The 2025 study found that advanced paternal age significantly raises the likelihood of pregnancy complications and developmental disorders, further confirming that men, too, face a biological time limit on their reproductive capabilities
For years, the concept of a "biological clock" has largely been associated with women - the idea that egg quality declines rapidly after the mid-30s, making conception harder and pregnancies riskier. But emerging scientific evidence now suggests the narrative may have been only half the truth: men, too, have a biological clock, as their sperm quality deteriorates with age.
According to new research published in Science Advances and reported by the Times of India and a New Scientist report, human eggs appear far more resilient to age-related mutations than previously believed, while men's sperm cells show a clear decline in quality over time.
Human eggs may not age the way we thought
Researchers at Penn State University found that human eggs do not accumulate mitochondrial DNA mutations with age - a type of genetic damage long thought to be a major reason behind women's declining fertility.
"When we think about age-related mutations, we think about older people having more mutations than younger people," said Kateryna Makova of Penn State University, who led the study.
The study, which examined 80 eggs from 22 women aged 20 to 42, found that mitochondrial mutations remained stable across all ages, unlike in blood or saliva cells, which accumulated 17 to 24 times more mutations.
"I think that we evolved a mechanism to somehow lower our mutation burden, because we can reproduce later in life," Makova added.
Mitochondria, the tiny energy producers inside cells, carry their own DNA and are passed only from mothers to their children. Although most mitochondrial mutations are harmless, they can cause health complications that affect high-energy organs such as muscles and nerves.
Ruth Lehmann, a biologist at the Massachusetts Institute of Technology who was not involved in the research, explained, "The oocyte [egg] provides this stockpile."
The findings challenge the long-held assumption that older women pass on more mitochondrial mutations to their offspring.
Previous research on macaques also revealed similar results: mitochondrial mutations in egg cells increased only up to the animals' reproductive prime, around age nine, and then remained constant.
"It would be interesting to also look at younger women; this might also be the case in humans," said Barbara Arbeithuber of Johannes Kepler University Linz, Austria, a member of the research team.
The overlooked male clock
While women's egg cells seem remarkably stable in their mitochondrial quality, studies show that men's sperm quality deteriorates with age, leading to increased risks of infertility, miscarriages, and even neurodevelopmental disorders such as autism in children.
The process of continuous sperm regeneration means genetic errors can accumulate over time, resulting in DNA mutations that negatively impact both fertility and offspring health.
The 2025 study found that advanced paternal age significantly raises the likelihood of pregnancy complications and developmental disorders, further confirming that men, too, face a biological time limit on their reproductive capabilities.
Why this matters
The findings mark a shift in understanding human fertility. While women's egg mitochondria remain stable for decades, their fertility can still decline due to mutations in nuclear DNA. Meanwhile, men's reproductive cells undergo a gradual decline in genetic quality with age.
This means fertility concerns are not exclusive to women - both partners play a biological role in conception success and offspring health.
Men who delay fatherhood may face increased risks similar to those women encounter as they age.
In short, the biological clock ticks for both - just in different ways.