A study based on Finnish twins shows that reproductive history is associated, at the population level, with women’s lifespan and biological aging. In the study, mothers of large families, women who had no children, or women who had their first child at a very young age appeared to age somewhat faster than other women.

The results from the University of Helsinki and the Minerva Foundation Institute for Medical Research suggest that both the number of children and the timing of pregnancies are reflected in women’s adult health and life expectancy.

Participants in the twin study were invited to complete a questionnaire in 1975, and their life course has been followed regularly up to the present day.

Based on the follow-up of nearly 15,000 participants, women with two to three children tend to live the longest. The timing of pregnancies also matters: according to the study, pregnancies occurring roughly between ages 24 and 38 were linked to more favorable aging and longevity patterns.

In particular, having an above-average number of children (more than four) was found to be associated with a shorter lifespan and accelerated biological aging. According to the researchers, this finding is well in line with life history theory in evolutionary biology developed in the 20th century.

“From an evolutionary biology perspective, organisms have limited resources such as time and energy. When a large amount of energy is invested in reproduction, it is taken away from bodily maintenance and repair mechanisms, which could reduce lifespan,” explains doctoral researcher Mikaela Hukkanen, who conducted the study.

Somewhat unexpectedly, the study also found that childless women showed faster aging than women with a few children. This result may be explained by other lifestyle or health-related factors whose effects could not be fully controlled for in the analyses.

The research group emphasizes that the findings apply only at the population level. They do not demonstrate cause–effect relationships, nor do they provide a basis for individual recommendations for women of reproductive age. For example, family size has decreased and the age at first birth has increased compared with the period covered by the study.

“An individual woman should therefore not consider changing her own plans or wishes regarding children based on these findings,” says the study lead, Dr. Miina Ollikainen.

Epigenetic age is associated with number of children

The potential evolutionary trade-off between reproduction and lifespan has interested researchers since the early 20th century, but numerous studies on the topic have produced conflicting results.

A novel aspect of this study was that aging was also measured biologically. Epigenetic clocks were determined from blood samples from more than one thousand participants. Epigenetic clocks aim to measure biological aging—that is, the gradual deterioration of cells and tissues. With such methods, aging-related changes can be detected years or even decades before death.

The results supported earlier conclusions based on mortality data. According to the epigenetic clocks, women who had either many children or no children at all were biologically somewhat older than their chronological age.

“A person who is biologically older than their calendar age is at a higher risk of death. Our results show that life history choices leave a lasting biological imprint that can be measured long before old age,” says Miina Ollikainen.
“In some of our analyses, having a child at a young age was also associated with biological aging. This too may relate to evolutionary theory, as natural selection may favor earlier reproduction that entails shorter overall generation times, even if it entails health-related costs associated with aging.”

The study was carried out through a collaboration between the Institute for Molecular Medicine Finland (FIMM) and the Institute of Biotechnology at the University of Helsinki, and the Minerva Foundation Institute for Medical Research. The results have been published in the journal Nature Communications. The findings are based on a longitudinal study covering nearly 15,000 female twins born between 1880 and 1957.