Joshua died aged four months

But Joshua died in his parents' arms at the age of four months.

Susan and Chris desperately wanted a second child but they were terrified that the same thing would happen again.

Although there is a test for CF, which can be done during pregnancy, the couple did not want to face the possibility of aborting a baby.

They consulted Hammersmith Hospital fertility expert Professor Robert Winston, who agreed to try PGD.

Although only 18% of PGD attempts are successful, Susan said: "We didn't really have any hope before - now we've got hope."

A large number of eggs are needed for fertilisation for PGD so Susan's ovaries were stimulated with a drug.

Fifteen eggs were collected in the operating theatre before being fertilised with sperm from Chris and placed in an incubator for three days.

A large number of eggs are collected

Six embryos resulted, each invisible to the naked eye.

From the embryos, one cell was carefully removed for testing to see whether or not it carried the CF gene.

The technique leaves a hole in the side of the embryo but does not prevent the embryo growing into a healthy human being.

The Hammersmith's teams first attempt at genetic analysis of the embryos failed and it seemed that the whole process might be doomed to failure.

On the second attempt, conditions allowed them to determine that there were two healthy embryos free from cystic fibrosis.

Late at night, before the embryos were too old, Susan was taken to theatre and the embryos placed in her uterus.

Ten days later, a test was carried out to see if the embryos had implanted - itself a complex process.

Selecting a cell leaves a hole in the embryo

As Professor Winston points out, "humans are among the most infertile mammals on the planet".

Even with healthy eggs and healthy sperm there is no guarantee of making a healthy embryo and implantation can only occur during 48 hours out of every month.

But the embryo transfer in this case was successful and Susan was pregnant.

Healthy son

Professor Winston, who created the embryo, also delivered the baby - another son, Ethan.

"Technology allowed Sue and Chris to have a new baby son, confident that he doesn't have cystic fibrosis and that the heartache they previously suffered won't be repeated," said Professor Winston.

He acknowledges that the work may be controversial, and that the new techniques present many ethical dilemmas. But he believes that techniques like PGD hold out hope that the next generation of children will be able to lead a healthier life.

"One day medicine will change the destiny of children before they are even conceived," he said.

Superhuman - The Baby Makers is broadcast on BBC One on Sunday 19 November at 2110 GMT