Diagnostic tests are usually recommended following earlier screening tests which might have suggested an abnormality in the developing fetus. There are a number of diagnostic tests which can be used. We will here look at the procedure, the diseases tested for as well as the risks and side effects.
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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 A needle is inserted into the womb and around twenty milliliters of amniotic fluid withdrawn.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 indicate neural tube defects such as:
- Spina bifida
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.
Lets see turner’s syndrome as an example.
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 a23 pairs of chromosomes results 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 person affect 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 suffers of this condition include what is called a webbed neck (excessive folds and creases in the neck region) and of the hands and feet (this condition appears as swelled extremities). 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 taking a blood sample from the child. The condition can also be diagnosed pre-birth by extracting samples of chorionic villi or amniotic fluid from the unborn child.
Karl M McDonald is a free lance writer specializing in the field of DNA testing genetics. More articles by the same author can be found in the article repository for: www.homeDNAdirect.ca