| Chromosomal abnormalities include Down''s syndrome (trisomy 21; trisomy means three chromosomes instead of two) and rarer, usually lethal syndromes. These include Edwards syndrome (trisomy 18) and Patau''s (trisomy 13). A chromosome is a ''bundle'' of genetic material, and the normal number contained in cells in our bodies is 46 (22 pairs=44, plus two sex chromosomes X and Y; XX=female, XY=male). In Down''s syndrome, which occurs more frequently in older mothers, an extra chromosome 21 is present which can give rise to congenital abnormalities and learning difficulties later on. An absence of a sex chromosome may cause Turner''s syndrome (45XO); this is not lethal but gives rise to short stature and poorly developed sexual organs. There is a whole host of other chromosomal conditions, most of them very rare. Although they can often be picked up on careful ultrasound screening, sometimes there are no obvious ''markers'' to see and ultrasound examination may be completely normal. |
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