What is Down syndrome?(DS)
DS is a chromosomal conditions characterized by the presence of an extra copy of genetic material on the 21st chromosome, either in whole (trisomy 21) or part (such as due to translocations). The effects and extent of the extra copy vary greatly among people, depending on genetic history, and pure chance .The incidence of DS is estimated at 1 per 733 births, although it is statistically more common with advanced maternal age, DS can occur in all age groups. Often DS is associated with some impairment of intelligence and physical growth, and a particular set of facial and physical characteristics. Individuals with DS tend to have a lower -than-average intelligence often ranging for mild to moderate disabilities.
Health concern for individuals or Health concern for an individual with DS include a higher risk for congenital heart defects, gastroesophageal reflux disease, recurrent ear infections, obstructive sleep apnea, and thyroid dysfunctions.Hemantologic malignancies such as leukaemia are more common in children with DS. These factors can contribute to a shorter life span for people with DS and in those who survive into middle-age ,a propensity to develop Alzheimer’s disease is present.
The human genetic material which is needed for proper development and maintenance of our bodies is stored in 23 chromosome pairs. There are about 20,000 and 25,000 human proten-coading genes gives signals for proteins building in our bodies . Normally, each human cell contains 23 pairs of different chromosomes. When a baby develops in morther's womb, it inherits genetic material from its parents. The genetic material is usually transferred to the baby in the form of 23 pairs of chromosomes, that is 23 chromosomes from father and another 23 from chromosomes from mother.
Screening Program for Down's Syndrome
All pregnant women, no matter what their age, should be given the choice to have their baby screened for Down's syndrome. Antematal screening for DS is carried out by examination of blood of pregnant women or an Ultrasound scan of fetus or both. This is known as a'combined test'.
Screening cannot diagnose conditions such as DS. However,what it does show is how likely it is that baby will develop the syndrome. If the risk of Down's syndrome is shown to be high, further tests can be arranged to confirm whether baby has the condition or not.
Maternal blood screening in 1st and 2nd trimester of pregnancy
During a blood test, a sample of blood of pregnant women is taken and tested to check the levels of certain proteins and hormones. If blood contains abnormal levels of these substances, there is an increased risk of having a baby with Down's syndrome. it can be performed in the 1 trimester or 2 trimester of pregnancy.
Genetic Ultrasound Screening in 1st and 2nd trimester pregnancy
All fetuses tend to collect fluid behind the neck normally. However, fetuses with Down's syndrome may have more fluid. Measuring the thickness of fluid will help to determine how likely it is that a baby has Down's syndrome. This is known as nuchal transluency scan, performed between 11 to 13 weeks of pregnancy. There are other soft markers seen in first trimester scan which are-Nasal bone size, flow across Tricuspid Valve, flow at Ductus Venosus and Fetal facial angle. There are also several soft markers for DS, which are seen in 2nd trimester scan.
Diagnostic tests for Down syndrome
Down syndrome cannot be diagnosed based on ultrasound findings or blood test alone. These findings suggest that there is a higher risk that the fetus may have Down Syndrome. Patient may be advised to undergo further testing. There are two types of tests that can diagnose genetic conditions while baby is still in the womb. These are chorionic villus sampling ( CVS ) and amniocentesis.
Chorionic Villus Sampling ( CVS )
CVS is used to identify chromosomal problems, such as Down syndrome. It can also detect other genetic diseases. The main advantage of CVS over amniocentesis is it done much earlier in pregnancy, at 10 to 12 weeks, rather than 15 to 20 weeks.
CVS is performed by taking a tiny piece of tissue from the chorionic villi. Under ultrasound guidance, the tissue is obtained either with a needle through the abdomen or a catheter inserted through the cervix. The tissue is then cultured and a karyotype analysis i.e. analysis of the chromosomal make-up of the cells is performed. It takes about 2 to 3 weeks to receive the results.
A small amount of amniotic fluid which contains fetal cells is extracted from amniotic sac surrounding a developing fetus , and the fetal chromo somes and DNA is examined for genetic abnormalities. It is usually performed between 15 to 20 weeks of pregnancy. With the aid of ultrasound-guidance, a needle is inserted into the amniotic sac in an area away from the fetus and approximately 20 ml of amniotic fluid. After the amniotic fluid is extracted, the fetal cells are separated from the sample. The cells are grown in a culture medium, then fixed and stained. Under a microscope the chromosomes are examined for abnormalities. The needle insersion site heals and the amniotic sac replenishes the liquid over the next 24-48 hours.
Results of diagnostic procedures may come out to be normal or abnormal. Normal results will assure that the baby is genetically normal. If it is abnormal, then it will be difficult news to deal with, therefore parents are counseled. A counselor will be able to make parents aware of the different options available, allowing parents to make a more informed decision about to proceed with the pregnancy.