Identical Twins Start Showing Differences in the Womb

International   |   Steven Ertelt   |   Jul 24, 2012   |   1:30PM   |   Melbourne, Australia

Identical twins may share the same womb and may have so few differences that it is often difficult to tell them apart, but are born with different alterations to their DNA that can affect the activity of individual genes, according to a new study from Australian researchers.

Previous studies have shown identical twins do have different sets of DNA modifications, known as epigenetic markers, but it was previous thought that these changes occurred after birth as the twins experience slightly different environments. But, in another study showing the fascination of human development before birth, scientists from Murdoch Children’s Research Institute, Australia show that differences are already apparent at birth.

Your genes determine much about you, but environment can have a strong influence on your genes even before birth, with consequences that can last a lifetime. In a study published online in Genome Research (, researchers have for the first time shown that the environment experienced in the womb defines the newborn epigenetic profile, the chemical modifications to DNA we are born with, that could have implications for disease risk later in life.

“Twins, like the rest of us, sit in their own amniotic sac and have their own individual experiences,” lead researcher Dr. Jeffrey Craig told International Business Times. “Sometimes one placenta could be in the best place in the womb, while the other twin might be shunted off to the side somewhere.”

Twin pairs, both monozygotic (identical) and dizygotic (fraternal), are ideal for epigenetic study because they share the same mother but have their own umbilical cord and amniotic sac, and in the case of identical twins, also share the same genetic make-up. Previous studies have shown that methylation can vary significantly at a single gene across multiple tissues of identical twins, but it is important to know what the DNA methylation landscape looks like across the genome.

In this report, an international team of researchers has for the first time analyzed genome-scale DNA methylation profiles of umbilical cord tissue, cord blood, and placenta of newborn identical and fraternal twin pairs to estimate how genes, the shared environment that their mother provides and the potentially different intrauterine environments experienced by each twin contribute to the epigenome. The group found that even in identical twins, there are widespread differences in the epigenetic profile of twins at birth.

“This must be due to events that happened to one twin and not the other,” said Dr. Craig.

Craig added that although twins share a womb, the influence of specific tissues like the placenta and umbilical cord can be different for each fetus, and likely affects the epigenetic profile.

Interestingly, the team found that methylated genes closely associated with birth weight in their cohort are genes known to play roles in growth, metabolism, and cardiovascular disease, lending further support to a known link between low birth weight and risk for diseases such as diabetes and heart disease. The authors explained that their findings suggest the unique environmental experiences in the womb may have a more profound effect on epigenetic factors that influence health throughout life than previously thought.

An understanding of how the intrauterine environment molds the human epigenome could provide critical information about disease risk to help manage health throughout life. An understanding of the epigenetic profile at birth could be a particularly powerful tool for managing future health.

“This has potential to identify and track disease risk early in life, said Dr. Richard Saffery of the MCRI and a co-senior author of the study, “or even to modify risk through specific environmental or dietary interventions.”