The procedure to open up a narrow aortic valve in the heart of the unborn baby is known as a foetal aortic valvuloplasty.
Doctors diagnosed the foetus as suffering from severe aortic stenosis, meaning the baby's aortic valve was very tight.
Blood was backing up in the left ventricle of the baby's heart, preventing it from pumping normally, 'Los Angeles Times' reported. Without the rare procedure the left ventricle would not develop properly, and the baby would likely be born with hypoplastic left heart syndrome (HLHS), which is a life-threatening condition, doctors said.
After practising a few times with a model of jello and a grape - the grape standing in for the heart, the jello standing in for the surrounding body - the doctors performed the procedure on September 25.
Both the baby and the mother were given local anaesthesia. The baby was also given a muscle relaxant so it wouldn't switch positions at an inopportune time.
Doctors performing the surgery at CHA Hollywood Presbyterian Medical Centre relied on ultrasound imaging to see what they were doing.
Only few weeks after the surgery, doctors reported that both the mother and the foetus are doing fine. "It's only been a week or two, but even initially after the procedure, we could see increased blood flow across the valve, and the heart was squeezing a bit better than before," foetal cardiologist Dr Jay Pruetz of Children's Hospital Los Angeles said.