During the early days of pregnancy, amniocentesis is carried out on pregnant woman to find out if the developing fetus is free from certain abnormalities. This procedure involves certain invasive methods like inserting a hollow needle into the uterus of the woman through her abdominal wall to withdraw a sample of amniotic fluid that surrounds the fetus. Using ultrasound, the location of the fetus floating in the amniotic fluid is determined. The abdominal skin surrounding the area is cleaned. The procedure may be conducted either by injecting a local anesthetic in that area, or by just applying a tropical anesthetic around the area through which the needle is to be inserted. About 1 cc per week of gestation of the fetus is taken out from the amniotic fluid.
This procedure is used as a tool which can detect chromosomal disorders like Downs Syndrome and many rare metabolic disorders which are normally inherited. Some structural defects like spina bifida and anencephaly can be detected using this test. This procedure is normally carried out for women over the age of thirty five. It is recommended for women who had undergone a triple screen blood test during pregnancy and for those who have a family history of birth defects or some inherited metabolic disorders. This procedure may be conducted even in later stages of pregnancy to detect suspected problems like Rh incompatibility or infection. This test is also used to determine lung maturity of the infant in the last trimester.
A cell culture is done with the fetal cells taken out during the procedure and the analysis of the DNA is conducted to detect chromosomal abnormalities. It may take up to a month for the results to arrive. This is rated as a highly accurate procedure.
After the withdrawal of the amniotic fluid which takes only a few minutes, the pregnant woman is advised to be in bed for several hours, though the entire procedure will last for about 45 minutes. Doctors check the heart beat of fetus to ensure that it is normal. It is wise to call the health care provider, if the woman experiences fever, bleeding or leakage of amniotic fluid for a long time. Though the chances of infection or injury to the fetus are not ruled out, they are quite rare. There is also a slight chance of miscarriage, but the percentage is negligible. During later stages of pregnancy, a Doppler ultrasound is safer than amniocentesis because it is non-invasive. Anemia in the fetus can easily be detected using ultrasound by measuring the velocity of the blood flow in the middle cerebral artery.
Chorionic Villus Sampling
The diagnostic procedure of taking out a sample tissue (Choroinic Villi) from the placenta to detect congenital abnormalities in a fetus is known as Chorionic Villus Sampling (CVS). With the guidance of ultrasound, the position of placenta is first determined. There are two methods - trans-cervical and trans-abdominal to perform this test. The position of the placenta helps the physician choose a suitable method. For trans-cervical CVS, parameters like the position of the uterus, the size of the gestational sac and the position of the placenta inside the uterus are first determined using abdominal ultrasound. Using a good antiseptic, the vulva, vagina and the cervix are cleansed. The abdomen is also cleansed for trans-abdominal procedure.
Trans cervical procedure: A thin plastic tube is inserted through the vagina and cervix for the trans-cervical procedure to reach the placenta. A tiny sample of chorionic villus tissue is taken out after locating the exact position of the placenta.
Trans-abdominal procedure: This procedure is similar to the earlier one, but a needle is inserted through the abdomen in this test to reach the uterus and then to the placenta. The chorionic villus sample tissue is drawn into the syringe, while the needle is guided by ultrasound.
This sample is then taken to the laboratory for evaluation. This procedure can be conducted even earlier than amniocentesis to detect any congenital defects present in the fetus. It is done at around 10 to 12 weeks after the last menstruation. Study of the DNA, chromosomes and enzymes of the fetus can be conducted using the sample taken out during the test. Results are available within a week or two. If there are any abnormalities found in the fetus, it is easy to conduct a therapeutic abortion, in case it is necessary. Pregnant women over the age of 35 who are at risk for giving birth to a baby with Downs Syndrome or those who have had birth defects in an earlier pregnancy are advised this test. For detecting neural tube defects and the Rh-incompatibility, amniocentesis is a better option. Hemoglobinopathies and Tay-Sachs disease can be detected through Chorionic Villus Sampling.
The risk involved in using CVS is slightly higher when compared to amniocentesis. Some complications like rupture of the amniotic membrane, miscarriage, infection, bleeding, Rh-incompatibility in the mother if she is Rh-negative and contamination of the sample with maternal cells can occur. When CVS is performed after 10 weeks of gestational period, there is a risk for limb defects in the fetus. If the mother's blood is Rh-negative, she has to receive Rho GAM to avoid Rh incompatibility. After the CVS, it is advised to have an ultrasound done after about two or four days to ensure the fetus is fine.
Monosomy is a rare chromosome anomaly. Human cells normally contain 23 pairs of chromosomes, with a total of 46 chromosomes in each cell. Monosomy refers to the loss of one chromosome in cells. Any such change of chromosomes shall cause problems pertaining to growth, development and function of the body's systems. Monosomy is a genetic defect caused by an incomplete set of chromosomes. The changes in chromosomes occur during the formation of reproductive cells in early fetal development.
Monosomy can be identified during prenatal testing, especially in women who are at high risk. Prenatal testing such as an amniocentesis can reveal monosomy. As the test results could be very complicated, it is important to receive genetic counseling before undertaking this test. While a negative result indicates that no abnormalities were detected, a positive result suggests that a problem may be present. Since false positives and negatives can also happen, follow up additional testing is also recommended.
Aneuploidy is the term used to refer to chromosomal defects, a gain or loss of chromosomes from the normal 46. In monosomy, which is a kind of anueploid, there is the loss of one chromosome in cells. Another common form of aneuploidy is trisomy where people have three copies of a particular chromosome 21 in each cell instead of the two copies. One common example of the condition caused by trisomy is Down Syndrome.
Turner syndrome is a known example of the condition caused by monosomy. In this syndrome, women typically have only one X chromosome instead of the usual two. Significantly, Turner syndrome is the only full monosomy that is found in human beings. In other full monosomy, the individual will not survive development.
Cri du chat syndrome and 1p36 Deletion Syndrome are instances of partial monosomy caused by deletion of the short p arm of chromosome 5 and chromosome 1 respectively.
Chi du chat syndrome is characterized by a number of symptoms and in particular a malformed larynx which causes the voice to sound strangely high pitched. Chromosome 1p36 deletion syndrome is considered one of the commonest chromosome deletion syndromes. It is characterized by features such as developmental delay, feeding difficulties, low muscle tone, distinctive facial features, hearing loss, heart problems, seizures, vision defects and a large fontanelle that is slow to close. The incidence of monosomy 1p36 has been estimated to be 1 in 5000 to 1 in 10000 live born children. Interestingly, more females than males have been reported.
Bibliography / Reference
Collection of Pages - Last revised Date: February 17, 2019