Rachel Lendyak-Peters says she probably spends more time up with her infant twin sons than their middle-of-the-night feeding requires, but she likes to watch them just to make sure they're still breathing. Sometimes the Verona woman prods them to get a physical reaction.

Her nervous vigilance is understandable. It's nothing short of a medical miracle that Greyson and Sawyer Peters, who turned 4 months old on Dec. 1, are not just alive, but healthy and thriving.

Make that three medical miracles.

They twice underwent laser surgery while still in the uterus, at 16 weeks and 22 weeks gestation, because they were suffering from twin-twin transfusion syndrome. It is a rare life-threatening disease occurring in just 10 percent of the pregnancies in which identical twins are in separate amniotic sacs but share a placenta.

They also were born about eight weeks premature and spent some two months in the neonatal intensive care unit at Magee-Womens Hospital of UPMC.

"It's hard to think about because it was so bad," said Ms. Lendyak-Peters, 26.

The first two medical miracles were the successful in utero surgeries for the dangerous TTTS.

With fetus twins, blood vessels normally fan out from each twin's umbilical cord and connect to the placenta. But sometimes vessels of one twin find vessels of the other and form connections. Blood passes freely from the "donor twin" to the "recipient twin," and twin-twin transfusion syndrome occurs.

The donor twin stops producing urine and amniotic fluid decreases, usually leading to death. The recipient gets increasingly high blood pressure, develops heart failure and also usually dies. The death of one twin usually results in the death of or injury to the other.

During the two surgeries on Greyson and Sawyer, maternal-fetal medicine specialist Stephen Emery, director of the Fetal Diagnosis and Treatment Center at Magee, found as many of the connections as he could and blocked them by burning them with a laser. Between the two surgeries, the donor twin and recipient twin had reversed roles, causing the occurrence of new vessels and the need for the second procedure.