About the time of fertilisation mistakes in the arrangement of the chromosomes can occur. Many are lethal - in other words the embryo will not survive - the best known is Down's Syndrome where the effect on the child varies considerably: other examples are Turner's and Klinefelter's syndromes both of which have a marked effect on fertility.
RISKS - for Down's syndrome and for total chromosomal abnormalities - All, in the table. In the table below the figures are expressed as chances - e.g. for a woman of 20 although the risk is low it is there - 1 in 667; for a woman of 49 the risk is high 1/11.
| Age | Downs | All |
| 20 | 1,667 | 526 |
| 25 | 1,250 | 476 |
| 30 | 952 | 385 |
| 35 | 378 | 192 |
| 40 | 106 | 66 |
| 41 | 82 | 53 |
| 42 | 63 | 42 |
| 43 | 49 | 33 |
| 44 | 38 | 26 |
| 45 | 30 | 21 |
| 49 | 11 | 8 |
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