(CNN) -- A woman in California delivered what may be the nation's second live-born set of octuplets on Monday morning, surprising doctors who expected seven babies.

The first three to seven days will be critical for the infants, Dr. Mandhir Gupta said.

The six boys and two girls -- ranging in weight from 1 pound 8 ounces to 3 pounds 4 ounces -- were generally doing well in incubators following their Caesarean-section delivery at Kaiser Permanente hospital in Bellflower, California, doctors said.

Three of the babies need breathing assistance, but otherwise the eight don't appear to have serious problems, doctors said at a news conference Monday evening.

"It was a truly amazing delivery," said Dr. Karen Maples, chief of the hospital's obstetrics and gynecology department.

Doctors initially believed the mother -- whom they did not identify -- was pregnant with seven fetuses. The woman was 23 weeks pregnant when she was hospitalized seven weeks ago and ordered to bed rest. Watch doctors explain what happened »

Over a seven-week period, a team of 46 physicians, nurses and other staff prepared for the births. When they started the delivery Monday -- more than nine weeks before the babies would be full term -- they were in for a surprise.

"After we got to Baby G, we were surprised by the discovery of a Baby H," Maples said.

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Getting the number correct with ultrasounds before delivery is difficult with so many babies, said Dr. Harold Henry, the hospital's chief of fetal medicine.

"It is quite easy to miss a baby when you're expecting seven," Henry said.

The hospital said the woman didn't want her personal information released to the news media, and it would not answer questions about whether she'd had fertility treatments.

Preliminary research indicates this is the second set of live-born octuplets in the United States, according to the hospital.

Eight babies believed to be the United States' first set of live octuplets were born in Houston, Texas, in 1998. One of the infants died days after birth. The seven other siblings recently celebrated their 10th birthday, the Houston Chronicle reported last month.

The first three to seven days will be critical for the California babies, said Dr. Mandhir Gupta, one of the doctors at the news conference. The infants could be in incubators for six to eight weeks and in the hospital for 10 weeks, Gupta said.

The mother is doing "very well" after the deliveries, which took about five minutes, Gupta said.

"She is very excited that she [has] all these babies and that the babies are looking good so far," Gupta said.

All About Pregnancy and Childbirth • California