Chromosomes are thread like structures situated in the nucleus of a cell and are made up of protein and DNA material. This is passed from parents to the offspring. Human beings have 46 chromosomes in each cell. It is important to maintain this number during cell division and formation of zygote which is the starting point for development of fetus. Due to some error in cell division baby may inherit less or more chromosomes, that is 45 or 46. This condition of having abnormal number of chromosomes is called aneuploidy.
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Chromosomes are arranged as 23 pairs and are labelled from chromosome 1 to 22 and the last pair is sex chromosomes which means there are 2 chromosomes in each pair. 1 pair of chromosome number 1, 1 pair of chromosome number 2 and so on. If there are three chromosomes instead of 2, it is called trisomy. For example if there are three chromosomes of 21 instead of 2, it is called trisomy 21. Trisomy 21 is called as Down syndrome. Likewise, trisomy 13 is called patau's syndrome, trisomy 18, Edward syndrome.
Babies with aneuploidy will have structural defects and neurodevelopment problems to a variable degree.
Chromosomal aneuploidies contribute to > 50% of early fetal losses, 6-11% of stillbirths and neonatal deaths.
What is screening ?
Screening is a process of surveying a population using a specific marker or markers and defined screening cut-off levels. Screening is done to identify the individuals in the population at a higher risk for a particular disorder.
All pregnant women should be offered screening test. Screening tests can be performed by fetal medicine doctor in Vijayawada after 10 weeks of pregnancy.
What are aneuploidy screening tests?
Among all the chromosomal aneuploidies, little more emphasis is laid on Down syndrome because, Trisomy 18 and trisomy 13 will present with major anomalies in 90% of the cases which can be detected by ultrasound. And more over the incidence is very low when compared to downs.
Incidence of Down syndrome (DS) is 1:800 livebirths.9 out of 10 survive the first year. Majority will have intellectual disability to a variable degree. Detection by ultrasound is only 30-50%.
Consequently, the detection of chromosomal abnormalities constitutes the most frequent indication for the invasive prenatal diagnosis like amniocentesis, which is associated with procedure related fetal loss rate.
Hence it is important to offer invasive testing to those cases where the risk of having a baby with DS is more than the risk of miscarriage. This is facilitated by Screening tests.
Screening tests for aneuploidy in Vijayawada are used to filter out those pregnancies which have less chance of having a baby with chromosomal abnormality and offer invasive test for those who have higher chance of having a baby with chromosomal abnormality.
Few of the biochemical analytes produced by placenta and fetus that enter the maternal blood circulation are used to assess the risk for aneuploidy, making it an indirect test to assess the fetal well being. Depending on the nature of the biochemical marker, the level of these biochemical markers increases or decreases as the gestational age advances in aneuploidy risk assessment, biochemical markers are analysed in relation to gestational age and biometry of the fetus .Hence, it is very important to perform the test at correct gestational age. Otherwise, there could be false positive or false negative results.
How is screening test done?
You can eat and drink normally before the test.
During the screening test, a member of the health care team takes a sample of blood by placing a needle into a vein in your arm. The blood sample is sent to a lab for examination. You can get back to your usual activities immediately.
Pre-test counselling:
Pre-test counselling can be given by the primary obstetrician, fetal medicine expert, clinical geneticist or a genetic counsellor. The following details are discussed during counselling session:
- Benefits of screening test
- Interpretation of test results and its implications.
- Possible options of management after obtaining the test report. No further test is recommended for women with low risk for aneuploidy. TIFFA scan at 20 weeks is advised. If the report shows high risk for aneuploidy, option of either NIPS or direct invasive testing (CVS/Amniocentesis) is discussed with the couple for further management.
It is the patient's choice to opt for the screening test or not to opt for screening test.
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