What is the difference between chromosomal, genetic and congenital abnormalities?

There are three main groups into which most fetal abnormalities fit into; identifying these abnormalities is the main reason for offering prenatal diagnosis. The first is known as congenital abnormalities. Congenital abnormalities are usually considered as those that have developed in the absence of a genetic or chromosomal cause, and occur when the baby has developed an abnormality in the womb often for no apparent reason. Rarely it is because of an infection (such as toxoplasmosis or german measles), dietary deficiencies or certain drugs that you might have been given early in pregnancy. These abnormalities include hare lip, cleft palate, heart defects, spina bifida, absent limbs or extra digits. Most of these abnormalities are detectable on ultrasound at 18-22 weeks. Often the baby is otherwise completely normal and the less severe congenital abnormalities such as hare lip, talipes (club foot) and extra digits can usually be dealt with by surgery to give a good cosmetic repair after the baby is born. Severe congenital abnormalities such as major heart and central nervous system (brain and spinal cord) malformations often result in miscarriage or death of the baby before 24 weeks; these defects can almost always be picked up with ultrasound screening. Genetic abnormalities are those that occur usually because of a defective gene (length of DNA portion containing a genetic blueprint) that you or your partner are carrying, but not necessarily suffering ill effects from, therefore may not know that you possess. If the genes then come together in certain combinations in the baby then the baby may suffer from the full condition. Examples of this are cystic fibrosis (a disease that causes lung and digestive system disease); you and your partner may carry the gene but be unaware of it. When the baby is born, you both may have passed on your ''abnormal'' gene to the baby and it would then suffer from cystic fibrosis; this particular condition is rare and occurs in around 1 in 2000 deliveries. Other more common ones include the diseases of red blood cell production sickle cell disease and thalassaemia; if the baby has the genes for these conditions it is normally perfectly well inside your womb, until it is born when the illness may show itself. This is why screening is often carried out on ''at risk'' groups at an early stage of pregnancy. ''Sex-linked'' conditions such as muscular dystrophy and haemophilia run in families so you would almost certainly know if you were at risk of passing these on; the mother is the carrier and male children only are affected. Chromosomal abnormalities include Down?s syndrome (trisomy 21; trisomy means three chromosomes instead of two) and rarer, usually lethal syndromes. 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 then very rare. Although they can often be picked up on careful ultrasound screening, sometimes there are no obvious ''markers'' to see and ultrasound examination is completely normal.
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