Most babies are born healthy but there is always a risk of that something can go wrong.
The risk will be greater for some couples than others depending on age and lifestyle issues and medical history.
One of the most common genetic problems that a baby can have is a genetic condition called Down syndrome. While women are generally advised to have genetic testing, the decision to do so is yours. It’s a good idea to think about why you want to be tested and the consequences for you and your family if you learn that your baby does have a genetic disorder.
You are more at risk of having a baby with a genetic disorder if:
- you are 37 years of age and over
- you or your partner have previously had a baby with a genetic problem.
Screening tests are best done in the first 16 weeks of pregnancy and cannot be done after 19 weeks.
There are two kinds of tests that can be done in pregnancy.
- Screening tests can tell you if you are at risk of having a baby with birth defects. These tests will not give you definite information about your fetus.
- Diagnostic tests can tell you if the fetus has a defect.
Women can choose whether or not to have tests to find out their risk of having a baby with a birth defect. Some hospitals have genetic counselling services who can discuss with you the implications of having testing done and what it might mean to you.
Some of these tests need to be done in early pregnancy, if you are a public patient you may need to organise testing them with your doctor (GP).
Screening tests
First trimester combined screening test
This test combines the results of a blood test taken at around 10-12 weeks and an ultrasound at 11-13 weeks. The test will show the risk or your chance of having a baby with Down syndrome or Trisomy 18. It will not tell you if your baby has Down syndrome.
If you are at increased risk you will be offered a diagnostic test, either a CVS (Chronic Villus Sampling) or amniocentesis.
Maternal serum screening
This is a blood test collected between 15-20 weeks of pregnancy. The test shows your risk of having a baby with Down syndrome, Trisomy 18 or neural tube defects such as spina bifida. If the test shows you are at an increased risk you will be offered amniocentesis and ultrasound.
Non-invasive prenatal test (NIPT)
This blood test is done after week 10 of pregnancy. It screens for Down Syndrome and certain other chromosomal irregularities in a baby. In Australia it is only available in some specialist centres.
Diagnostic tests
A diagnostic test is testing the fetus’s genetic material and can therefore tell whether the fetus actually has a genetic disorder.
Chorionic Villus Sampling (CVS) (11 to 12 weeks)
In this test a small sample is taken from the placenta. The sample from the placenta can be tested for Down syndrome or in some cases other genetic conditions such as cystic fibrosis. One woman in one hundred (1%) will have a miscarriage as a result of this test.
Amniocentesis (15 to 18 weeks)
A sample of the amniotic fluid that surrounds the baby is collected and can be used to diagnose Down syndrome or some other genetic conditions. Amniocentesis has a one in two hundred risk of causing a miscarriage.
Ultrasound scans (18 to 20 weeks)
This second trimester scan is used to identify physical and structural abnormalities including spina bifida, heart and limb defects.
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Overview
First trimester screening is a prenatal test that offers early information about a baby's risk of certain chromosomal conditions, specifically, Down syndrome (trisomy 21) and extra sequences of chromosome 18 (trisomy 18).
First trimester screening, also called the first trimester combined test, has two steps:
- A blood test to measure levels of two pregnancy-specific substances in the mother's blood — pregnancy-associated plasma protein-A (PAPP-A) and human chorionic gonadotropin (HCG)
- An ultrasound exam to measure the size of the clear space in the tissue at the back of the baby's neck (nuchal translucency)
Typically, first trimester screening is done between weeks 11 and 14 of pregnancy.
Using your age and the results of the blood test and the ultrasound, your health care provider can gauge your risk of carrying a baby with Down syndrome or trisomy 18.
If results show that your risk level is moderate or high, you might choose to follow first trimester screening with another test that's more definitive.
Why it's done
First trimester screening is done to evaluate your risk of carrying a baby with Down syndrome. The test also provides information about the risk of trisomy 18.
Down syndrome causes lifelong impairments in mental and social development, as well as various physical concerns. Trisomy 18 causes more severe delays and is often fatal by age 1.
First trimester screening doesn't evaluate the risk of neural tube defects, such as spina bifida.
Because first trimester screening can be done earlier than most other prenatal screening tests, you'll have the results early in your pregnancy. This will give you more time to make decisions about further diagnostic tests, the course of the pregnancy, medical treatment and management during and after delivery. If your baby has a higher risk of Down syndrome, you'll also have more time to prepare for the possibility of caring for a child who has special needs.
Other screening tests can be done later in pregnancy. An example is the quad screen, a blood test that's typically done between weeks 15 and 20 of pregnancy. The quad screen can evaluate your risk of carrying a baby with Down syndrome or trisomy 18, as well as neural tube defects, such as spina bifida. Some health care providers choose to combine the results of first trimester screening with the quad screen. This is called integrated screening. This can improve the detection rate of Down syndrome.
First trimester screening is optional. Test results indicate only whether you have an increased risk of carrying a baby with Down syndrome or trisomy 18, not whether your baby actually has one of these conditions.
Before the screening, think about what the results will mean to you. Consider whether the screening will be worth any anxiety it might cause, or whether you'll manage your pregnancy differently depending on the results. You might also consider what level of risk would be enough for you to choose a more invasive follow-up test.
Risks
First trimester screening is a routine prenatal screening test. The screening poses no risk of miscarriage or other pregnancy complications.
How you prepare
You don't need to do anything special to prepare for first trimester screening. You can eat and drink normally before both the blood test and the ultrasound exam.
What you can expect
First trimester screening includes a blood draw and an ultrasound exam.
During the blood test, a member of your health care team takes a sample of blood by inserting a needle into a vein in your arm. The blood sample is sent to a lab for analysis. You can return to your usual activities immediately.
For the ultrasound exam, you'll lie on your back on an exam table. Your health care provider or an ultrasound technician will place a transducer — a small plastic device that sends and receives sound waves — over your abdomen. The reflected sound waves will be digitally converted into images on a monitor. Your health care provider or the technician will use these images to measure the size of the clear space in the tissue at the back of your baby's neck.
The ultrasound doesn't hurt, and you can return to your usual activities immediately.
Results
Your health care provider will use your age and the results of the blood test and ultrasound exam to gauge your risk of carrying a baby with Down syndrome or trisomy 18. Other factors — such as a prior Down syndrome pregnancy — also might affect your risk.
First trimester screening results are given as positive or negative and also as a probability, such as a 1 in 250 risk of carrying a baby with Down syndrome.
First trimester screening correctly identifies about 85 percent of women who are carrying a baby with Down syndrome. About 5 percent of women have a false-positive result, meaning that the test result is positive but the baby doesn't actually have Down syndrome.
When you consider your test results, remember that first trimester screening indicates only your overall risk of carrying a baby with Down syndrome or trisomy 18. A low-risk result doesn't guarantee that your baby won't have one of these conditions. Likewise, a high-risk result doesn't guarantee that your baby will be born with one of these conditions.
If you have a positive test result, your health care provider and a genetics professional will discuss your options, including additional testing. For example:
- Prenatal cell-free DNA (cfDNA) screening. This is a sophisticated blood test that examines fetal DNA in the maternal bloodstream to determine whether your baby is at risk of Down syndrome, extra sequences of chromosome 13 (trisomy 13) or extra sequences of chromosome 18 (trisomy 18). Some forms of cfDNA screening also screen for other chromosome problems and provide information about fetal sex. A normal result might eliminate the need for a more invasive prenatal diagnostic test.
- Chorionic villus sampling (CVS). CVS can be used to diagnose chromosomal conditions, such as Down syndrome. During CVS, which is usually done during the first trimester, a sample of tissue from the placenta is removed for testing. CVS poses a small risk of miscarriage.
- Amniocentesis. Amniocentesis can be used to diagnose both chromosomal conditions, such as Down syndrome, and neural tube defects, such as spina bifida. During amniocentesis, which is usually done during the second trimester, a sample of amniotic fluid is removed from the uterus for testing. Like CVS, amniocentesis poses a small risk of miscarriage.
Your health care provider or a genetic counselor will help you understand your test results and what the results mean for your pregnancy.
Nov. 17, 2020