Here's all you need to know about the three-parent baby.

Scientists have announced the birth of the first baby using a controversial new three-parent technique. The boy, whose parents are Jordanian, has two biological mums and one biological dad.

The technique was used so that the mother could have a child with her DNA without passing on a genetic disease.

The 36-year-old woman is a carrier for a fatal nervous system disorder called Leigh syndrome. Tragically, she’s already had two children die from the disease, one at the age of eight months and the other at the age of six. She’s also had four miscarriages.

The boy born using this technique is now five months old and doing well.

Got questions? We’ve got answers.

How did they do it?

A team, led by New York’s Dr John Zhang, took the nucleus from one of the mother’s eggs and implanted it into a donor egg, which had had its nucleus removed. That means, the egg got the nuclear DNA from the mum, which wasn’t carrying the genes for the disorder, but not the mitochondrial DNA, which was.

The egg was fertilised with the father’s sperm. It was implanted into the mother, and she had an “uneventful” pregnancy.

Dr John Zhang and the baby boy. Photo via YouTube.

So will the baby have more DNA from the mother or the egg donor?

The mother. The nucleus has about 20,000 genes and the mitochondria only has about 37. But it's not known exactly how important mitochondrial DNA are. Some researchers have suggested they might influence a person's IQ and athletic ability, as well as how long a person lives.

Is the technique safe?

It's a bit early to say. The treatment was carried out in Mexico, because it's not yet legal in the US. Harvard University ethics specialist Hank Greely says the technique “hasn’t been sufficiently proven safe enough to try to use to make a baby”.

In the 1990s, some three-parent babies were born using a slightly different technique. Mitochondria from healthy eggs was injected into the eggs of women who had been unsuccessful getting pregnant through IVF. It was tried 30 times. Two foetuses developed a genetic disorder. One was miscarried, the other was aborted. The technique fell out of favour.

The baby boy's health will need to be constantly monitored, but Dr Zhang is convinced he did the right thing.

“To save lives is the ethical thing to do,” he says.