Advice on why foetal abnormality tests are carried out during pregnancy, with ultrasound, blood and genetics screening methods explained.
It is likely that you will be offered screening tests for Downs Syndrome, a chromosome disorder, and spina bifida, a problem with the brain or spinal cord, early in your pregnancy. Several different kinds of test are used. They involve blood tests, a special ultrasound scan or a combination of both. The test results will tell you if you are at high or low risk of having an abnormal baby. If you at high risk you will be offered a diagnostic test to find out if your baby is affected. If you are at low risk you will normally not be offered further tests, although there is no guarantee that your baby will be normal. Diagnostic tests can only ever give a prediction of risk and your baby could be the 1 in 1000 that is affected.
AFP Maternal serum alpha-feto protein
Screening for anencephaly, absences of the skull and open spina bifida.
Blood is taken, ideally at around sixteen weeks for this test. If you are unsure of your dates then it is always best to have dating scan to make the test more accurate.
Maternal serum screening
A blood test for Downs Syndrome carried out between 14 and 21 weeks. The test can be performed on the blood sample taken to measure AFP so it is often done at 16 weeks. When the tests are combined this is called a triple test. Your risk of having a baby with Downs Syndrome is calculated from your age, the level of AFP and other substances in the blood.
Ultra sound screening
A 'nuchal translucency scan' or NT is a special kind of ultrasound scan carried out at around 11 to 13 weeks. A measurement of the amount of fluid at the back of the baby's neck is recorded. A computer programme uses this measurement along with measurements of the size of your baby and your age to calculate the risk of Downs Syndrome.
Blood and ultrasound testing
You may be offered a combined screening test, which uses the NT scan and blood taken at 10 and 15 weeks. The result is worked out after the second blood test.
With all of the above tests the hospital should tell you how long it will take to get your results and how you will be informed. If you have a high risk result your baby may well be normal, but you will be offered further testing. Three tests are available, a high resolution ultrasound scan, chronic villus sampling, performed from 11 to 14 weeks, or amniocentesis carried out from 16 weeks onwards. Not everyone chooses to have these tests because of the risks involved. You may prefer to continue your pregnancy and let nature take it's course.
High resolution ultrasound
A foetal anatomy scan may well be offered to you when you are around 19 weeks pregnant regardless of whether or not your pregnancy is considered to be high risk. For most women this is a part of standard care. If you are high risk of having an abnormal baby a scan may be performed earlier, although this is not always possible to see your baby clearly and a rescan may be suggested. Ultrasound is highly successful at detecting spina bifida and anencephaly.
Scans are used to check when your baby is due, check the position of the placenta and make sure you are not expecting twins or more as well as making sure your baby's development looks normal.
You do not have to have a scan if you do not want one. Scams certainly do not cause severe problems but without further research we cannot be absolutely certain that they are completely safe.
Chorionic villus sampling or CVS
A fine needle is passed either through the abdomen or through the vagina under the guidance of an ultrasound scan. A small sample of tissue is taken from the placenta. This is analysed in a laboratory and the baby's chromosomes are counted. Very occasionally the test does not produce a result and a repeat test may be offered.
Using ultrasound scan a fine needle is inserted through your abdomen into the womb. A sample is then taken from the fluid surround your baby. This fluid will contain some of your baby's cells, which will be analysed in the laboratory. About 1% of samples do not produce a result and you may have a repeat test if you wish.
This test is not without risk. 1% of women who have an amniocentesis will miscarry. Both of these tests can be uncomfortable but they should not be painful. You may need to rest for a day or two afterwards.
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