Scientists have found only a few medical differences between children born as a result of assisted reproduction and kids conceived in the traditional way.

More than three million children have been born worldwide as a result of assisted reproductive technologies since the birth of the first "test tube baby" in 1978.

While the majority of these kids are healthy and normal, as a group they are at greater risk of certain kinds of birth defects and being low birth weight, which is linked to obesity, hypertension and type 2 diabetes later in life.

In the new study, Carmen Sapienza, a geneticist at Temple University School of Medicine, investigated the effect of the environment on genes by comparing one type of chromosome modification, called "DNA methylation" between children conceived in the traditional way with children born as a result of assisted reproduction.

He said: "We found that 5-10 per cent of these chromosome modifications were different in children born through assisted reproduction, and this altered the expression of nearby genes. "But we have not yet distinguished whether this is caused by assisted reproductive technologies or other factors such as the couple's infertility."

Since some of the affected genes are involved in the development of fat tissue and the metabolism of glucose, Sapienza says it will be important to monitor these children long-term to find if they have higher rates of obesity or diabetes.

He notes that although there were measurable differences in DNA methylation and gene expression between the two groups, only a small fraction of the assisted reproduction children were found to be outside the "normal" range.