What are the odds of that ?

Two sets of sextuplets have been born in different states less than a day apart.

It's a rare occurrence, but one that fertility experts said could become increasingly common as more couples seek artificial methods of conceiving babies.

Brianna Morrison, who used fertility drugs, gave birth just before midnight Sunday in Minneapolis. About ten hours later, Jenny Masche, who used artificial insemination, gave birth in Phoenix by Caesarean section.

The tiny new Minnesotans -- four boys and two girls -- were listed in critical condition.

Five of the Arizona babies were still on ventilators. The three boys and three girls were almost 10 weeks premature. All but one weighed less than 3 pounds.

Experts said the chances of spontaneously conceiving sextuplets is one in 4.7 billion, although the odds improve significantly with fertility treatment.