It’s not eggs-actly the news you want to hear.
A new study has pinpointed the age when female fertility starts to plummet — and it happens earlier than you might think.
Researchers say the findings could pave the way for future treatments that help keep women’s biological clocks ticking longer.
The stakes are high: as more American women wait longer to start families, the chances of encountering fertility problems rise.
Back in 1970, the average age of first-time mothers in the US was 21.4. But by 2023, it had jumped to 27.5 — with many women putting off motherhood well into their 30s and 40s, according to the CDC.
While options like egg freezing and IVF can help women work around reproductive challenges, researchers say that understanding why fertility declines in the first place could eventually help lead to ways to extend it.
In the study, scientists from Jilin University in China analyzed data from more than 15,000 embryos created through IVF.
They found that in women ages 20 to 32, about 1 in 5 eggs carried a chromosome error. But after 32, that number started climbing fast.
By a woman’s mid-thirties, more than half of her eggs contained too many or too few chromosomes — a leading cause of miscarriage, infertility and conditions like Down syndrome.
And with each passing year, the risk of chromosomal abnormalities continued to rise.
So what’s behind this shift? It’s at least partly due to a ring-shaped protein called cohesin, which acts like molecular glue to hold chromosomes together as eggs develop.
But cohesin doesn’t last forever. The scientists found that as women age, levels of this vital protein drop. Eggs from women over 40 had up to a third less than eggs from women in their twenties.
The pattern held in mice as well. By 17 months of age, roughly the equivalent of a woman’s late 30s, more than 95% of their cohesin was gone.
Without enough cohesin, chromosomes can’t stay paired. As a result, they can split too early, drift to the wrong places and leave some eggs with too many or too few chromosomes.
Cohesin also helps repair DNA. When levels drop, DNA damage builds up, repair slows down and errors like chromosome loss and mutations increase. These mistakes can raise the risk of cancer and developmental problems in offspring.
Scientists are still figuring out why cohesin levels drop with age, but they have a few leads.
For example, protective proteins that normally guard cohesin fade over time, and oxidative stress that builds up in the body can damage it. At the same time, certain cellular signals that help keep cohesin in place also weaken as eggs get older.
Two pathways, in particular, stand out.
One, called mTOR, controls cell growth and cohesin attachment. Studies in yeast suggest that modifying it could strengthen cohesin levels, though it’s still unknown if the same approach would work in humans.
Another, called ATM, coordinates DNA repair and helps cohesin hold chromosomes together. But in older eggs, ATM becomes less efficient, speeding up cohesin loss.
The researchers suggested that future studies explore how these pathways influence cohesin, which could point to new ways to preserve egg quality as women age.
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