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AM I PREGNANT? Pregnancy tests III - Prenatal tests- do I need them? - February 2009. (Continuation of last month’s Newsletter on Pregnancy tests) Once you are pregnant you always wonder whether it is a boy or a girl or will the baby have your husband’s or your features etc. All these questions provide a lively material for speculations & debates. It is fun thinking about all these. But there is one question that most parents fear: is the baby all right? Until recently, that question could be answered only at birth. Today it is possible to be sure that everything is okay (except in a few cases) with your baby. This is because of the prenatal tests. Prenatal tests are a group of tests done to detect birth defects. Birth defects may affect the baby's health, or his or her ability to function. Down’s syndrome, one of the common causes of mental retardation is the other abnormality that is detected by these tests. In more than 95% of cases, prenatal diagnosis detects no abnormalities. In the remainder, the test may be detecting some defect. In that event, along with genetic counseling, the information can be used to make vital decisions about this & future pregnancies. Possible options include:
The commonest test done for diagnosing fetal abnormalities is an ultrasound examination. Ultrasound scanning uses sound waves to create “a picture” of the baby. The test detects structural abnormalities. Commonly it is ordered in the fifth month. Most of the structures are developed by then & can be assessed. Most of the defects are picked by the ultrasound exam. However a detailed study of the heart is possible only after 24 weeks. The rationale to do the ultrasound scan before 20 weeks is to identify the fetuses that have major anomalies & if termination has to be done it is allowed only up to 20 weeks. The other screening tests assess the risk of the baby being born with certain conditions, such as spina bifida or Down's syndrome. These tests are:
First trimester screening test:
This test is combined with the results of a special ultrasound test, called nuchal translucency screening. It involves an ultrasound scan to measure the thickness of the layer of fluid at the back of the baby's neck. Babies with Down's syndrome have a thicker layer. If it is thicker than the average, further tests will be needed for confirmation. The above tests pose no risk to the mother or fetus.
Second trimester screening test: This is a blood test ordered between 16-20 weeks. A positive result does not always mean there is a defect. Sometimes it means there is a greater risk of one. If the results of screening show a risk, a diagnostic test - amniocentesis may be done.
Amniocentesis:
The procedure is done as a day care procedure & does not need Anesthesia. Under the ultrasound guidance a needle is pushed into the amniotic sac and some amount of amniotic fluid is aspirated. It may be up to four weeks before you get the result. Being “ on hold” until you know the result is distressing. Therefore another special test called FISH technique (that gives results in 2-3 days) is also ordered. The test is extremely accurate with an overall error rate of 0.4%. There is however a risk of fetal loss. The risk varies from 5% to 9%. The other minor troubles are mild cramping, vaginal bleeding or amniotic fluid leak.
Chorionic Villus Biopsy: The potential risks include miscarriage, occasionally limb deformities. And there is a slim risk that a pregnancy could be terminated on the basis of incorrect information. Since an abnormality called mosaicism may be detected in the villi when it does not exist in the fetus. Fetal blood sampling & fetal skin biopsy are the other rarely performed tests. These tests have specific indications.
A word of caution:
Ethics of prenatal testing: There are lots of controversies about some of these prenatal tests. Our esteemed readers are free to seek any clarification with me at skadharao@gmail.com Best wishes; take care and ensure that baby born in your family gets the very best attention, starting from the time of the ‘fetus’ stage itself.
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Second trimester screening test measures different hormones or proteins, or combinations of these, including alpha-fetoprotein (AFP), human chorionic gonadotrophin (hCG), oestriol and inhibin A. Alpha fetoprotein is a substance produced by the fetus. Elevated levels can indicate if your fetus has certain defects, such as spina bifida (a deformity of the spinal column) or anencephaly (the absence of all or part of the brain). Abnormally low levels suggest an increased risk of Down syndrome or other chromosomal defect.
This is a special test that will be ordered only if there is a possibility (if there is a family history or if the previous pregnancy resulted in an abnormal baby) inborn errors of metabolism& Down’s syndrome& other chromosomal abnormalities. The test is done between 9-12 weeks. The advantage of this test is that it avoids a late pregnancy termination. Abortion after you feel the baby moving would be traumatic. It does not help in identifying spina bifida & other structural abnormalities though.
Under ultrasound guidance a fine, hollow needle is used to remove a small piece of the developing placenta. The cells are removed & tested. The sampling of cells is taken viw the vagina & the cervix or via a needle inserted in the abdominal wall. The second one is the preferred route.
There is some debate going on whether routine prenatal tests which seek to discover and “destroy” abnormal fetuses is justified. Even though discrimination against people with disabilities is wrong the impact of a disabled child on the lives, relationships & financial situation cannot be overlooked. Therefore the prenatal testing is done only when the parents wish to.
