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Maritime Medical Genetics Service
Prenatal Tests
The following information is to review prenatal screening and testing options available to you during your pregnancy. Remember, the choice of whether to have any of these tests is up to you. We encourage you to share this information with your partner or another individual that is involved in the pregnancy. The role of your health care provider is to give you information and offer you support in any decisions you make. At no time should you feel pressured to have a test that you do not want.
Before discussing options available during your pregnancy, it is important to review some basic concepts:
Chromosomes are units of inheritance found inside the cells in our bodies. They contain all of the genetic information and act as a blueprint for how an individual will grow and develop. An individual usually has 46 chromosomes (23 pairs) in every cell of their body. It is important that an individual have the correct number of chromosomes for proper growth and development.
A chromosome problem occurs when there is extra or missing chromosome material in an individual. This is a random event that can happen to any pregnancy and there is nothing we can do prior to pregnancy to prevent this.
Age-related risk, or background risk, is the concept that every woman has a chance of having a pregnancy with a chromosome problem based on their age. The risk for having an abnormal number of chromosomes in a pregnancy increases for women who are older. Therefore, the older the woman, the higher the risk.
Prenatal screening and diagnostic testing
1. Screening versus diagnostic
Screening tests provide an estimate of the likelihood that a baby may have certain type of problems. Screening tests may modify a woman’s “age-related” risk of having a child with a chromosome abnormality. This risk may increase, decrease or stay the same. All pregnant women are eligible for screening tests. Screening tests involve testing the mom’s blood, doing ultrasounds, or a combination of both. There are no risks to the pregnancy if a woman has a screening test. Although screening tests cannot definitively rule our or confirm a chromosomal abnormality, they can change the risk that would be predicted by age alone. The types of screening tests that are available differ depending on whether a pregnant woman is considered to be at ‘high risk’ for an abnormality based on age, past history, and other factors. Screening options will be reviewed for women in the general population as well as women considered at “increased risk”.
Diagnostic tests provide a definite “yes” or “no” answer about whether or not a baby has a specific condition. This can include chromosome differences, such as Down syndrome, or other inherited genetic conditions when indicated. There are two diagnostic tests: chorionic villus sampling (CVS) and amniocentesis. Both carry a small risk of causing a miscarriage. Which test is offered or chosen depends on many factors including: how far along the pregnancy is, the condition in question, the procedure-related risk and the preference of the woman. Diagnostic testing is offered to women who are considered at increased risk based on personal factors and screening results.
Having prenatal screening or diagnostic testing is your choice. Some individuals may decide to not have any testing at all. However, they must realize that if their baby does have an abnormality, it will not be detected until after the baby is born.
2. Prenatal screening
Screening options for pregnant women consist of biochemistry (which is done through a blood test in the mother), ultrasounds, or a combination of both.
Biochemical testing:
This is a blood test that is offered to all pregnant women. It measures the levels of substances that are produced by the pregnancy. Several factors, including how far along the pregnancy is, the levels of the substances, and the age of the pregnant woman, are used to calculate the chance that the baby will have certain genetic conditions such as Down syndrome (a condition that affects individuals’ mental and physical development) and Trisomy 18 (a severe condition where babies are stillborn or die shortly after birth).
Occasionally an increased chance for other chromosome problems may be detected by biochemical screening. It is important to remember that this test does not screen for all chromosome differences or genetic conditions. This screening test is offered between 10-13 weeks gestation and/or at 16-18 weeks gestation. Many women will have a second biochemical test combined with the early biochemical test (and nuchal translucency results if this was done; see below) to give one final risk figure, which will be reviewed with you. Check with your health care provider ordering your biochemistry to confirm how your results will be given to you.
If this test identifies a pregnancy that is at increased risk for one of the conditions listed above, you will be offered an appointment for counselling and a detailed ultrasound. Remember, an increased risk does not mean there is a problem with the baby. The test is a guide to identify “at-risk” pregnancies, most of which are in fact normal. For most pregnant women, the biochemistry will be reassuring and actually show a decrease in the chance that the baby has one of these conditions.
Ultrasound:
An ultrasound is used to obtain an image of the baby inside the mother’s womb. It is safe for both the baby and the mother. All pregnant women are offered a detailed ultrasound, which is discussed in the next section. However, some women who are considered ‘at risk’ may also be offered an earlier ultrasound between 11-13 weeks of the pregnancy. During this early ultrasound, a nuchal translucency is obtained. This is a measurement of a small fluid layer at the back of the baby’s neck. This measurement is used along with other information such as maternal age, to calculate a likelihood of a chromosome problem.
Detailed Ultrasound
An ultrasound is offered to all pregnant women usually between 18-21 weeks of a pregnancy. Most women will have their ultrasound performed at their local hospital. This ultrasound looks at the anatomy of the developing baby (heart, kidneys, head, spine, etc) and measurements of the baby are taken to confirm the gestational age of the baby and that the baby is growing properly. It is important to note that while an ultrasound is a special time to see the developing baby, the ultrasound is both a screening and diagnostic test for certain conditions.
“Soft markers” are ultrasound features that are seen more commonly in babies that have chromosome problems but they are also seen in normal healthy babies. Soft markers are not considered a physical abnormality. The finding of soft markers does not mean that the baby actually has a chromosome problem, only that the risk is increased. The finding of a soft marker, your biochemical screening result, your age, and possibly other factors, are combined to determine the overall chance for your baby to have certain problems. If the overall risk is increased above a certain level, further options for investigations are discussed with the couple.
Check with your health care provider to determine if you are considered to be at increased risk. There are several screening and diagnostic options for women considered at increased risk that are different than for women in the general population.
Diagnostic Options:
Women who are considered at increased risk may decide to have diagnostic testing. Whether or not to have a diagnostic test is a personal decision.
Diagnostic tests include:
Chorionic Villi Sampling (CVS)
This test involves the removal of a small sample of placental tissue, chorionic villi, which contain cells that are usually the same as the cells in the baby. It is usually performed between 11-13 weeks of pregnancy. Ultrasound is used before and during the procedure to enable the doctor to locate the baby and the placenta. Depending on factors such as the location of the placenta and the baby, this procedure may be done either by inserting a needle through the mother’s abdomen (like an amniocentesis) or by small biopsy forceps along the cervix (set up similar to a PAP smear). CVS can be used to detect a chromosome abnormality. In some circumstances it may also be used to detect other genetic conditions that have previously been identified in a family.
Women who have CVS have an additional 1/100 or 1% (procedure-related) risk to have a miscarriage above the baseline risk at that gestational age. Chromosome results are usually available in 2-3 weeks. For other genetic conditions the results can sometimes take a bit longer. Check with your health care provider to ask how your results will be given to you.
2.Amniocentesis
This test involves the removal of a small sample of amniotic fluid (liquid that surrounds the baby) from the uterus. Within the amniotic fluid are cells from the baby. An amniocentesis is usually done between 16-20 weeks of the pregnancy. Ultrasound is used before and during the procedure and enables the physician to locate the baby, placenta and a pocket of amniotic fluid. A needle is guided through the mother’s abdomen (not the belly button) and into the uterus. A small sample of fluid is withdrawn. An amniocentesis can detect a chromosome abnormality. It may also be able to detect other genetic conditions for individuals whose baby has a higher risk.
The risk of miscarriage for any woman at this stage of pregnancy is about 2-3%. Women who have an amniocentesis have an additional 1/200 or 0.5% (procedure related) risk of miscarriage. It usually takes 2-3 weeks to obtain the results of chromosome testing. Results for other genetic conditions may take longer. Check with your health care provider to ask how your results will be communicated to you.
3. What does this mean for you?
For any couple embarking on a pregnancy, there is a 2-3% chance of having a child born with a problem. There is nothing you, or your physicians can do to completely remove this risk. However, it is possible to decrease the risk of having a child born with an open neural tube defect by 70% by taking at least 0.4 mg of folic acid once daily, for the month before becoming pregnant and through the first 13 weeks of the pregnancy. As pregnancies can be difficult to predict, we recommend that all women in their childbearing ages take folic acid.
Remember that screening and diagnostic tests cannot rule out all possible inherited conditions, health complications or developmental problems in a baby.
Ask your doctor about prenatal screening options early. All decisions concerning screening and/or diagnostic tests are personal. Your health care provider can give you information about the various options that are available to you but should not decide for you.
You can contact the Maritime Coordinator for Prenatal Screening and Diagnosis at 902-470-8321 or claire.blight@iwk.nshealth.ca if you have any questions about the information found here.
