How does a surgeon operate on a fetus without damaging the amniotic sac ?

The sac can reseal itself from a puncture. That’s why amniocentesis can be performed, because a needle is inserted into the sac and extracts a small amount of fluid, which is then analyzed for the baby’s chromosomes.

There is risk though. There is a percentage of women who miscarry after amnio. Small, but it happens. I actually know someone it happened to. A wife of someone my husband worked with.

With miniaturized surgical equipment, and the use of remotely controlling the surgery, there is no need for a very large incision.

Fetal surgery is done every now and then. The sac is closed afterwards (how, I do not know).

The risk of miscarriage after amnio is 1:400. We researched it thoroughly before agreeing to have it done.