SALEM — They're still new to the world. They flail their little arms and legs, blink and look around a bit. Gavin Hansen grunts more than his twin brother, Gabriel, and he's smaller and has a small mark above his left eyebrow. For now that's how Christopher and Janis Hansen tell them apart.

But Christopher and Janis are just happy to have to tell them apart. Gavin and Gabriel were diagnosed with twin-to-twin transfusion syndrome, a disorder in which the movement of blood and other fluid between the twins in the womb becomes imbalanced, creating massive stress on both babies.

TTTS only happens in identical twins who share the same placenta, but have separate sacs, said Dr. Robert Ball, a maternal-fetal medicine specialist at St. Mark's Hospital in Salt Lake City. Blood vessels on the surface of the placenta provide a connection between the babies' circulations. How the babies respond to dehydration or over-hydration also affects the outcome of the syndrome.

Ball and his partner, Dr. Michael Belfort, are the only physicians in the region who could perform in utero operations to try to save the twins. There are two types of surgeries: amnioreduction, where the doctor pokes a hole in the membrane between the twins and allows the fluid to equalize, and a laser option, where doctors use a laser to close the shared blood vessels. While it can cure the twins, the laser option can cause brain damage to the surviving babies.

For twins with TTTS, if the case is serious enough and nothing is done, the survival rate is in the zero to 10 percent range for either baby, Ball said. The chance that at least one twin will survive to 6 weeks old is 60 percent with the amnioreduction and a little more than 70 percent with the laser therapy, Ball said.

Ball discovered a discrepancy in the fluid with the Hansens' twins when Janis was only about 12 weeks along, he said. By the time she was 16 or 17 weeks, he knew there was a serious problem and at about 18 weeks, the Hansens decided to have the laser therapy.

Born on April 2 at 34 weeks, Gavin and Gabriel spent 40 days in the Newborn Intensive Care Unit at Timpanogos Regional Hospital in Orem. Gavin, who was the "donor" baby — dehydrated and with less blood — weighed 3 pounds 11 ounces, while Gabriel weighed 5 pounds 3 ounces, Janis said.

Gavin has nearly doubled in weight and Gabriel gained 2 more pounds and continues to grow.

"We feel so fortunate," Janis said.

She added that they were grateful for Ball and Belfort's help during the difficult process.