Most healthy expectant mothers only need simple, non-invasive tests to check the health of their unborn child, says Dr Thewin Dejthevaporn
“She has her mother’s eyes” or “He is his father’s son” are the kind of comments new parents often hear. The similarities people see between parents and their children can be attributed to chromosomes, which could be said to hold the keys to our lives. Height, weight, hair color, skin color, eye color, length of limbs, length of nose are all determined by our chromosomes, or more specifically, our DNA. Thus, any problem with chromosomes could have a serious impact on our lives. Chromosomal abnormalities are usually caused by errors in cell division, although some may be inherited from the parental DNA. One in 150 babies are reportedly born with some kind of chromosomal abnormality, the most well-known being Down’s syndrome.
Doctors conduct two kinds of tests to check babies for chromosomal abnormalities — screening and diagnostic tests. Screening tests are routinely conducted on all pregnant women, while diagnostic tests are only administered if there is a positive result on the screening test or high risk factors are identified. Healthy women under the age of 35 usually undergo only non-invasive screening tests. Older women or those with a history of congenital abnormalities may be asked to undergo amniocentesis and other procedures to determine the health of the fetus. This helps detect abnormalities early. None of these tests are 100 percent accurate and have been known to offer both false positive and false negative results, although this is rare. Doctors, therefore, use a screening test for all pregnant women and then recommend more tests as necessary.
This test is carried out in the first trimester and includes an ultrasound scan of the fetus and a blood test for the mother. The ultrasound scan measures and checks the fetus’ neck region for fluid retention and/or other abnormalities. The mother’s blood serum is tested for levels of specific 2 placenta and hormones; abnormal levels are indicators for further diagnostic tests. This test is safe and does not pose any threat to the mother or the fetus. It has a 85 percent detection rate and is used around the world. However, since its efficiency rate falls short of 100 percent, doctors sometimes recommend diagnostic procedures for high-risk mothers, even if these tests are negative.
This is an analysis of cell-free fetal DNA that is found in the mother’s blood. Targeting three of the most common abnormalities, the test checks fetal DNA for three chromosomes — trisomy 21, trisomy 18, and trisomy 13. This is also a screening test and would have to be followed by amniocentesis in case of a positive result. However, it does offer a 99 percent detection rate with less than 0.5 percent false negative/positive results. Since the mother’s blood is used for this test, it poses no risks for the fetus. This test is usually carried out between 10 and 20 weeks, although it can be conducted anytime during the pregnancy.
If any of the earlier tests prove positive or if the mother falls in the high-risk group due to Down’s syndrome in the family, doctors will recommend amniocentesis. In this process, a fine needle is inserted into the mother’s abdomen and amniotic fluid is drawn for testing. This fluid contains the baby’s cells. Culture of the baby’s cells helps doctor to detect chromosomal anomalies. However, this is a risky procedure with the chances of miscarriage at 1 in 300. Hence, it is not routinely recommended to all mothers. However, it has been found to be very effective in identifying Down’s syndrome, which could delay a child’s physical and mental growth. If you have had an amniocentesis done, please rest before and after the test. In case of any vaginal bleeding or discharge, see your doctor immediately.
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M.D., Faculty of Medicine, Ramathibodi Hospital, Mahidol University, 1989.