Pregnancy – Special Tests
Although most babies are born perfectly normal, around 2% are born with a major congenital abnormality. Many of these abnormalities cannot be identified in pregnancy, but there are two classes of abnormality for which screening is available. These are:
· Open neural tube defects. The best known of these is spina bifida.
· Chromosome abnormalities. The best known of these is Down’s Syndrome, otherwise known as Trisomy 21.
Screening for open neural tube defects.
In most areas you will be offered a blood test at around the 16th week of pregnancy in order to detect raised levels of a substance called alpha-fetoprotein (AFP). This is produced by the fetal liver and minute amounts can be detected in your blood from quite early in pregnancy. If levels are found to be raised, you will be offered a detailed ultrasound scan to examine your baby in greater detail. You may have to attend a specialist hospital for this. Not every area offers maternal AFP screening, so you will need to ask your midwife or doctor if you are keen to have it.
How accurate is screening for open neural tube defects?
Most, but not all, neural tube defects will be picked up in this way. Some mild cases of spina bifida will be missed.
Screening for chromosome abnormalities.
There are several ways to screen for chromosome abnormalities during pregnancy:
· Measurement of nuchal translucency. Around 11 to 14 weeks of pregnancy an ultrasound scan is used to measure the fluid space at the back of the baby’s neck. Babies with some chromosome abnormalities have larger spaces than those without. This test is only available in certain specialist centres and you may have to pay for it.
· Chorion villus sampling. This test can be performed from the 11th week of pregnancy onwards. A small sample of the developing placenta is taken for chromosomal analysis. This test is only available in specialist centres and carries an approximately 2% risk of miscarriage. It has been suggested that there is an increased risk of your baby being born with limb abnormalities following this test, but this has not been proven. The test results are available in a few days.
Both the above tests need to be performed early in pregnancy and are only available in a few centres. If you are keen to have either of these tests, you will need to discuss this with your midwife or doctor who will tell you where your nearest centre is.
· Serum screening. This is a blood test performed between the 15th and 20th week of pregnancy. There are a number of different tests available such as the Triple test or the Bart’s test. They all measure the levels of several hormones in your blood. Combining these results gives the risk of the baby having a chromosomal abnormality. If you are considered low risk, nothing further is necessary. However, if the result suggests that you are high risk, you may be offered an amniocentesis.
· Amniocentesis. This involves taking a sample of the amniotic fluid surrounding the baby via a needle inserted through your abdomen. The sample is then sent for chromosomal analysis. The risk of miscarriage associated with this procedure is approximately 1%. It is usually done around the 16th week of pregnancy and the results take two to three weeks to come back.
How accurate is screening for chromosomal abnormalities?
Serum screening alone detects approximately two thirds of affected pregnancies, with an overall false positive rate of 5%. Nuchal translucency screening plus serum screening detects between 80% and 90% of affected pregnancies, but this varies considerably between centres. Both amniocentesis and chorion villus sampling should detect all affected pregnancies, but, occasionally, it will not be possible to culture the baby’s cells and the test may need to be repeated.
Many women have screening in pregnancy to reassure themselves that their baby is all right. However, before you have any screening test you need to think about what you will do if the baby is found to have a problem. You will almost certainly be offered a termination of pregnancy – a course of action you may find unacceptable.