Moreover, says Fisk, the procedure is still far from perfect. Trial results suggest that two-thirds of the time, laser ablation leads to brain damage or death for at least one twin. Meanwhile, 70% or more of twins with the mildest form of TTTS do better with no intervention or a single amnioreduction, and predicting which fetuses will progress to a more severe stage after 25 weeks (the cutoff for laser ablation because of the risk of rupturing vessels) is extremely difficult. “So you’re left with the question of how to treat early-stage disease,” Fisk says.

One answer may come from researchers at Children’s in Philadelphia who have just developed a technique involving echocardiograms and Doppler ultrasound measurements that can evaluate the severity of TTTS and the likelihood it will progress. “With a new, 20-point scoring system, we can observe a characteristic series of changes that occur in the vessels and the heart that tell us this is TTTS and not something else,” says Johnson. “It lets us know when to intervene and how to tailor treatment more effectively.”

Knowing whether to intervene in utero to repair hypoplastic left heart syndrome, a grossly underdeveloped and nonfunctioning left ventricle and aortic valve, is murkier yet. The rare heart defect is detected during routine ultrasound in fewer than half of the fetuses that have it. And some cases of HLHS are better dealt with after birth, if at all. “HLHS surgery is sensational, but no one knows which babies need it and which don’t,” says pediatric surgeon Alan Flake, a fetal-surgery pioneer and director of the Children’s Center for Fetal Research at the Children’s Hospital of Philadelphia. “I’m not knocking it—I’ve been in this field a long time and think it’s a good thing. But, in general, very few kids qualify.”

Gynecologist/obstetrician Sally Grogono, however, is grateful for the procedure, which opened the aortic valve in the grape-size heart of her second son, who would be named Anders when he was born 17 weeks later. Grogono traveled from her home in Texas to Children’s Hospital Boston for the experimental fetal surgery. Chief of cardiology James Lock and a team of five fetal specialists inserted a five-inch needle into Grogono’s abdominal wall, passing it through the uterine wall, the amniotic space and Anders’s chest to pierce his heart. Guided only by ultrasound, Lock manipulated a wire the thickness of a hair through the needle’s hollow core, advanced a balloon and inflated it to four millimeters, splitting open the leaflets of the valve.

Without the fetal surgery, Anders likely would have needed repeated open-heart surgery early in life to make his right ventricle the sole pumping chamber. Even then, his remodeled heart would probably fail before he reached age 40. Now a year old, he has had a repeat balloon-dilation procedure and two open-heart surgeries. He weighs 15 pounds and has a feeding tube—but he does have a partially functioning left ventricle. “It’s not perfect, but Anders is far better off than if he hadn’t had the intervention,” Grogono says. “We’re so thankful there was a way to give him a chance for a better life.”

Spina bifida—a hole in the spinal column—is rarely lethal, making it unique among conditions treated with fetal surgery. Yet it is unquestionably devastating, often resulting in bowel and bladder incontinence, paralysis and cognitive disorders, and parents have latched on to the idea that early surgical intervention might forestall many symptoms.

First performed in 1997 at Vanderbilt University in Nashville, the procedure involves cutting the mother’s abdomen, lifting and opening the uterus, and partially removing the fetus. Surgeons push the exposed spinal cord and protective membranes into place, sew the opening shut and return the fetus to the womb and the womb to the mother.