Angela M. Sahin had just one thought.

“Please, just fix her.”

Her baby was just 8 days old when she was sped by special ambulance from Worcester to Children’s Hospital Boston. The doctors explained to her and her husband, Mark, that Angelina had a congenital heart defect. She would need open-heart surgery — three times — to overcome the problem.

“I couldn’t handle the details,” she said. “At first I was in shock. Then I was in denial. The doctors terrified me so much I would leave the room when they came in.”

Now Mrs. Sahin patiently explains hypoplastic left heart syndrome, or HLHS, and the purpose of the third operation that is planned for her little girl in May. For reasons not well understood, the left side of the heart fails to develop properly in children with HLHS. A solution must be fashioned to make the right ventricle take over the left ventricle’s job, which is to pump blood to the body.

Now 20 months old, Angelina romped through her grandmother’s Worcester apartment recently, showing few signs of the morning nap she missed, much less the heart defect that looms so large in her young life. She is small for her age, like many children with heart problems, but otherwise healthy, climbing into her grandmother’s lap to point out characters in a book called “The Angel’s Lullaby.”

Angelina is one of about 1 million Americans who have structural problems with their hearts that developed before they were born. The American Heart Associations estimates that eight out of every 1,000 babies born in the United States will have some form of congenital heart disorder. Most of them are mild. But severe heart defects, such as Angelina’s, are the leading cause of death in the first year of life. Almost twice as many children die from congenital heart disease every year as from childhood cancer, the AHA reports.

The picture is improving, though, at a pace that brings hope to patients and the people who care for them.

The outlook for HLHS has changed enormously, said Dr. Lars C. Erickson of Child Health Associates in Worcester, who is Angelina’s cardiologist. Sixteen years ago, when he was a fellow at Children’s (where the first successful surgery for HLHS was performed), the condition was considered inoperable in most parts of the country. Some children were considered for heart transplants. Other children were sent home with comfort care until they died.

Now the mortality rate at Children’s for the first and most difficult of the three HLHS operations — in which surgeons reroute blood vessels in infants soon after birth — is between 1 percent and 2 percent for children with no other complications. The three- to five-year survival rate after all three operations is better than 90 percent.

Advances are coming not just in HLHS treatment. Overall survival rates are above 98 percent.

“I believe that this improvement in survival rates for serious congenital heart disease in the last 30 years may be the single most striking success story in all of medicine,” said Dr. James E. Lock, cardiologist-in-chief at Children’s. “Before the ’70s, mortality rates were extremely high and long-term survival rates were extremely poor. Since then, essentially every form of congenital heart disease can now be successfully managed for decades, using surgical techniques, catheterization techniques and medical management.”

An example came last month, when a team of cardiologists from Children’s including Dr. Lock and high-risk obstetric specialists from Brigham & Women’s reported how they placed a tiny stent in the heart of a 30-week fetus. Grace VanDerwerken, who was born Jan. 10, has HLHS, but her case was complicated because she also had no hole between the two upper chambers of her heart. Without intervention, at birth blood would back up immediately into her lungs. Instead, she was born healthy and went home to Ashburn, Va., after having the first of her three HLHS surgeries.

Fetal surgery, which threads a catheter into tiny blood vessels to reach the heart, had been performed before to create an opening between the heart’s chambers, but inserting a stent, a device normally used to prop open adults’ clogged coronary arteries, was a first.

Cardiac catheterization has changed from being a tool for diagnosis to a method of repairing defects that used to require open-heart surgery, Dr. Erickson said. Devices inserted to patch holes in the heart have held up over time, which was a concern 15 years ago.

Minimally invasive surgery, called video assisted thoracoscopy, is being used to make very small incisions in the child’s chest, rather than the large cut from chin to the bottom of the breastbone. Diagnostic techniques have expanded to include tiny implants the size of a grain of rice that can be placed under the skin to monitor heart rhythm problems. The development of replacement tissue grown from the patient’s own cells to create new heart valves is being researched now, with the idea of placing them by catheter.

Improvements in surgical technique have allowed children to be operated on at younger ages. Fifteen years ago, the third of three surgeries for HLHS, for example, wasn’t performed on children under 2 years old. Now a refinement in the procedure relieves some of the pressure of blood flowing through the lungs, helping children recover more rapidly.

One of the reasons for the success in treating congenital heart disease lies in its anatomical nature.

“If you can fix the anatomy, especially if you can fix it early in life, you can reverse the disease process,” Dr. Lock said.

The first survivor of the three surgeries for HLHS is now in his 20s. The future looks good for children who followed him into HLHS surgery, although sports such as weight-lifting or football might be out.

“This is something that people can lead quite normal lives with,” Dr. Erickson said. “What’s going to happen in the next 20, 30, 40 years is impossible really to predict, but people are very optimistic that the outlook for these kids is going to be quite good.”

Angelina is doing well now, with medication and periodic visits to her cardiologist and Nancy M. Hagberg, her nurse practitioner, to monitor her progress.

“This is the way we want it to happen,” Ms. Hagberg said. “She’s sailing.”

Angelina’s mother is being monitored, too. She is expecting another child in July. Because there appears to be a genetic component to congenital heart defects, she will receive fetal echocardiograms, which are tests that use sound waves to study the heart’s structure and function.

Mrs. Sahin has learned from her daughter.

“She’s opened my eyes to a lot of things, like the importance of awareness and research,” she said. “It makes you compassionate. It doesn’t matter what race you are, how much money you have. It affects everyone, from all walks of life.”

While optimistic, she still worries.

“It’s very nerve-wracking. There are no guarantees,” she said. “They are so little, when you think of the things they go through.”