Scientists have found evidence of effects before birth that could account for differences in identical twins.
On paper, identical twins that share the same womb and set of genes should be indistinguishable.
But the activity of their genes can be affected by environmental factors unique to each of them.
Researchers identified widespread differences of this kind in the genes of identical twins at birth.
They believe they can be traced to the influence of tissues such as the placenta and umbilical cord that are specific to each twin.
Genes are affected by environmental factors through a chemical process called epigenetic tagging, which acts like a switch turning them on or off.
In the new study, scientists looked at the epigenetic profiles of umbilical cord and placenta tissue, and cord blood from pairs of new born identical and non-identical twins.
Even in identical twins, they were significantly different.
"This must be due to events that happened to one twin and not the other," said lead research Dr Jeffrey Craig, of the Murdoch Children's Research Institute (MCRI) in Australia.
The study is reported online in the journal Genome Research.
Further evidence supporting the known link between low birth weight and the risk of conditions such as diabetes and heart disease was found by the scientists.
Tagged genes associated with birth weight were also those that played roles in growth, metabolism and cardiovascular disease.
"This has potential to identify and track disease risk early in life, or even to modify risk through specific environmental or dietary interventions," said co-author Dr Richard Saffery, also from the MCRI.