If the fetus has a sacrococcygeal teratoma, the ultrasound will show a large mass. The mass may be either cystic or solid in appearance or commonly a mixture of both. Measurements will be taken to determine how far into the abdomen the teratoma extends and what organs it may have affected. Many times the mass can cause pressure on internal organs like the bladder, and may lead to obstruction and enlargement of the kidneys (hydronephrosis)

The tumor that is mostly solid has a large amount of blood vessels, and this causes the fetus' heart to pump extra hard to circulate blood through both his body and the tumor. This extra work may cause the fetus to develop heart failure (hydrops). This will be monitored closely by fetal echocardiography, which looks directly at fetal heart function.

There are two types of prenatal interventions. Your doctor will discuss with you which of them is right for you. For the first procedure, the mother's uterus is opened, and the baby is taken out of the uterus, and, while the fetus is still attached to and sustained by the mother's placenta, the tumor is removed. The fetus is then returned to the mother's uterus to be carried as close to term as possible. Open fetal surgery can cause preterm labor and early delivery, so mothers are monitored closely after surgery.

The second procedure, which does not involve opening the uterus, is called radio frequency ablation. During this procedure, a needle is placed into the mother's uterus and into the tumor. Radio frequency waves are sent through the needle to destroy the blood vessels that lead to the tumor. Without blood flow to the tumor, it does not grow and the hydrops resolves. After delivery of the baby, the mass is completely removed in the operating room, and reconstruction is performed if needed.