Motherhood is a difficult job. In fact, the results of a new study suggest that, historically, taking on the role early in life was linked to shorter lifespans. A report published online this week by the Proceedings of the National Academy of Sciences indicates that mothers who gave birth at a young age in the 18th and 19th century also tended to die young. The results suggest that natural selection may have sacrificed a woman's longevity for reproductive success.

Jenni E. Pettay of the University of Turku in Finland and her colleagues analyzed medical records of four generations of Finns born between 1745 and 1903. They found that mothers and daughters tended to share a number of similarities, including the age at which they first gave birth and the number of children they had who survived to adulthood. Across generations, a family's lifespan was relatively consistent, with women who delayed childbirth living longer than women who had their first child at a younger age. In addition, women who waited longer between births lived longer than did mothers who gave birth in quick succession. For males, meanwhile, there was no significant link between the age of first fatherhood and longevity.

Passing along a gene for longevity would help women live long enough to help out with their grandchildren, thus giving their genes a better chance to survive, the authors point out, but their results indicate the opposite may be true. The findings, they write, "are also interesting because they suggest some underlying genetic mechanism affecting age of first reproduction and longevity and another mechanism affecting rate of reproduction and longevity."