The reason behind why older women are more likely to produce abnormal eggs may be discovered by scientists.
The Newcastle University team saw a fall in levels of proteins called cohesins, essential for chromosomes to divide properly for fertilisation.
Abnormal eggs are linked to infertility, miscarriage and conditions including Down's Syndrome.
It was already known that pregnancy problems in older women can be linked to eggs containing the wrong number of chromosomes, but not why this occurred.
If there is too little cohesin, the structure can be too "floppy" for division to happen equally.
The researchers looked at eggs from young and old mice - and found cohesin levels declined with age.
By tracking chromosomes during division in the egg, the Newcastle team found that the reduced cohesin in eggs from older females resulted in some chromosomes becoming trapped and unable to divide properly.
"Reproductive fitness in women declines dramatically from the mid-thirties onwards. Our findings point to cohesin being a major culprit in this," the BBC quoted lead researcher Dr Mary Herbert, of the Centre for Life at Newcastle University, as saying.
The study has been written in Current Biology.