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Older women face a lower chance of fertility treatment working, even when using young donor eggs - with a marked drop-off from around the age of 49, new research suggests.
Experts studying 1,774 women say the findings challenge the idea that donor eggs can fully "reset" the reproductive clock, but should not put older couples off trying.
Women aged 49 and over had double the risk of miscarriage than those aged 35 to 40, and were also less likely to get pregnant.
Researchers believe age-related changes in the womb lining may be the cause and could be treatable in the future.
It might be possible to find ways to predict, prevent or improve womb age, they say.
Lead researcher Dr Beatrice Crestani, from an assisted reproduction medical institute in Italy, external, says that reproductive ageing has been seen as an ovarian issue, and that if you replace older eggs with younger donor ones, you "reset" the reproductive clock.
"Our findings suggest the picture is more complex," adds Dr Crestani.
In the study, women in their mid to late 30s had a 54% chance of getting pregnant with donor eggs and IVF treatment; that fell to around 43% for those aged 49 or older.
Live birth rates declined from 46% to 32%, while miscarriage rates increased from 24% to 38%.
The researchers also noticed age-related changes to the womb lining or endometrium - where a fertilised egg or embryo implants to grow - in the older women.
While the thickness was similar, the condition declined with age.
Dr Crestani added: "These findings should not discourage women from pursuing donor-egg treatment, because success rates remain meaningful even at advanced ages.
"However, patients should be counselled that donor eggs cannot completely eliminate the effects of reproductive ageing, particularly beyond 49 years."
In the study, the live birth rate was around 80% for women aged 35-40 and 62.5% among those aged 49 or older who transferred all available embryos.
