The scarring on the uterus from the two operations makes it likely that she will need cesareans for future births, and also increases the risk of placental problems that can be life-threatening. The small slits for the fetoscopes are thought to reduce these risks.

To develop their fetoscopic procedure, Dr. Belfort and Dr. Whitehead operated on sheep and spent several hundred hours over the course of two years practicing on a simulator that they had created. It consisted of a rubber kickball, about the size of a basketball — like a uterus at 24 weeks of pregnancy — with a doll inside, wrapped in chicken skin that they cut to mimic the defect in spina bifida.

They would insert fetoscopes into the ball and, eyes on the monitor, work together to sew up the chicken skin. They completed more than 30 simulated operations, including two sessions in an operating room, with a full surgical team assembled. They still use the simulator at least twice a month to keep up their skills, Dr. Belfort said.

They operated on their first patient in July 2014. In August, in the journal Obstetrics and Gynecology, they reported on their first 28 cases. So far, the results have been good, though the numbers are small.

No fetuses have died, few have needed shunts, and some of the mothers have been able to have vaginal deliveries. Their pregnancies appear to last longer, coming closer to full-term than with the open procedure. More research is needed, but other medical centers have begun adopting the technique. Surgeons at Johns Hopkins have used it to treat five patients, and Dr. Belfort is helping to train colleagues at Stanford.

Doctors who practice the open procedure are critical, and warn that the carbon dioxide pumped into the uterus may harm the fetus and cause neurological problems. Dr. Belfort said there has been no evidence of harm. But time will tell.