Multiple births such as the octuplets in California have become more common in the last few decades, mainly thanks to advances in fertility treatments.

Britain has seen several high-profile cases of multiple birth in recent years, including Mandy Allwood, from Warwickshire, who conceived octuplets in the mid-1990s but lost all eight, and Janet Walton, from Liverpool, who gave birth to sextuplets in 1983 after she and her husband Graham had been trying for a baby for five years. Amazingly, all six girls survived.

Multiple births are risky as the uterus is only capable of carrying, feeding and providing oxygen for a certain number of foetuses. They increase the likelihood of babies being born prematurely, developing health problems or dying.

Even twins are more likely to suffer birth defects, such as blindness and cerebral palsy, and are less likely to survive than single births.

To reduce the risks, doctors try to get mothers to have the babies one at a time, but this is not possible in all cases.

The multiple births website Oneatatime said that 19,049 babies were born as part of a multiple birth in 2004 alone, amounting to one in every 67 births. The vast majority were twins.

There are two principal ways in which multiple births can arise. Multiple eggs can be released by the ovary, or implanted during fertility treatment, with each egg being fertilised separately. This gives rise to non-identical siblings who can be of either sex.

Alternatively, a fertilised egg can split, producing two, three or in some cases more identical embryos, always of the same gender.

Jane Denton, director of the Multiple Births Foundation, said: “It is extremely rare to have anything higher than quads. We have never seen octuplets in the UK so far.

“There is a great deal we just don’t know about the conception and the embryo division.

“The biggest risk is that babies can be born early when they are not fully developed.”

The womb and placenta would also find it difficult to provide enough room and food to successfully raise large multiple births.

Feeding and caring for eight babies once they have born is also likely to be very hard for the parents, especially when the mother intends to breastfeed each baby as the mum in the California octuplets case has promised to do.

Ms Denton added: “To nourish eight babies is a tremendously unusual and demanding task for the human body.”

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