The world's only known octuplets, six girls and two boys, are fighting for their lives in a hospital in Houston, Texas, where their condition remains critical.

Seven of the eight babies were born some ten weeks premature on Sunday, weighing between three-hundred and eight-hundred grammes.

The first was born two weeks ago -- she is the only one who is already breathing on her own.

Their mother, a Nigerian-born American, Nkem Chukwu, is described as stable after undergoing emergency surgery.