Naturally, the occurrence of twins amongst humans is rare and somehow linked to genetics.

For example, in a Nigerian tribe, the twins' occurrence is very common.

Triplets are quite an exception amongst humans.

But the hormonal bouleversement induced by former use of contraceptive pills or fertility treatments have induced ultimately rabbit fertility in human females.

Two sets of sextuplets have been born in different states less than a day apart, an astonishing occurrence that could turn increasingly common with the artificial techniques of "in vitro" fertilization.

Brianna Morrison, 24, who resorted to fertility drugs, gave birth just before midnight on Sunday in Minneapolis. About 10 hours later, Jenny Masche, 32, who employed artificial insemination and medication to stimulate her ovulation cycle, gave birth on Monday in Phoenix by Cesarean section. "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 prematurely delivered in critical condition at just 22 weeks and their weights varied between 11 ounces (301 g) and 1 pound, 3 ounces (562 g).

The Masche sextuplets - three boys and three girls - were also born premature, almost 10 weeks earlier, and all but one weighed less than three pounds (1.35 kg). Five of the six were put on ventilators to ensure their breathing. "Their tiny lungs are underdeveloped," said Dr. Jordan Leonard, at Phoenix Children's Hospital.

The babies could 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.

Doctors had advised the Morissons to opt for selective reduction, a procedure by which women carrying multiple fetuses decrease the number of viable fetuses to two, but they rejected the option.