The researchers developed a mouse model of "aneuploidy" - where some cells in the embryo contain an abnormal number of chromosomes.  Normally, each cell in the human embryo should contain 23 pairs of chromosomes - 22 pairs of chromosomes and one pair of sex chromosomes, but some can carry multiple copies that can lead to developmental disorders. For example, children born with three copies of chromosome 21 will develop Down's Syndrome.  Pregnant women - particularly older mothers, whose offspring are at greatest risk of developing such disorders - are offered tests to predict the likelihood of genetic abnormalities.  Between the 11th and 14th weeks of pregnancy, mothers may be offered chorionic villus sampling (CVS), a test that involves removing and analysing cells from the placenta.  A later test, known as amniocentesis, involves analysing cells shed by the foetus into the surrounding amniotic fluid. That test is more accurate, but is usually carried out during weeks 15 to 20 of the pregnancy, when the foetus is further developed.  Study senior author Prof Zernicka-Goetz, said she was inspired to carry out the research following her own experience when pregnant with her second child.  A CVS test at the time found that as many as a quarter of the cells in the placenta that joined her and her developing baby were abnormal.  When Prof Zernicka-Goetz spoke to geneticists about the potential implications, she found that very little was understood about the fate of embryos containing abnormal cells and about the fate of these abnormal cells within the developing embryos.  Fortunately for the Professor, her son, Simon, was born healthy.  She said: "I know how lucky I was and how happy I felt when Simon was born healthy. Many expectant mothers have to make a difficult choice about their pregnancy based on a test whose results we don't fully understand.  "What does it mean if a quarter of the cells from the placenta carry a genetic abnormality? How likely is it that the child will have cells with this abnormality, too? This is the question we wanted to answer.  "Given that the average age at which women have their children is rising, this is a question that will become increasingly important."  Co senior author Professor Thierry Voet, of the Wellcome Trust Sanger Institute, said: "In fact, abnormal cells with numerical and/or structural anomalies of chromosomes have been observed in as many as 80 to 90 per cent of human early stage embryos following in vitro fertilisation (IVF) and CSV tests may expose some degree of these abnormalities."  • World's oldest living man with Down's Syndrome celebrates 76th birthday
Cambridge professor inspired by her own experience discovers abnormal cells   can be eliminated and develop into healthy children