• Question: Hi Katie, Is there a set date when a womans eggs start going dodgy? Like, if they turned 41 would they automatically start going dodgy or is it a bit more complicated than that? Loved doing the live chat with you this morning. You answered loads of my questions ^.^ xxx

    Asked by busybodyscientist to Katie on 22 Nov 2012.
    • Photo: Katie Howe

      Katie Howe answered on 22 Nov 2012:


      Im glad you enjoyed the chat! It is really fun when people get interested in your work! If you look at my profile page you can see a graph which might explain this better. Here is a link:


      If you look at the graph then after the age of about 35 there is an dramatic increase in the chance of having children with the wrong number of chromosomes (for example the graph shows the increased chance of ‘trisomy’ which is 3 copies of one chromosome instead of 2 like there are supposed to be- in Downs syndrome there are 3 copies of chromosome number 21 so it is often called trisomy 21)

      The reason why a womens eggs start going dodgy when she gets older is because her eggs are also old. That is because when a baby girl is born she already has all the eggs she will ever get and no more are ever made! So when a women is 40 her eggs are also about 40 years old! This means they start making mistakes and ending up with the wrong number of chromosomes. If those eggs get fertilised by the sperm then the baby that results will also have the wrong number of chromosomes whcih causes all sorts of problems for the baby.

      But this doesnt mean that your eggs have some kind of switch in them which makes them go dodgy at a certain time. It depends a bit on the individual person – we just know that there is an increased chance of getting dodgy eggs when a woman gets older. For example my mum was 40 when she had my brother and while she was pregnant the doctors told her that he had Downs syndrome which made sense as she was a bit older. But when he was born he was actually perfectly healthy! – that is what got me interested in this area to begin with!

      I hope that answered your question – please ask me again if you want me to explain anything that does not make sense! 😀