Expectant mothers are encouraged to switch to a healthy diet and stop smoking, but scientific studies are showing by then it may be too late: the parents' pre-pregnancy lifestyles, even including a father's bad diet, are not forgotten by the genes.
One brand-new and shocking study has found that when adult mice were forced to become obese or develop type-2 diabetes, their offspring ended up with an innate susceptibility to these disorders.
An earlier study in Norway found that children whose fathers had smoked before they were born were at a higher risk of asthma.
The new science is upsetting an old assumption that children only "catch" self-destructive behaviour from their parents by watching it. Instead, some vices may be hard-coded into the genes from birth.
Although smoking and a high-fat diet most likely cause no mutations in the genes themselves (DNA), they affect the mechanisms of action and regulation of certain gene sequences. These so-called epigenetic - as opposed to genetic - factors are also heritable.
"There's now no doubt that not only gene sequences are passed on from generation to generation, but also the range of gene regulation," said Thomas Meitinger, director of the Institute of Human Genetics at the Munich Institute of Technology Hospital.
This has been shown in experiments on animals, he said, adding that studies on humans were more difficult, not least because scientists have to wait about 25 years for each next generation of test cases.
So far scientists have mainly studied fathers' role in epigenetic inheritance, since sperm are easier to obtain than ova.
The new study, on diet-induced obesity and diabetes in mice, was led by scientists from the German Research Centre for Environmental Health (HZM) and published in the journal Nature Genetics.
It found that mothers' role is greater.
"The parents' pre-pregnancy constitution transfers to the next generation," said Martin Hrabe de Angelis, initiator of the study and director of the HZM's Institute of Experimental Genetics (IEG).
Though "fat parents have fat kids" is a familiar rule of thumb, he noted, blaming one's obesity on "bad genes" has often been scorned as a flimsy excuse offered by people who over-eat.
"It's now clear this inheritance really does happen via the germ cells (egg and sperm cells). At any rate, the effect is massive in animal experiments," remarked Hrabe de Angelis.
"It could be another cause of the dramatic global increase in type-2 diabetes," which he said could hardly be explained by mutations in the genes themselves, because the increase has occurred too fast.
In his theories on heredity and evolution, British naturalist Charles Darwin proposed that traits which parents acquired during their lifetime through interaction with the environment would be passed on to offspring, noted the IEG's Johannes Beckers, who directed the study.
It has also been proposed that psychological stress, for example due to traumatic war experiences, can be passed on.
Since epigenetic inheritance – in contrast to genetic inheritance – is in principle reversible, positive lifestyle changes could lower the incidence of obesity and type 2 diabetes down the generational road.
"That," Hrabe de Angelis said, "gives us hope."