A woman who was given permission to have embryo screening treatment in a bid to save her son is carrying the UK's first "designer baby".
Stem cells from the new baby could save its brother's life
Julie Fletcher and husband Joe, from Moira, County Down, won the go-ahead earlier this year to begin treatment.
Their two-year-old son Joshua has a potentially fatal blood disorder.
Diamond Blackfan anaemia (DBA) can be treated by using stem cells from a sibling to stimulate his body to produce healthy red blood cells.
WHAT IS INVOLVED
Doctors test embryos to find perfect tissue match for Joshua
Mrs Fletcher carries the embryo to term
At the time of birth, blood from the cord would be extracted
This would be given to Joshua, triggering his own body to produce the red cells he needs
Dr Mohammed Taranissi, the director of the Assisted Reproduction and Gynaecology Centre in London, who has treated the Fletchers, said it was "very early days".
He said: "She tested positively for the pregnancy, but it has only been a few days, so we still have a long way to go."
The couple were given the go-ahead in September to begin controversial embryo screening treatment which could save the life of their son.
It followed a decision in July by the UK's fertility watchdog to relax the rules on the creation of so-called "designer babies" to help sick siblings.
The Human Fertilisation and Embryology Authority said embryos could be selected which were free of disease and which could provide blood cell transplants to treat sick brothers and sisters.
Dr Taranissi said: "At the time of birth, we can actually take the cord blood, which is something which is dispensed with anyway.
"This blood has got stem cells, which are very primitive cells, that can be transfused into the affected child.
"This, hopefully, will give him a very good chance of a complete cure."
Dr Taranissi said the chances of success were about 85%.
He said that the term "designer baby" was misleading as no embryos were being designed.
"The embryos develop at random in the laboratory. What we do is test them for certain potential characteristics and then use them to help people who are seriously ill," he said.