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SCREENING TESTS IN PREGNANCY

What are Screening Tests in Pregnancy ?

Much of your antenatal care by your midwife and doctors consists of screening tests. These aim to pick up at the earliest stage any possible problems in your pregnancy which may affect either you or your baby. The simplest screening tests are really the questions asked by your midwife and doctors about your general health, the health of your family, and any previous children, and what happened in any previous pregnancies. The answers to these questions may suggest problems which may occur in your pregnancy, or particular tests which then need to be done. The general examination by your doctor is also a screening test for disorders such as high blood pressure, diabetes or heart disease which may complicate the pregnancy.

Antenatal clinic blood tests

A blood sample is taken to check that you are not anaemic. Your blood group and Rhesus factor will be checked. Rhesus factor is important. You may be Rhesus negative and your baby positive. If some of your baby's blood enters your blood system, then your body can react by making chemicals called antibodies. These antibodies enter the baby's system again and can actually harm your baby's blood cells. If you are Rhesus negative (and 1 out of 7 people are), you will be checked for antibodies at your antenatal booking visit, and then again when you are 28 and 32 weeks pregnant. Your blood will be checked to make sure that you do not have any infections, such as syphilis and hepatitis B, which would harm the baby. In most hospitals, you will also be offered screening for HIV. This is because transfer of the virus to the baby can be reduced, if a mother is known to be infected. If you are from Africa, Asia or the Mediterranean, your blood will also be studied to make sure that you do not have sickle cell disease or thalassaemia conditions affecting the blood cells which could affect both your pregnancy and the baby. Your blood will also be screened to find out if you have had rubella (German measles). If you have not had rubella, and you are in contact with the disease in early pregnancy, then your baby could be born with birth defects. To avoid this risk in any future pregnancy, you will be offered a vaccination once your baby is born. You may also be offered an alpha-fetoprotein (spina bifida) blood test or a double/triple test (for Down's syndrome). These tests do NOT tell you whether your baby has spina bifida or Down's syndrome. What they do tell you is whether your baby's risk of having these conditions is increased or reduced you should use the results (after discussing them with your doctor) to decide whether further tests are needed.

Checking your baby is normal

About one in every forty babies is born with an abnormality. This may be anything from a minor abnormality, such as an extra toe, or a more serious problem such as a heart defect. It is not possible to detect all abnormalities before birth. There are several tests which can be performed: Ultrasound scan : The advantage of this test (which is like a radar picture) is that it is safe and will not harm your baby. An ultrasound scan in the first 4 months of your pregnancy can be useful in dating your pregnancy. Some hospitals offer a scan at 11 to 14 weeks, to assess the risk of the baby having Down’s syndrome. This scan is called a nuchal translucency scan. It measures a rim of fluid behind the neck of the baby. The thicker the rim of fluid, the greater are the chances that the baby could have this syndrome. Like the triple/double test, this scan can only give a broad indication of risk : it can not tell absolutely whether the baby has Down’s syndrome.

However, a more detailed scan is performed between 18 and 20 weeks. From this scan, the structure of your baby's face, brain, spine, heart, bowel, kidneys, bladder and limbs can be studied. In addition to looking for actual problems with how your baby is formed, your scan will also look for what are called « markers ». These include inward curvature of the little finger or minor swellings of the kidney. Although these are not serious problems in themselves, they might point to the presence of another problem such as Down’s syndrome. Before you have your scan, it is important for you to be sure that you want to know about these markers. If you don’t, dfo tell the ultrasonographer, so that she can concentrate only on structural problems. Sometimes, ultrasound alone cannot make a diagnosis. This is indeed the case for conditions such as Down’s syndrome. These disorders are termed « chromosomal ». This means that the baby has too much or too little genetic information. Other conditions like cystic fibrosis occurs because the genetic information is incorrectly coded. To get more information, a sample which allows one to look at cells themselves must be taken. This involves one of two tests; amniocentesis or chorionic venous sampling. Amniocentesis : This is the removal of a sample of fluid which cushions your baby. By performing amniocentesis, cells which came from the baby can be cultivated. Abnormalities of the genes and chromosomes which control development of a baby can then be checked. Using this method, abnormalities with missing or extra chromosomes (like Down's syndrome) can be found. The number of conditions which can be "picked up" in this way is increasing all the time. After 2 to 3 weeks, the cells have grown and a result is known. You may also be told the sex of your baby (although you may choose not to know this until the baby is born). Occasionally the sex of the baby is very important some diseases are passed from mother to son but not from mother to daughter.

Unfortunately amniocentesis can lead to a miscarriage (which may be of a normal baby) in one in every 150 procedures. Other problems consist of the cells failing to grow, so that the test might need to be repeated. Cells may be mistakenly sampled from you, rather than your baby, by mistake. Because of the problems of amniocentesis, it will not be performed without a good reason. These include a family history or previous baby with a chromosome abnormality like Down's syndrome, an increased risk shown by the double/triple test, or if you are having a baby in later years of your life. Chorion villous sampling (CVS) : CVS is an alternative to amniocentesis. Instead of sampling the fluid around the baby, some of the placenta is sampled. Ultrasound scanning is again used to guide the needle, which can either be passed through the skin of your abdomen (like amniocentesis) or through the vagina. The needle does not enter the sac where your baby is.The advantage of CVS is that it can be done earlier in pregnancy, and the results are obtained faster, as there is no need to wait for cells to grow. This avoids the long wait after the test has been done. If the result of the test shows an abnormality and you decide to have an abortion, it is easier to do this early in the pregnancy. CVS can be performed at any stage after 10 weeks. It can be offered instead of amniocentesis. (the risk of miscarriage is one to two percent at 10 to 13 weeks, and one percent after 14 weeks.)

Screening in late pregnancy

There are several simple tests that will be performed. You yourself can check that the baby is moving at least ten times a day (you may be given a kick chart to record the baby's movements). Your midwife or doctor will regularly measure your blood pressure to screen for high blood pressure in pregnancy, and test your urine for protein or sugar. You may be given a sugar load test, in which you are given a sugar drink and the blood samples are taken to test how your body copes with the sugar. This is a screening test for diabetes. By measuring the size of your womb, your midwife or doctor will be able to check that your baby is growing well. If you are Rhesus negative, your blood will be rechecked for antibodies at 28 and 32 weeks.

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