Against all the odds, a young American woman has delivered her second set of triplets in less than two years.

Crystal Cornick, 19, who did not take any fertility drugs, gave birth naturally - without Caesarian section - on Tuesday to Ariel, Tyree and Tyler at the University of Maryland Hospital in Baltimore.

The babies weigh between 3 pounds 14 ounces (1.75kg) and 5 pounds (2.3kg), and are being kept in the hospital's intensive care unit.

Doctors at the hospital said mother and babies and doing very well and should be sent home soon.

"They'll be in the unit one or two weeks - just for safety's sake. Their prognosis is excellent," said Dr Ira Gewolb.

The couple's first set of triplets, two girls and a boy, are now about 20 months old.

Dumbfounded

She and her fiancé, 22-year-old Richard Williams, said they were dumbfounded when they discovered in September that they would be having triplets again.

"I was at work, she called me ... and said, 'We're having triplets.' I said, 'No, you're lying, stop lying.'"

Ms Cornick said her pastor helped reassure her.

"He said, 'Your children are a blessing and you should feel lucky to have them, because some people can't have children and you're lucky to be blessed with three at once,"' Ms Cornick said.

"So that kind of put me at ease." Dr Gewolb, the director of the neonatal department of the hospital, said the odds of Ms Cornick having a second set of triplets were very high.

"It's rare. It's pretty rare. The odds are one in many millions at least, possibly one in 10 million."

Ms Cornick was studying finance, and had been planning to return to her studies in September. Mr Williams is a truck driver.

The young parents were working with their local church and the hospital's social work office, but officials said the couple would be grateful for any donations from the community.