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Last Updated: Tuesday, 26 December 2006, 01:37 GMT
Large families 'bad for parents'
Pregnant woman
The more babies a woman has, the higher her chance of dying early
Having a large number of children is bad for parents' health - particularly that of mothers, a study suggests.

US researchers looked at 21,000 couples living in Utah between 1860 and 1985, who bore a total of 174,000 children.

It was found the more children couples had, the worse their health and the more likely they were to die early.

The Proceedings of the National Academy of Science study is historical, but the experts say it helps explain both the menopause and modern family planning.

Menopause appears to allow mothers to live longer and rear more offspring to adulthood
Dr Dustin Penn and Dr Ken Smith

In other species, the high physical costs of bearing and raising offspring explain why having as many offspring as possible is not ideal - even though it might appear to be the most successful way of continuing the species' existence.

Research had not concluded whether or not the same was true for human reproduction.

Physical costs

The researchers, from the University of Utah, analysed nineteenth century data from the Utah Population Database.

They found that the couples had an average of eight children each, but family size ranged from one to 14 or more children.

The data showed that the more children a couple produced, the higher their risk of early death.

The situation was worst for women, because they were affected by the physical costs of bearing the children.

Fathers' mortality risk increased the more children they had, but never exceeded that of mothers.

The team looked at deaths after the last child was born and found mothers were also more likely than fathers to die after the last child was born.

They found 1,414 women died within a year of the last child's birth, and another 988 by the time the child was five.

In comparison, 613 men died in the first year after their last child was born, with another 1,083 dying within five years.

And the larger the family, the more likely children were to die before the age of 18, particularly if they were among the youngest.

Reproductive control

The team, led by Dr Dustin Penn and Dr Ken Smith, say the findings do shed light on human reproduction which are still relevant today.

Humans are one of the few species where the female goes through a menopause which ends her reproductive years.

The researchers say: "Menopause appears to allow mothers to live longer and rear more offspring to adulthood, and this unusual life history probably evolved in our species because, as we found, offspring so extremely depend on their mother's survival."

They add the findings also suggest why women now tend to have fewer children.

"If women have generally incurred greater fitness costs of reproduction, this could explain why they generally prefer fewer offspring than their husbands and reduce their fertility when they obtain more reproductive autonomy."

Peers give clues to human evolution
11 Jun 03 |  Science/Nature

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