UK scientists believe in the future they will be curing babies in the womb of serious diseases with the use of gene therapy.
The aim is to cure diseases before they can cause damage
The work is controversial not just because of the ethics but also safety concerns.
A few years ago France and the US suspended gene therapy trials after a child who had undergone treatment at the age of three developed cancer.
The British Society for Gene Therapy heard how trials were progressing.
Gene therapy is a way of treating disease by either replacing damaged or abnormal genes with normal ones or by providing new genetic instructions to help fight disease.
These therapeutic genes can be transferred into the patient attached to a non-threatening virus or similar carrier "vector" which is injected it into the body.
Scientists have already successfully treated patients with haemophilia and infants with rare "bubble boy" disease, who have no immune system, using gene therapy.
But gene therapy in children and adults has faced problems because the recipient's body can mount an immune response and make antibodies that prevent the treatment working.
And there is also the risk that the treatment may trigger other diseases, like cancer.
The hope is that, at the foetal stage, the immune system is not yet developed sufficiently to prevent the effect of the implanted genes.
Scientists believe the treatment could also be more powerful in babies who are still developing and whose cells are rapidly multiplying.
Plus, it could provide a cure before the disease has had chance to cause any damage in the unborn child.
Benefits vs risks
Dr Simon Waddington of University College London explained: "There are several advantages. For example, in cystic fibrosis, lung damage is actually occurring before birth.
"So, if you can get your gene therapy in before then, you might be able to stop the disease from happening.
"If you are going to treat adults it is often too late to reverse some of the damage."
He stressed this was not meddling with gene traits to be passed on to future generations. "We are not modifying the children's children, only treating that patient."
He and colleagues at Imperial College have already successfully implanted corrective genes in foetal mice with disease.
And they have received a grant to test a gene therapy cure for the blood disorder haemophilia in unborn primates.
Dr Waddington said they still had another five years of testing in animals to do before they could think about move the technique into humans.
But he added: "The technologies are there to deliver and inject genes into babies if we find it is effective enough and safe enough."
The scientists cannot yet exclude whether the genes might pass into the mother as well as the baby, for example.
Co-researcher Professor Charles Coutelle, at Imperial College, said: "The safety issue is something that will take time...and needs looking at very carefully."
A Department of Health spokesperson said: "No human clinical trials of in utero gene therapy have ever taken place in the UK, nor is this considered to be feasible in the next few years."