While the majority of babies are born without any abnormality, 3-4% of pregnancies result in babies with abnormalities.
Some but not all fetal abnormalities can be detected during pregnancy. No single test checks for everything.
Different types of tests are available. They vary in how accurate they are, how safe they are, when they are performed, and what information they give.
A woman may decide to have testing:
- Because she wants to know more about her pregnancy
- Because she wants the option of terminating an affected pregnancy, or
- So she can prepare herself for a baby with an abnormality.
A woman may decide not to have any prenatal testing. Testing for a fetal abnormality is not compulsory
Which Abnormalities are tested for?
Regardless of a mother’s age, the vast majority of babies born do not have any of these abnormalities.
For normal human development we need 46 chromosomes (23 pairs). Any extra or missing chromosome, or any change in the structure or arrangement of the chromosomes, may affect normal development.
Down syndrome (Trisomy 21)
- The risk increases with a woman’s age, however, pregnancies of younger women can also be affected
- It is the most common chromosomal abnormality (1 in every 400 pregnancies). It is caused by having an extra chromosome (chromosome 21) and results in a number of characteristic physical, medical and intellectual features
- People with Down syndrome typically have a degree of intellectual impairment but can participate in school, work and social life. Some are severely intellectually disabled. It is not possible during the pregnancy to predict the level of disability
- Many pregnancies with Down syndrome miscarry
- There is no cure for Down syndrome
Edwards syndrome (Trisomy 18)
It is less common than Down syndrome (1 in every 1,600 pregnancies) and is caused by having an extra chromosome (chromosome 18).
Patau syndrome (Trisomy 13)
It is less common than Edwards syndrome (1 in every 3,400 pregnancies) and is caused by having an extra chromosome (chromosome 13).
- Pregnancies with Edwards syndrome or Patau syndrome usually miscarry, and babies that are live born rarely survive for long. All babies with Patau syndrome or Edwards syndrome have severe developmental problems.
- The risk of having a pregnancy affected by Edwards syndrome or Patau syndrome increases with a woman’s age, but pregnancies of younger women can also be affected.
Sex chromosomal abnormalities
- The chromosome count for typical development of a girl is 46XX and for a boy is 46XY.
- Sex chromosomal abnormalities occurs when there is a missing or an extra copy of the X chromosome or an extra copy of the Y chromosome.
- These abnormalities are not known to be associated with a woman’s age. The risk of having a pregnancy with a chromosomal abnormality decreases as the pregnancy develops. This is because many miscarry naturally during pregnancy, without any intervention.
Some, but not all, physical abnormalities can be detected during pregnancy by ultrasound examination. Many of the abnormalities can be treated once the child is born. Most physical abnormalities are not related to the mother’s age, but some can be related to chromosomal abnormalities or other syndromes. The vast majority of babies born, regardless of a mother’s age, do not have any of these physical abnormalities.
Neural tube defects (Spina bifida)
Occur early in pregnancy when the spine fails to form properly and the spinal cord and/or brain can be affected. Taking Folate before and during early pregnancy is known to reduce the occurrence of neural tube defects.
Cardiac abnormalities (mal-development of the blood vessels or the heart’s structure)
Digestive system abnormalities (narrowing of the gut)
Renal abnormalities (mal-development of the kidneys or blockages in the outflow of urine)
What are the Different Types of Prenatal Tests?
The tests available fall into two groups: screening tests and diagnostic tests.
Screening tests can be blood tests of the pregnant woman and/or ultrasound examinations of the fetus. These tests are available to women of all ages.
Diagnostic tests are tests that examine samples of the developing placenta or fluid taken from around the baby. It is done by inserting a needle into the woman’s abdomen to take the sample, guided by ultrasound. These tests are available to women who have been identified as being at “increased risk” because of their age or the results of screening rests.
All pregnant women should offered prenatal screening, regardless of their age. Screening tests do not have any additional risk of miscarriage.
A screening test will tell you if your baby has an increased risk or low risk of having the condition, but will not give you a “yes” or “no” answer. “Low risk” does not mean “no risk”, and screening tests will miss some pregnancies with Down syndrome or other abnormalities. The result can be expressed in words or numbers. Detection rates vary depending on the test and the age of the woman.
Screening tests are more accurate in women who are older.
Combined First Trimester Screening
The information from two screening tests is combined with the woman’s age and gestation to provide a risk estimate for Down syndrome. These tests, performed early in pregnancy, are:
Blood taken from the pregnant woman between 9 and 13 weeks that measures a hormone and a protein in the woman’s blood.
An ultrasound scan (from 11 weeks to 13 weeks and 6 days) that measures the thickness behind the baby’s neck (nuchal translucency)
Combined First Trimester Screening identifies more pregnancies with Down syndrome than using nuchal translucency alone. It detects 85 to 90% of Down syndrome pregnancies; about 10 to 15% are missed. Most fetuses identified as being at “increased risk” for Down syndrome will NOT have Down syndrome.
Second Trimester Maternal Serum Screening
Blood is taken from the pregnant woman between 15 and 17 weeks gestation. The test measures the levels of three or four hormones in the woman’s blood and is combined with the woman’s age, weight and weeks of pregnancy. Pregnancy dates must be accurate.
Maternal Serum Screening test can detect about 75 to 80 % of Down syndrome pregnancies; about 20 to 25% are missed.
Second Trimester Ultrasound
This is an ultrasound examination that is performed between 18 and 20 weeks of pregnancy. Many fetal physical abnormalities can be detected using this ultrasound examination.
Risk factors for chromosomal abnormalities can also be detected on this ultrasound. Sometimes called “soft markers”, these are findings on the ultrasound that do not give definite information but may indicate the increased risk of a problem. A diagnostic test is needed to exclude a chromosomal abnormality.
This ultrasound is not a reliable screening test for Down syndrome.
Understanding Screening Results
The waiting time for screening tests usually takes up to one week
The results of screening tests do not give “no” or “yes” answers. The risk results use
- Words like “Low risk” or “increased risk”
- Numbers like “1 in 1,000” or “1 in 200”
Risks are based on a cut-off level to determine whether or not further investigation with a diagnostic test is indicated. The risk cut-off level for Down syndrome is 1 in 250, or 1 in 300, depending on the test.
If a test result is a “low” risk, it does not mean NO risk, but it does mean that diagnostic testing is less likely to be indicated.
In order for screening tests to have the highest detection rate possible, the laboratory sets a cut-off level that makes about one in 20 (5%) of all pregnancies tested fall into the “increased risk” category for Down syndrome. The majority of these results will be in pregnancies that are not affected with Down syndrome, but a diagnostic test would be needed to find this out in pregnancy.
Of the pregnancies identified as at “increased risk” for Down syndrome, only two in 100 (2%) will be affected.
For all risk results, the numbers are telling you about the chance of having a fetal abnormality as well as the chance of NOT having a fetal abnormality. For example, if a risk result is 1 in 100, this also means a chance of 99 out of 100 the pregnancy will NOT be affected.
If the results of the screening test estimate “Low risk for Down syndrome” then no further testing is indicated.
If an “increased risk” result is received, a diagnostic test is offered to provide reassurance that the fetus does not have a chromosomal abnormality or to confirm that the fetus is affected. It is important to remember that it is more likely that the fetus will NOT have a chromosomal abnormality.
Two diagnostic procedures are available: chorionic villus sampling and amniocentesis. Both procedures confirm whether there is or is not a chromosomal abnormality. These procedures have different miscarriage risks and are performed at different stages of pregnancy.
Chorionic Villus Sampling (CVS)
This procedure involves the insertion of a fine needle through the woman’s abdomen. Under the guidance of an ultrasound, cells from the developing placenta are obtained and sent to the laboratory to be grown and analysed. It can be performed through the vagina, but only if the position of the placenta prevents an abdominal approach.
CVS is the earlier diagnostic procedure, available from 11 to 13 weeks of pregnancy. CVS can give a definite diagnosis of chromosomal abnormalities.
CVS has a 1% to 3% risk of miscarriage, in addition to the risk of miscarriage for all pregnancies.
Uncommonly, another test may be required if there were difficulties with growing the cells or with analysing the results.
CVS is available to women whose pregnancy is at an “increased risk” of a chromosomal abnormality (such as Down syndrome), on the basis of screening test results or their age.
Amniocentesis involves the insertion of a fine needle through the woman’s abdomen. Under the guidance of ultrasound, a small sample of the fluid from around the fetus is obtained and sent to the laboratory for the fetal cells to be grown and analysed.
This is the later diagnostic procedure and is done from 15 to 19 weeks gestation. It can give a definite diagnosis of chromosomal abnormalities.
Amniocentesis has a 0.5% to 1% risk of miscarriage, in addition to the risk of miscarriage for all pregnancies
Uncommonly, another amniocentesis may be required due to difficulties with growing the cells.
Amniocentesis is available to women whose pregnancy is identified as being at an “increased risk” of a chromosomal abnormality (such as Down syndrome), on the basis of screening test results or their age.
The final results of a CVS or an amniocentesis can take up to two weeks. In some cases a preliminary short-term result can be made available in 24 to 48 hours (FISH test).
If no abnormalities are detected then no further testing for chromosomal abnormalities is required.
If a chromosomal abnormality is confirmed, a woman may want to continue with the pregnancy or to terminate the pregnancy. Your decision will be respected and supported.
If the pregnancy is terminated, the method used will depend on the woman’s stage of pregnancy. Early in pregnancy, the procedures used are called a Dilatation and Curettage (D & C) or a Dilatation and Evacuation (D & E). The procedure is performed under anaesthesia, and the woman usually goes home the same day. Later in pregnancy the procedure used involves labour being induced. It may take 1 to 3 days. Different types of pain relief are used according to individual needs. Following a termination, follow-up care is important to assist in the woman’s physical and emotional recovery.
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