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Delaying motherhood: the risks and the science of reproductive ageing

By Katie Griffiths, Young People’s Programme Assistant at the British Science Association


Women need to be better informed about the potential risks of waiting until they’re in their late-30s to start a family, Professor Mary Herbert, Dr Jane Stewart and Professor Judith Rankin told the British Science Festival today.   

Over the past few decades there has been an increasing trend towards women trying to have children later in life. In 1986 just 6.8% of births were to woman aged 35-39 years, but by 2008 that number had increased to 17%. And these women are the lucky ones, according to Dr Stewart: many women in their late 30s and early 40s struggle to conceive at all. Evidence shows that fertility rates fall as women get older. Data from unrestricted populations, where no family planning or birth control is used, clearly show that women are less likely to conceive later in life, and an increase in the age of women who are looking into fertility treatment also backs up this trend.

However, this isn’t the only issue. Professor Rankin explained that increased maternal age is also linked to a higher likelihood of multiple births, higher incidence of emergency caesareans, and an increased chance of chromosomal disorders such as Down’s syndrome. In fact, in 2008, 65% of babies born with Down’s syndrome were born to women that were over the age of 35.

So why is it the case that women have more problems reproducing as they get older?  Professor Mary Herbert explained that it’s to do with what happens within eggs as they age. A fertilised embryo requires 23 chromosomes from each parent; that’s 23 from mum and 23 from dad. The process of creating a sperm or an egg cell with these 23 chromosomes is known as meiosis. In women, meiosis begins when the baby is still in the womb, and is not fully completed until the egg becomes fertilised.  A woman’s eggs spend the majority of her fertile life suspended in action.

During this suspended state, chromosomes exist in groups known as tetrads. These tetrads are composed of two chromosomes joined together and they are necessary to make sure that when the cell finally splits, each cell has the right 23 chromosomes. Tetrads themselves are held together by molecules of cohesin. What research has uncovered is that, in mice, this cohesin starts to fall apart as they age. As a result, when meiosis is completed the chromosomes don’t always go exactly where they’re supposed to go, which can lead to chromosomal disorders, or infertility.  

Currently, there are no treatments available that can reverse or delay this age-related infertility. Professor Herbert and her colleagues therefore believe that it is important for women to understand the facts about reproductive ageing, so that they can consider them early and make educated and informed choices about their family planning.

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