The world's first "three parent baby" was born five months ago to Jordanian parents. The treatment, which seeks to avoid mothers passing defective mitochondrial DNA to their offspring, was performed in Mexico.

The news, revealed by the New Scientist, is already hailed as revolutionary advance in the field of reproductive medicine. The procedure - based on a new In vitro fertilisation technique that incorporates DNA from three parents - was necessary because the baby's mother carries mitochondrial DNA mutations for Leigh syndrome, a fatal neurological disorder.

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Because of this, she and her husband had already lost two children, so they asked a US-based team of scientists – at New Hope Fertility Center in New York City – for their help. The doctors, led by John Zhang, designed a new "three-parent" technique.

The traditional way to do this is to use a method known as pronuclear transfer, which involves fertilising both the mother's egg and a donor's egg with the father's sperm. When both eggs start dividing into early-stage embryos, scientists remove the nucleus. The nucleus from the donor's egg is discarded, and the one from the mother is inserted in its place. This is the technique that was legally approved in the UK in 2015.

However, the couple rejected this technique for religious reasons – they didn't want an embryo to be discarded. Thus, another "three parent" technique, known as spindle nuclear transfer, was used: the nucleus was first removed from the mother's egg and inserted into a donor's egg, after removing its own nucleus. Only then was the resulting egg fertilised with the father's sperm.

The complete findings will be presented at the American Society for Reproductive Medicine's Scientific Congress in Salt Lake City in October, but already it represents a huge step forward for a technique that has often attracted a lot of heated debates.

The idea of three parent babies previously sparked criticism from some groups, which claimed it could pave the way for "designer babies". At the time of its approval in the UK, England's chief medical officer Dame Sally Davies commented: "The intention in making these regulations is to ensure that mothers who carry faulty mitochondria can have healthy children free from devastating and often deadly conditions caused by serious mitochondrial disease.

"It is important to remember that mitochondrial DNA represents less than 0.054 per cent of the total DNA, and is not part of the nuclear DNA, which determines our personal characteristics and traits such as personality, hair and eye colour."

What is mitochondrial DNA?

Mitochondria are organelles responsible for creating the energy needed by the cells to function. They are found in the body of every living cells, separated from the nucleus which holds the nuclear DNA – the genetic characteristics inherited both from the mother and the father.

But mitochondria also have a small amount of their own DNA. Mitochondrial DNA contains 37 genes passed down to us from our mothers. Genetic mutations in mitochondrial genes can lead to devastating diseases for offspring. Mutations in one of several different mitochondrial genes can cause Leigh syndrome, a progressive disorder of the nervous system that usually appears in infancy.

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