Grannies exist because of a clever evolutionary plan to provide carers to look after children while younger women have more, say experts.
Humans are almost unique among mammals because women live on long past the age of child-bearing.
Finnish researchers said the "childcare" theory explained why this was the case.
Writing in Nature, they said the study showed grandparents had an influence on their family's reproductive success.
Most mammals - aside from some rare types of elephant and whale - do not go through a menopause and can go on reproducing until they die.
But even in populations where average life expectancy is well below 40, research has found girls who survive childhood live past their child-bearing age.
Survival chances improved
The researchers looked at 2,800 women living in two 18th and 19th century farming communities in Finland and Canada.
They wanted to see how long women lived after their menopause and what effect that might have on how many babies their own children had.
The data showed women had two extra grandchildren for each extra decade they lived after 50.
They found the link was true whether the women had sons or daughters, and was valid even after social and cultural differences between the two communities had been taken into account.
Women were more likely to have children at a younger age if their own mothers were still alive, and the grandchildren were more likely to survive, the study found.
Children who reached the age of two were more likely to reach the age of five if they had a living grandmother.
Around 12% more grandchildren survived to adulthood when their grandmother was under 60, though the benefit was reduced to 3% if the grandmother was older.
Writing in Nature the researchers, led by Dr Mirkka Lahdenpera, said: "Although grandmother effects alone are suggested to be insufficient to account for the evolution of the menopause, our results suggest that they can be sufficient to account for the evolution of the substantially prolonged post-reproductive lifespan observed in humans."
They added that mortality rates in older women accelerated when their own children stopped reproducing and could become the future generation of helpers.
Gordon Lishman, Director-General of Age Concern, said modern families were known to benefit from the presence of grandparents.
He told BBC News Online: "They are the main source of childcare for families with children in Britain.
"Six out of 10 families turn to grandparents as carers, saving a possible £5 billion per year in childcare costs.
"Modern grandparents are central to family life. British families are changing rapidly, but we have consistently found that grandparents are a constant source of support both to their own children, and their grandchildren."
He added: "This report demonstrates the strength of the bond between grandmothers and their daughters, who frequently draw on the experience of older members of the family when raising their own children.
"Grandparents provide care, love, advice and laughter to their families and the importance of this can not be valued highly enough."