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."
Meanwhile, Medical News Today recently learned that researchers in Japan urge greater awareness after finding that children who are exposed to secondhand smoke as they grow have double the risk of tooth decay. In The BMJ, the researchers explain how they found children exposed to secondhand smoke at 4 months of age have a higher risk of tooth decay at age 3 years.