Mums told, put off your first baby and you may live longer.
Women who delay the birth of their first child may live longer than those who become mothers at a young age. A new study points to an evolutionary trade-off between motherhood and long life.
The pattern was spotted trickling down four generations of more than 5,000 Finns born during the 18th and 19th centuries.
Scientists used a model normally employed to study animals to analyse the trend.
Controlling for environmental and cultural effects, they found that daughters inherited certain reproductive traits from their mothers.
These included the number of children they raised to adulthood and the age at which they had their first child.
Lifespans were also linked within families over generations. Women who delayed childbirth, or waited longer between births, tended to live longer.
Women who live longer have a better chance of raising both their children and grandchildren to become adults with their own offspring. Genes for longevity are therefore likely to be passed on to future generations.
At the same time, there is a distinct selective advantage in a woman giving birth at a young age.
Babies born to young mothers are generally healthier and likely to be better fed.
In the case of the young mothers in the study, natural selection appears to have sacrificed longevity in favour of reproductive success.
The age at which the women had their first child ranged from about 18 to between 30 and 40.
This was at a time before assisted conception, when it was unusual for women to give birth in their 30s.
The researchers, led by Dr Jenny Pettay, from the University of Turku in Finland, reported their findings in the journal Proceedings of the National Academy of Sciences.
They wrote, 'We found significant genetic trade-offs between key female life-history traits, specifically longevity and AFR (age at first reproduction), and longevity and MIBI (mean interbirth interval).
'Their existence implies that females who start to breed relatively late, or who have longer interbirth intervals, will have relatively longer lifespans, supporting the hypothesis that the rate of reproduction should trade off with longevity.
'These trade-offs may have had important implications for the evolution of human life history, because human females may gain fitness benefits by outliving their own reproductive capacity, by improving the reproductive success of their offspring and the survival of their grand-offspring.
'Such positive fitness effects of post-reproductive survival would intensify the selection of genes increasing longevity, but our results suggest the existence of constraints on any response to such selection imposed by countervailing selection favouring early or frequent breeding.'
Men's lifespans were unrelated to the age at which they had children.
The researchers said this may illustrate the fact that a man's reproductive fitness largely depends on his partner. Men do not give birth or produce milk to feed offspring.
The extent to which the findings apply to general populations remains to be seen.
Dr Pettay said, 'It's hard to say how general a trend it might be. There haven't been any other studies like this.
'We need to have studies in other populations before any conclusions like that can be drawn.'