They started lining up outside the Children's Museum of Pittsburgh an hour early yesterday: babies, squirmy toddlers and antsy kindergartners in singles and sets of twins, triplets, and at least one set of quadruplets. There were some 400 of them in all, accompanied by enough family members to push attendance past 1,300.

It was a reunion for little ones who had "graduated" in the past five years from the neonatal intensive care unit of the Western Pennsylvania Hospital. They did so after stays ranging from a day or two to months, with the average stay about 16 days.

But, looking at most of them, it was difficult to imagine they had ever been sick a day in their young lives.

Inside the North Side facility they climbed toward the ceiling through a vertical maze; ran at a tilt across the dizzying floor of the Gravity Room; "drove" the real-life Mini Cooper in the Garage Workshop; swished down a very fast slide; jumped up and down as a light threw their shadows across a wall; and chowed down -- if they were old enough to chew -- on hot dogs and pizza.

Prematurity -- with the accompanying need to grow and mature their lungs -- is the biggest reason some 500 infants are admitted annually to the West Penn NICU, said unit manager Lisa Graper, and "over time they catch up. Usually it takes a few years. They don't do it all at once."

Some don't develop as quickly as others, especially those with additional, serious health issues.

Joseph Paul Long was born eight weeks early by Caesarean section to Nicole and Evan Long of Worthington on Feb. 28. Mrs. Long had suffered from pre-eclampsia, a condition marked by high blood pressure and a high level of protein in the urine.

Joseph weighed just 3 pounds 5 ounces but was remarkably healthy.

"He was never sick, just little," his mother said. "[His lungs] were actually good. They were amazed with him ... He was just anxious [to be born]."

He was in the West Penn NICU about a month and went home two days after Easter.

Now Joseph, a handsome blond, weighs 16 pounds and laughs a lot.

"He's a very happy baby," Mrs. Long said. "He loves attention."

Dominic and Shelby Seybert were born to Tanya and Mike Seybert of New Kensington six weeks early, on March 15, 2006, by Caesarean because they had lined up side by side "trying to come out first," as Mrs Seybert put it. Dominic weighed 5 pounds 12 ounces; Shelby, 5 pounds.

They were admitted to the NICU because neither was eating fully and both had jaundice. They went home three weeks later.

"We have since then not had any health problems at all," the twins' mother said.

The McGrath triplets of Murrysville have had to battle many more problems.

The triplets, all fraternal, the children of Tony and Dana McGrath of Murrysville, were born, also by C-section, at 25 weeks gestation, or 15 weeks early, on Oct. 3, 2006. Molly, at 2 pounds, was the largest; Bridget and Shannon were 1 pound 13 ounces each.

Molly got out of the hospital two days before Christmas. Bridget went home about the third week of January, and Shannon finally joined her sisters at home Feb. 10, 2007.

"They had multiple health problems," Mrs. McGrath said.

"Bridget had a brain hemorrhage. ... [She] also had abdominal surgery when she was 9 days old for a perforation." Both conditions are common in preemies.

"Shannon had to have a heart procedure, a PDA ligation," her mother added. That's a surgery needed for a baby to come off a ventilator.

"Molly was what they called 'the grower.' She had the least issues. She just had to catch up in size."

Now as they approach their second birthday, the three little blondes -- dressed up in red and white frocks for the reunion -- look like the picture of health. Molly, at 34 pounds, is by far the biggest, with Bridget and Shannon at 25 pounds each. Their energy is boundless.

"They're walkers," Mrs. McGrath said. "It's just that we're out-numbered. We can't chase them because they all go in different directions."

Robert "Bobby" McWilliams, 3, the son of Crystal and Lewis McWilliams of Verona, spent 7¹/₂ months in the NICU after his birth April 23, 2005. He was born at 25 weeks gestation and weighed just 1 pound 9 ounces. Now he weighs nearly 38 pounds.

"He's a miracle," Mrs. McWilliams said. "I was given only a 30 percent chance for him to be born alive."

But Bobby has a long way to go before he catches up because he was born with a number of serious problems. They included a chronic lung disease called bronchopulmonary dysplasia that kept him on a ventilator for a long time; an underdeveloped throat that required a tracheotomy and, later, throat reconstruction; a reflux condition that requires him to be fed through a gastric tube; and cerebral palsy. He has had 21 surgeries and now needs to have his adenoids removed.

"Before his tracheotomy tube came out in August 2007, we had never heard him at all," Mrs. McWilliams said. "We never heard him cough. We never heard him laugh."

But now he's learning both to talk and to walk with the aid of a walker and is home-schooled in preschool by the Penn Hills School District.

"Through time and working, he should get on track," his mother said.

"There's nothing wrong up here," she added, pointing to her brain. "It's all physical. He's just the happiest guy you've ever seen. He's always smiling."

Lisa Graper loves to hear testimonials like that.

"We're celebrating," she said, pointing to the other NICU workers attending the reunion. "This is the joy of our work. It makes us do what we do.

"Some still have struggles, but they have a joy in life, and they bring joy to their families."