Two sets of sextuplets were born in different states less than a day apart, a rare occurrence but one that fertility experts say could become increasingly common as more couples seek artificial methods of conceiving babies.

Brianna Morrison, 24, who used fertility drugs, gave birth just before midnight Sunday in Minneapolis. About 10 hours later, Jenny Masche, 32, who used artificial insemination, gave birth Monday in Phoenix by Caesarean section, the first successful sextuplet delivery in Arizona.

"It is something that we're going to be dealing with more and more," unless doctors learn how to reduce the risk of women having four or more babies, said Dr. F. Sessions Cole, a pediatrics professor at Washington University in St. Louis.

Morrison's four boys and two girls were premature, delivered after just 22 weeks. Doctors at Children's Hospital listed them in critical condition Monday afternoon, with weights ranging between 11 ounces and 1 pound, 3 ounces.

"The babies arrived sooner than we'd hoped for, but we are optimistic," father Ryan Morrison said in a statement. "Brianna is doing well. Thanks to all who are praying for our family. We are very happy to be parents."

The Masche sextuplets — three boys and three girls — were almost 10 weeks premature, and all but one weighed less than three pounds. On Monday, five of the six were on ventilators to help them breathe.

Their tiny lungs are underdeveloped, said Dr. Jordan Leonard, who is overseeing the Masche babies at Phoenix Children's Hospital. He said medications the mother was taking to prevent labor had a side effect of making them sleepy.

"So they come out a little sedated," he said. The babies should leave the hospital in six to eight weeks.

The chances of spontaneously conceiving sextuplets is one in 4.7 billion, although the odds improve significantly with fertility treatment, said Dr. Helain Landy of Georgetown University Hospital's Obstetrics and Gynecology Department.

The Morrisons, of the Minneapolis suburb of St. Louis Park, spent more than a year trying to conceive before agreeing to fertility drugs, according to the couple's personal Web site.

Upon learning Brianna Morrison was carrying sextuplets, doctors advised the couple to opt for selective reduction, in which women carrying multiple fetuses reduce the number of viable fetuses to two.

"However, we knew right away that this is not an option for us," the couple wrote. "We understand that the risk is high, but we also understand that these little ones are much more than six fetuses.

"Each one of them is a miracle given to us by God."

They named them Lucia Rae, Cadence Alana, Bennet Ryan, Tryg Benton, Lincoln Sean and Sylas Christopher.

In Arizona, Jenny Masche used artificial insemination and medication to stimulate her ovulation cycle, Leonard said. Their children are named Bailey Elizabeth, Savannah Jane, Molli Grace, Cole Robert, Blake Nickolas and Grant William.

Father Bryan Masche, 29, said in an interview last week that the couple were terrified when they learned in December they were going to have six babies.

"We're blessed and excited," he said. "I keep coming back to the Bible verse that says, 'God will never leave or forsake us."'