The charity Twins Trust, which supported the study, said the new approach could make a real difference.

"Any procedure that is non-invasive and can potentially identify TTTS earlier and improve outcomes for our families with this life-threatening condition could be a turning point," said Helen Peck, head of healthcare engagement and research.

Weeks after Brioney's scan, it was clear the procedure had worked - the blood flow to the babies had been rebalanced, although there were other issues during the course of her pregnancy.

Scans showed Margo, who did not have enough fluid around her, "was in a much better position", Brioney said, and "the strain on Nancy's heart had eased".

The twins were born at nearly 34 weeks - Nancy weighing 3lbs 7 and Margo 3lbs 3 ounces.

"They were both healthy, and Margo wasn't as small as we worried she was going to be," Brioney said.

In September, they start primary school. "They're funny, smart, energetic little girls that just fit right in with their age group," Brioney said.