Diagnostic tests are usually recommended following earlier screening tests which might have suggested a chromosomal abnormality in the developing fetus. There are a number of diagnostic tests which can be used. The article here looks at the procedure, the diseases tested for, as well as the risks and side effects.
Amniocentesis
Amniocentesis is carried out at around 13 weeks of pregnancy. The procedure lasts around ten minutes and is done under localized anesthetic on the belly area. A needle is inserted into the womb and around twenty milliliters of amniotic fluid withdrawn. Amniotic fluid serves to protect the baby and is rich in dead fetal cells which naturally detach from the surface of the fetus’s skin as dead cells get replaced with new cells. Samples of amniotic fluid can then be tested to determine fetal anomalies. DNA in amniotic fluid degrades very quickly so it is imperative the testing begins within 24 hours of sample collection. Amniocentesis is also ideal for testing levels of alpha-fetoprotein which cannot be done using an earlier fetal DNA sampling method known as chorionic villus sampling. High levels of alpha-fetoprotein (AFP) indicate neural tube defects such as:
- encephaloceles
- Spina bifida
- hydranencephaly
- iniencephaly
Chorionic Villus sampling
Chorionic villus sampling is a prenatal biopsy which involves taking a very small sample of the tissue that lines the wall of the placenta (this layer of tissue is referred to as a chorionic villus).The test can be carried out at around 11 weeks of pregnancy and is ideal for early diagnosis of chromosomal disorders or genetic illnesses. The tissue sample is collected in two ways:
- One possible method is via insertion of a catheter through the opening to the womb
- Another procedure used is via insertion of a needle through the abdomen.
Both the above two methods of sample collection require an ultrasound in order to collect the sample from the correct place and avoid any contact whatsoever with the baby.
Prenatal Paternity Testing
Amniocentesis as well as chorionic villus sampling are also used in prenatal testing for paternity. Since both these methods of fetal DNA extraction carry certain risks, they are slowly being phased out for use in prenatal testing for paternity and being replaced by noninvasive prenatal testing using maternal blood samples. Scientists are nowadays able to successfully identify the DNA of the unborn fetus in the mother’s peripheral blood supply in a sample taken be means of a standard blood draw. The DNA extracted is usually cell-free fetal DNA (cffDNA). Noninvasive prenatal paternity testing using cffDNA is 99.9% accurate but besides the levels of accuracy, it is the risk -free nature of the test that is turning it into the standard prenatal testing method.
Down Syndrome
Down syndrome is caused by the presence of an extra chromosome. It is the most common genetic disorder in the USA. We would normally have two copies of chromosome 21 but Down syndrome individuals will have 3. The condition is in fact medically referred to as trisomy 21. The extra copy of chromosome 21 may manifest itself as a complete chromosome or just in part. This means that collectively, the Down syndrome sufferer will have an increased number of genes from chromosome 21, stemming from the presence of 3 chromosomes. As a result of the presence of the extra copy of their 21st chromosome, they suffer physical and mental developmental delays and disabilities.
Down syndrome can be diagnosed in the prenatal stage of a pregnancy. The first indications would come about during a standard ultrasound at around 11 weeks where the doctor would look for Nuchal Translucency, a collection of fluid in the neck area that, although seen in all babies, is present in higher volumes in children with Down syndrome. Other screening tests include non-invasive prenatal screening test (NIPT) which is done via a simple medical blood draw. Diagnostic tests, such as amniocentesis, are usually recommended based on the results of screening tests already carried out. They are recommended by your doctor as follow-up tests where abnormal results are witnessed. Chorionic villus sampling (CVS) and cordocentesis (Percutaneous umbilical blood sampling, PUBS) are other additional tests which may be recommended by the doctor to detect Down syndrome during the prenatal stage.
CVS is done between the 9th and 14th weeks of pregnancy. The doctor takes sample cells from the placenta for analysis of the presence of Down syndrome in the fetal cells.
PUBS (percutaneous umbilical cord sampling) is done past the 18th week of pregnancy and mainly involves taking a blood sample from the umbilical cord for examination which is then analysed in a laboratory.
Turner’s syndrome: What genes cause it?
Turner’s syndrome is a chromosomal abnormality specific to the X chromosome mainly affecting females. Often, the condition is characterized by monosomy X which means that the female does not have a pair of X chromosomes as would usually occur. The condition is not believed to be hereditary and is actually attributed to an error in cell division. Normally, chromosomes exist in pairs, with each human having 23 pairs of chromosomes resulting in a total of 46. Turner’s syndrome is caused by an unpaired X chromosome (a single X chromosome). This means that the chromosomes in the body of the affected person are 45 (44 paired chromosomes plus a single unpaired chromosome).
The disease adversely affects the ovaries and ovarian function leading to infertility. The degeneration of ovarian tissue begins before birth. Physical characteristics of sufferers of this condition include what is called a webbed neck (excessive folds and creases in the neck region), growth of nails that are narrow and turned upward, and swollen hands and feet, among others, especially at birth. Heart defects are also not uncommon in children with the condition, affecting around one in every three kids.
Diagnosis of Turner’s syndrome
The condition can be diagnosed by a blood test known as a karyotype. The test is done to analyze the chromosomal composition of the female. The condition can also be diagnosed pre-birth by extracting samples of chorionic villi (CVS) or amniotic fluid (Amniocentesis) from the unborn child for chromosomal analysis.
Edwards’ syndrome
Edwards’ syndrome is a genetic disorder caused by the presence of an additional copy of the 18th chromosome in all or some of the body cells. A normal cell contains 23 pairs of chromosome, but a baby with Edwards’ syndrome, instead of the normally paired cells, has three copies of chromosome 18.
The presence of the 3 chromosomal copies disrupts the normal development of the baby and could result in miscarriage or stillbirth. The babies are often born with heart defects and are underweight. Other symptoms include clenched fists with long overlapping fingers and underdeveloped thumbs, a small head with a small jaw and mouth. The babies also develop heart and kidney problems, slow or stunted growth, bone abnormalities and acute learning disability.
The two main types of Edwards’ syndrome are the full form and the mosaic trisomy 18. The full form is where each of the baby’s body cells has three copies of chromosome 18 while for the mosaic trisomy 18 has only some of the baby’s cells with the extra copy of chromosome 18.
Diagnosis of Edwards’ syndrome
A screening test is done between the 10th and 14th week of pregnancy comprising of a nuchal translucency ultrasound scan and a blood test to determine the chances of the baby having the genetic disorder.
Another scan is offered when the mother is between 18 and 21 weeks pregnant. This is done to check for physical abnormalities in the developing baby as well as the Edwards’ syndrome and other rare genetic disorders. If there is a high risk of the baby having the syndrome, the chronic villus sampling and/or amniocentesis are/is offered as invasive diagnostic tests to check if the baby has the Edwards’ syndrome. Non-invasive forms of diagnosis are also available. It involves taking a blood sample from the mother and testing the DNA of the baby found in it. It is referred to as Non-invasive prenatal diagnosis.
References
http://www.healthline.com/health/down-syndrome#screening6
http://www.mayoclinic.org/diseases-conditions/turner-syndrome/basics/symptoms/con-20032572
https://www.genome.gov/19519119/#al-3
http://www.nhs.uk/conditions/edwards-syndrome/Pages/Introduction.aspxhttps://en.wikipedia.org/wiki/Edwards_syndrome
https://en.wikipedia.org/wiki/Edwards_syndrome
