The findings from the University of Helsinki and the Minerva Foundation Medical Research Institute suggest that both the number of children and the timing of pregnancies influence the health andlife expectancy of women in adulthood. The results have been published in the journal Nature Communications. The findings are based on a longitudinal study involving nearly 15,000 twins born between 1880 and 1957.
Participants in the twin study were invited to complete a questionnaire in 1975, and their life trajectories have been periodically monitored up to the present.
According to the follow-up of nearly 15,000 participants, women with two or three children tend to live longer. The timing of pregnancies is also important: pregnancies that occurred approximately between the ages of 24 and 38 were associated with more favorable aging and longevity patterns, according to the study.
In particular, it was found that having a number of children above the average (more than four) was associated with a shorter life expectancy and accelerated biological aging. According to the researchers, this finding is fully consistent with the life history theory of evolutionary biology developed in the 20th century.
«From an evolutionary biology perspective, organisms have limited resources, such as time and energy. When a significant amount of energy is invested in reproduction, it is taken away from the organism's maintenance and repair mechanisms, which could reduce life expectancy,» explains researcher Mikaela Hukkanen, who led the study.
Somewhat unexpectedly, the study also revealed that women without children showed faster aging than women with few children. This result may be explained by other factors related to lifestyle or health whose effects could not be fully controlled in the analyses.
The research group emphasizes that the findings are only applicable at a population level. They do not demonstrate cause-and-effect relationships, nor do they provide a basis for individual recommendations aimed at women of reproductive age. For example, family sizes have decreased and the age at first childbirth has increased compared to the period covered by the study.
«Therefore, a woman should not consider changing her own plans or desires regarding having children based on these findings», states the study's director, Miina Ollikainen.
Epigenetic Age is Associated with the Number of Children
The potential evolutionary trade-off between reproduction and life expectancy has interested researchers since the early 20th century, but numerous studies on the subject have yielded conflicting results.
A novel aspect of this study was that aging was also biologically measured. Epigenetic clocks were determined from blood samples of over a thousand participants. Epigenetic clocks aim to measure biological aging, that is, the gradual deterioration of cells and tissues. With these methods, aging-related changes can be detected years or even decades before death.
The results supported previous conclusions based on mortality data. According to the epigenetic clocks, women who had many children or none at all were, biologically, somewhat older than their chronological age.
«A person who is biologically older than their chronological age has a higher risk of death. Our results show that decisions made throughout life leave a lasting biological imprint that can be measured long before old age», states Miina Ollikainen.
«In some of our analyses, having a child at a young age was also associated with biological aging. This may also be related to evolutionary theory, as natural selection may favor earlier reproduction that entails shorter generation times overall, even if it involves health costs associated with aging».
The study was conducted through collaboration between the Institute for Molecular Medicine Finland (FIMM), the Biotechnology Institute of the University of Helsinki, and the Minerva Foundation Medical Research Institute.