The triplets' mother, Allison Penn, was impregnated with just one embryo through in-vitro fertilization, said Dr. Victor Klein, a specialist in multiple births and high-risk pregnancies who delivered the boys.
That embryo split in half, and then one half of that split again, he said.
"This is the first one we're aware of in the literature in the country in which they only put back one embryo" and a woman gave birth to triplets, said Klein. "Most people put back two or three embryos, and you just never know."
Identical triplets are born at a rate between one in 60,000 and one in 200 million, depending on the research, Klein said.
Allison Penn, 31, said she and her husband, Tom, 46, had tried to have a baby since they got married about four years ago. Although she once thought of having several children, the disappointments over four years revised her dreams downward.
"When it took us so long to get pregnant, I just assumed we were going to have one, and that would probably be it," she said. "So I thought one would be good."
And when she and her husband were told three youngsters were on the way?
"I looked over at Allison and her mouth was wide open and her eyes were like saucers, and she didn't say a word," Tom Penn said. "Then I realized that it was possible, and then I started to laugh."
He confessed he couldn't get over the irony.
"Everything we had done was to have one baby," he said. "Anybody who says God doesn't have a sense of humor -- everything we did was just for having one baby, and now we have three."
To help tell them apart, the boys have a dot of maroon nail polish on their fingers. Logan Thomas, who weighed 4 pounds, 12 ounces, has a mark on his thumb; Eli Kirkwood, a 4-pounder, has polish on his forefinger, and Collin McGuire, at 4 pounds, 11 ounces, has a mark on his middle finger. Logan may have a problem with a non-functioning kidney, but the other children are healthy, doctors said.
Allison, an education specialist for the U.S. Fish and Wildlife Service, said she has not decided whether to return to work.
"That's one of those 'one-day-at-time' issues," she said.