Doctors at the Texas Children's Hospital upgraded the condition of the two largest octuplets from critical to serious on Wednesday.

Baby Echerem and his brother Jioke have both been moved from the hospital's neonatal intensive care unit to a nursery where they will continue to receive continuous medical care.

The other five children remain in critical but stable condition. Doctors said last week that all seven should be able to leave the hospital by late March if they continue to make steady progress.

Fertility treatment

The mother, Nkem Chukwu, 27, was treated with fertility drugs and gave birth to the first of the octuplets on 8 December.

The others were delivered by Caesarean section on 20 December.

The children were 10 and 12 weeks premature. The smallest, a girl nicknamed Odera, weighed just 320 grams at birth.

She died on 27 December from heart and lung failure.

Ms Chukwu and the children's father, Iyke Lous Udobi, 41, are both US citizens who originally came from Nigeria.

Doctors estimate that medical bills for each child will amount to $250,000 by the time they are strong enough to leave the hospital.

The family live in a four-bedroom house, but local politician Sheila Jackson Lee, a Texas Democrat, is asking corporate donors to help the family of nine buy a van and a larger house.