Melissa Paske was 29 weeks pregnant with a girl when she asked her doctor for a second ultrasound. Her belly had been scanned at 24 weeks, and there was no medical reason to take additional images. But, says Paske, "something in me felt that I needed to see her again." That something turned out to be a devastating diagnosis for her baby: a badly clogged aortic valve, which doctors feared could lead to a massively underdeveloped left side of the heart, a condition called hypoplastic left heart syndrome (HLHS). The news, says Paske, "was like a truck backing up over me."

Babies with HLHS typically undergo a complex series of surgeries after birth to compensate for the defect. But a team of doctors at Children's Hospital Boston and neighboring Brigham and Women's Hospital offered Melissa Paske and her husband, Travis, of Spokane, Wash., something new: a revolutionary procedure performed while their baby was still floating in Melissa's womb. Within days the Paskes were flying across the country. In her 30th week of pregnancy, doctors inserted a needle through Melissa's uterus into her fetus's heart, where they dilated a balloon and opened the blocked valve. Six weeks later the Paskes' daughter, Camryn, was born HLHS-free. Last week the slender little girl with eight tiny teeth and a mop of light brown hair celebrated her 1st birthday. "She's just beautiful," says Melissa.

She's also a medical pioneer. Thirty years ago, babies diagnosed with HLHS were doomed to die within the first days to weeks of life. Today, thanks to earlier detection through ultrasound and postnatal surgery, the majority of the 1,500-plus HLHS babies born every year now survive; the oldest are in their early 20s. The medical journey, however, is far from easy. Ten to 25 percent of infants die during or after their first operation, and those who do make it require lifelong care; some will need pacemakers or even heart transplants. The condition is so severe that many couples choose to terminate their pregnancies. The Boston team is now trying to change all that by waging a pre-emptive strike through fetal intervention. The procedure is experimental, but the potential payoff is huge: warding off HLHS before it even develops. It's way too early to claim victory, but Dr. Jim Lock, Children's cardiologist in chief, is optimistic: "This is clearly the wave of the future."

Development of the human heart is nothing short of anatomical wizardry. The organ starts out as a narrow tube, then twists and turns during the first eight weeks of gestation to create its elegant final design: four chambers, four valves, two walls or septums and an assortment of veins and arteries. The two lower chambers of the heart operate as pumping stations: the right ventricle squeezes blood out to the lungs, and the left--responsible for the lion's share of the work--pumps to the rest of the body. Nobody knows precisely what causes HLHS, but one key factor is the plumbing problem that Camryn suffered early on in development: a narrowing of the aortic valve. The malfunction causes blood to back up in the left ventricle, forcing the developing heart muscle to squeeze extra hard. Eventually the muscle tires, stops pumping and growing, and withers. The outcome is dire: babies with HLHS have one working ventricle instead of two, making their hearts incapable of sustaining life.

As of now, the best treatment for HLHS is a three-stage open-heart surgery--performed at birth, at 6 months and around the age of 2 or 3--which re-engineers the heart's plumbing, making the right side take on the pumping job of the left side as well. (A complete heart transplant is also an option, but few infant hearts are available for donation.) "We don't fix the heart," says Dr. Jack Rychik, director of the Fetal Heart Program at Children's Hospital of Philadelphia (CHOP). "We cheat nature by redirecting the blood." Since it was first attempted in the early 1980s, the procedure has been a godsend, saving the lives of thousands. But it's not a cure. The goal among fetal experts is to redirect the heart toward normal development and get rid of HLHS altogether.

Operating in the womb raises the surgical stakes. For starters, there are two patients--mother and fetus--and one of them is floating in fluid. A team of doctors, including anesthesiologists (both mother and fetus are given medication to relieve pain), echocardiographers, obstetricians, radiologists and cardiologists, must work together, using snowy ultrasound images as their only road map. A needle must be carefully guided through the mother's abdomen and uterine wall into the grape-size fetal heart and then on to the aortic valve, about the diameter of spaghetti, where doctors dilate a balloon to stretch the opening. Along with the technical challenges come operational dangers. Both mother and fetus are susceptible to complications from anesthesia. The mother could develop blood clots or an infection or go into premature labor. Her fetus could be injured by the needle, suffer cardiac distress, even die in utero. "There are risks with no guarantee that the intervention will work," says Lock's colleague Dr. Wayne Tworetzky, who counsels prospective parents. "We have to be very straightforward with families."

Since 2000, the Boston team has performed just over 50 procedures. Most have been aortic-valve dilations, but the team is using a similar approach to try to fix blocked pulmonary valves, which can lead to a mirror image of HLHS called hypoplastic right heart syndrome. And the team is intervening in an even more dangerous problem that strikes some fetuses who have already developed HLHS: a blockage in the atrial septum, which divides the two upper chambers of the heart. In addition to withered left hearts, these babies are susceptible to lung damage. Not all the procedures have been technically successful--the needle hasn't gone in properly or the balloon hasn't dilated. Some of the sickest babies have died despite the intervention; others have developed HLHS. But seven babies who had successful aortic-valve dilations have been spared the syndrome, a home run in congenital heart disease. "We now have significant evidence that we can remodel hearts before birth," says Lock. "This represents a major potential breakthrough in the field."

There are still challenges ahead. The womb is new territory, the learning curve is steep and the parents, desperate to help their babies, are vulnerable. Doctors can't predict with certainty which babies will benefit from the procedure, making patient selection tricky. And even if a procedure does seem warranted, there's no guarantee that the baby's outcome will be better than what standard surgery now offers. CHOP's Rychik believes more research is needed on the evolution of congenital heart disease before those dilemmas can be solved. "I'm encouraged by the Boston experience, and I applaud it," says Rychik, who is launching a nationwide study to document fetal heart development, but "we're still very early in all of this." Other clinics are taking the plunge, albeit in very small numbers. Doctors at the University of California, San Francisco, have performed one fetal procedure; the Cleveland Clinic has completed two. "We're seeing the fetus as a patient, and we're prepared to do everything in our power to make things better," says Cleveland's Dr. Stephen Emery.

Fetal intervention is still in its infancy. The number of affected babies is small, and the technique requires highly skilled doctors at topnotch clinics. But the odds for sick babies could improve immediately simply through better detection of abnormalities during pregnancy. Alex Osborne of Lebanon, Pa., is proof of that. His heart defect was identified during a routine 19-week ultrasound, allowing his parents, Julie and David, to seek out cardiac experts at CHOP well before their baby's due date. Doctors were able to counsel the couple about Alex's condition--he had HLHS complicated by a blocked atrial septum--and prepare them for surgical intervention. And a team of experts, including obstetricians, pediatric cardiologists and cardiac surgeons, could be assembled for Julie's delivery, ready to provide immediate care. "We delivered him and rushed him to the OR," says Rychik. Just two hours after birth, Alex had his first surgery. Today he's 7 years old and a four-foot-tall, 50-pound budding paleontologist. "It's unlikely he would have survived if we hadn't known about his condition ahead of time," says Rychik.

Alex, however, is in the minority. Today more than 90 percent of women receive midtrimester ultrasounds, and yet only 20 to 30 percent of congenital heart defects are picked up, says Dr. Lisa Hornberger, director of UCSF's Fetal Cardiovascular Program. That means doctors are delivering babies with ticking time bombs inside: they look perfectly healthy at birth, then suddenly crash, gasping for air. Ultrasound works, says Hornberger; "the problem is, people don't know what to look for." Today Hornberger is conducting seminars for obstetricians and other diagnosticians, teaching them to identify the earliest signs of trouble. Once they do, babies can be scheduled for surgery after birth, or perhaps even referred for fetal intervention. "We're at the tip of the iceberg," she says. "We need to be improving detection, or we're not going to be making a difference."

Camryn Paske is making a difference in her own little 1-year-old way. "She's crawling and getting into everything," says Melissa. "I call her my 'terror on knees'." She's also laughing and playing around with pet beagles Copper and Bagel. Camryn's heart isn't perfect. She takes three medications a day to make sure it pumps efficiently, and one day she may need her damaged aortic valve repaired or replaced. But she's been spared the worst. "I feel so fortunate and indebted to the doctors," says Melissa. Her family may live in Spokane, but "we're Red Sox fans now," she says. Rooting not just for baseball--but for every fragile little heart that makes its way to Boston.