Baby Tanner weighed just 13 ounces and had a heart the size of a peanut. So the delicacy of a watchmaker was required for a pioneering operation. David Whitehouse reports
Esther Durham's pregnancy had not been without concern. She suffered from hypertension and was confined to bed for long periods: a precautionary but not especially worrying measure. She was looking forward, at 28, to the birth of her first baby.

Tanner, her son, was born 12 weeks prematurely last month, weighing 13 ounces. It soon became apparent that in addition to the expected difficulties of a premature baby he also had multiple heart problems, the most serious of which is called coarctation of the aorta. For Esther it was a bolt out of the blue; there was nothing in her pregnancy or in her family history that prepared her for this.

Tanner's aorta, the artery that supplies blood to the lower two-thirds of his body, was blocked by a membrane. This was restricting the blood flow to his kidneys and vital organs. Left uncorrected, a stroke and brain damage was inevitable and death probable. It is a common condition: one in 100 babies is born with a heart defect. Of those, 7 per cent - one in every 5,000 live births - have coarctation of the aorta. Many parents know what Esther went through.

Time was not on Tanner's side and something had to be done. But an open- heart operation would have been too traumatic. "Because of his small size, open heart surgery would have been too risky to attempt," says Zuhdi Lababidi, the surgeon who eventually saved Tanner. "If we were to operate then we had to be like watchmakers."

The procedure Mr Lababidi, a paediatric cardiologist at the University of Missouri Children's Hospital, had in mind is called coarctation angioplasty, which he has spent the past 14 years pioneering. He has carried out the operation on 300 babies and children throughout the world, but this one would be the most demanding of them all.

Before Tanner Durham the next smallest baby to have such a delicate heart problem corrected was a 2lb boy in 1987. Other cardiologists have performed the operation on infants as small as 4lbs. No one had carried out the operation on an infant who weighed under 1lb.

At three weeks old, after some of his other problems had stabilised, Tanner was flown by helicopter to the University of Missouri Children's Hospital for more intensive treatment. He was very small and fragile and the doctors wanted to try everything else first fearing that he might not survive any kind of operation.

But after a week of drugs tiny Tanner was developing congestive heart failure. Without some form of action he was going to die. Mr Lababidi flew in and within hours Tanner was being prepared for a record-breaking operation.

With conventional angioplasty a needle is inserted through a small incision into an adult's groin. A fine flexible wire, with a deflated balloon at the end, is threaded through the needle, then pushed up the artery and into the heart. Using X-ray images to help them, surgeons position the balloon and inflate it to force the artery open. It has become a relatively straightforward procedure in adults - but for Tanner?

The arteries in Tanner's leg were too small for the finest needle and his heart was the size of a peanut. Mr Lababidi had to insert the catheter through the umbilicus, or tummy button, into the umbilical artery, which is bigger, then guide it through a 180 degree turn and thread it up through the aorta towards the heart. But getting the balloon to the right spot was only half the problem.

Tanner's aortic wall was paper thin, the aorta barely thicker than a toothpick. Inflating the balloon was tricky: one wrong move could have torn the wall and killed Tanner instantly. "Because of his small blood volume any blood loss would have been life threatening. If we lost just 8 millilitres of blood - one- and-a-half teaspoons' worth - he would have gone into shock and died," notes Mr Lababidi.

It took less than an hour to remove the blockage. The results were immediately apparent. The blood pressure in Tanner's lower body immediately rose to normal levels. His mother watched with relief as for the first time colour flooded throughout his tiny body.

Although the procedure saved Tanner's life, he still faces an uphill battle. He had another operation on 2 March to tie off a blood vessel that normally closes shortly after birth. He is doing very well and his doctors are trying to wheen him off his ventilator. His mother is looking forward to taking him home.

The writer is the BBC's science correspondent.