In the early 90's a groundbreaking prenatal surgery for the crippling disease spina bifida started to become popular. It did cause some problems such as premature deliveries so researchers wanted to test it against postnatal surgery. Collaborating with surgeons across the country, researchers basically brought this surgery to a halt anywhere but at the study hospitals. This means that over the course of the 7 year study, the only way to even hope for a chance at the surgery families would have to agree to be part of the study. At that point they would be randomly assigned to either the prenatal or postnatal surgery.
Where I think it became unethical is that the only reason they did the study was to prove that the prenatal surgery was more effective than the postnatal. They already knew that it worked, yet they deprived many families of any chance at giving their children a halfway normal life with this disease.
It should have been up to the families to decide if the surgery was worth the risks and not researchers.