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CAR: Anomalies - Chromosome
Down's Syndrome

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Introduction Antenatal Postnatal West Midlands Data


Down's Syndrome is caused by an additional copy of chromosome 21 and is the most common condition associated with autosomal aneuploidy at birth. The majority of Down's Syndrome conceptions are lost through spontaneous miscarriage, mainly very early on, but with decreasing frequency throughout pregnancy. Image

Down's Syndrome is an important cause of structural anomalies and it is the biggest single cause of learning difficulties.
The vast majority of cases of Down's Syndrome are due to non-disjunction (a failure of the normal separation of a pair of chromosomes during cell division). Trisomy 21, as well as other autosomal trisomies, is more common with increasing maternal age .

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Biochemical screening for Down's Syndrome was developed during the 1980's. The principle of this method is to refine the risk for an individual by measuring pregnancy related hormones and proteins, which are present in different quantities in a population of trisomy 21 pregnancies, compared with unaffected ones. Biochemical screening has traditionally been undertaken at 16 to 18 weeks of pregnancy. Measuring the levels of serum hormones and comparing them to median values for the same gestation allows a multiple of the median (MoM) to be calculated. The risk can be further refined by standardising for maternal weight and ethnic group.

Ultrasound is increasingly used to examine the fetus for structural anomalies. Ultrasound screening is often undertaken at 18 to 22 weeks gestation, with the express purpose of identifying structural congenital anomalies. Some specific diagnoses are commonly associated with trisomy 21; these include atrioventricular septal defects, Fallot's tetralogy, exomphalos, hydrocephalus, duodenal atresia and diaphragmatic hernia. These conditions, where there is a high chance of fetal trisomy, are known as "hard markers".

The other circumstance where ultrasound can raise suspicion of trisomy is when a number of sonographic "soft markers" are seen. These are not, in the real sense, congenital anomalies, and if they are not associated with trisomy, there is no known adverse outcome for the pregnancy. These include brachycephaly, nuchal pad, choroid plexus cysts, echogenic foci in the heart, echogenic bowel, mildly dilated renal pelvis, short femur, sandal gap, clinodactyly, and hypoplastic middle phalanx of little finger. Some of these findings may raise concerns other than trisomy, but a combination of these soft markers in a fetus is associated with an increasing risk of trisomy.

Screening for trisomy 21 using nuchal translucency measurements by ultrasound is a relatively recent development. The principle is the same as for serum screening but is undertaken at 10 to 14 weeks. A measurement is taken of the sonolucent layer within the skin at the back of the fetal neck and compared with a median value for that gestation. The age-related risk is then adjusted up or down to give an individual risk for that pregnancy.

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A main concern for parents is the fact that people with Down's Syndrome have moderate to severe learning difficulties. Other congenital malformations are common in babies with Down's Syndrome, particularly congenital heart disease (including atrioventricular septal defect, common atrioventricular canal and patent ductus arteriosus), but also duodenal atresia. Other common findings include short stature and squint.

Survival in Down's Syndrome has improved considerably over the years, particularly with improvements in the correction of congenital heart defects. Overall, 85% of infants now survive to one year, compared with 70% of those with congenital heart disease. More than 50% now live beyond 50 years. Other deaths are mainly due to other congenital malformations and respiratory infections.

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WM Data

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© Perinatal Institute 2011