The six boys and two girls were doing well and were in stable condition in the neonatal intensive care unit, said Dr Karen Maples at the Kaiser Permanente Medical Centre in the Los Angeles suburb of Bellflower.

But two needed some help to breath with ventilators, she told a press conference.

The eight babies were born nine weeks prematurely by Caesarean section over a five-minute period, stunning a 46-member medical team that was expecting only seven babies.

They weighed between 1 pound 8 ounces (680 grams) and 3 pounds 4 ounces (1.47 kg) and doctors initially identified them by the letters A through H as they were born.

"We decided to proceed with the delivery in anticipation of seven babies. We had done some drills, some preliminary dry runs," Maples said.

"Lo and behold, after we got to Baby G, which is what we expected, we were surprised by Baby H."

Maples said she had been following the mother, who was not identified, since the first trimester of her pregnancy.

Citing patient confidentiality rules, the hospital declined to say whether the mother had become pregnant through fertility treatments, which can raise the likelihood of multiple births.

"It was a shock, especially with the eighth baby," Maples said.

The mother plans to breast feed all eight babies, her doctors said.