Genetic (Prenatal) Screening
We will discuss the following topics in this section:
Chorionic Vilus Ssmpling (CVS)
Cystic Fibrosis Carrier Testing
(also available as a PDF)
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Moral Considerations
Our mission is to provide you with excellent medical care. In keeping with this mission, the information below explains your prenatal testing options. While prenatal screening and diagnostic tests are available for all of our patients, please know that they are not 100% accurate. We caution you that the results may suppress the joy of your pregnancy, and/or lead to unnecessary invasive procedures. While these tests can offer useful information, they are sometimes used to justify terminating a pregnancy. As you know, Tepeyac Family Center is dedicated to the sanctity and dignity of all human life, and does not believe in or perform abortions. As you read this information and decide whether or not to pursue testing, please know that Tepeyac Family Center will walk you through this process in the context of respect for you and your baby, and sound medical advice. Feel free to discuss these options with your provider(s).
Prenatal screening is done to assess whether or not your baby is at increased risk of being born with a genetic abnormality or neural tube defect. Diagnostic testing is performed if a screening test comes back positive. Some families find this information very useful, while others are happy to wait for this information to arrive with the baby at birth. At Tepeyac Family Center, we would like to offer you what you need, and so present this information so that you can decide what is best for your family. Each test has its strengths and limitations as well as a time frame when it can be performed. The following sections give you information on each test.
First Trimester Screen
What does the screening look for?
The first trimester screen looks for genetic disorders called trisomies, the most common of which is Trisomy 21, or Down syndrome. Two other trisomies are 18 and 13. Trisomies 18 and 13 are conditions that cause significant birth defects, including severe mental retardation, as well as health problems involving nearly every organ system in the baby's body. Unfortunately, the baby diagnosed with either of these two trisomies usually dies in infancy.
How common are trisomies?
The incidence of Down syndrome is estimated at 1/1000 births for women under 30 years of age, 1/400 for women who are 35 years old, 1/100 for 40 year old women, and 1/30 for women at age 45. Trisomies 18 and 13 are much rarer with incidence rates of 1/3000 and 1/5000 live births, respectively.
How is the screening performed?
The screening test consists of blood work and an ultrasound. The blood work measures the concentration of two proteins in the mother's blood, the free beta HCG and PAPP-A. The ultrasound measures the thickness of the back of the baby's neck, or nuchal translucency (NT). The thicker the NT is, the greater the risk of significant chromosomal abnormalities. It also notes the presence or absence of the nasal bone.
When is the screening performed?
The first trimester screen takes place between 11-13 weeks of gestation.
How accurate are the results?
The first trimester screen has an accuracy rate of 95% and a false positive rate of 2% for detecting Down syndrome and 97% for detecting Trisomy 18 and Trisomy 13. (A false positive rate means that the test indicates a problem when the baby is actually fine.)
How would the screening results affect my obstetric care?
Should your screening come back positive, you may be advised to have a chorionic villus sampling or amniocentesis performed. Should those results come back confirming the abnormality, you may be referred to a specialist, delivered at a tertiary care center, or be enrolled in our perinatal hospice program.
Does the First Trimester Screen detect open neural tube defects (ONTDs)?
No. The alpha-fetoprotein and a level II sonogram can identify ONTDs.
Where is the screening performed?
A number of specialty centers in the area perform the first trimester screen.
Go to top…Alpha-fetoprotein (AFP)
What does AFP look for?
Alpha-fetoprotein looks for open neural tube defects (ONTDs). An ONTD is a condition, such as spina bifida or anencephaly, where the baby’s spine or skull does not close completely during fetal development resulting in severe impairment, disability, or even death.
How is the screening performed?
AFP is tested through a sample of maternal blood. AFP is elevated in women carrying babies with an ONTD.
When is the screening performed?
AFP is tested between 16-20 wks of gestation.
How accurate are the results?
The second trimester screen has an accuracy rate of 98% for detecting ONTDs.
How would the screening results affect my obstetric care?
Should there be a high suspicion of an ONTD, you may be asked to consult specialists for immediate treatment after birth, you may be delivered at a tertiary care center, or enrolled in our perinatal hospice program.
Go to top…Quad Screen
What does the screening look for?
The Quad screen looks for genetic disorders called trisomies, the most common of which is Trisomy 21, or Down syndrome. Another is trisomy 18. Trisomy 18 is a condition that causes significant birth defects, including severe mental retardation, as well as health problems involving nearly every organ system in the baby’s body. Unfortunately, the baby diagnosed with this trisomy usually dies in infancy. The screening test also looks for open neural tube defects (ONTDs). An ONTD is a condition, such as spina bifida or anencephaly, where the baby’s spine or skull does not close completely during fetal development resulting in severe impairment, disability, or even death.
How common are trisomies and ONTDs?
The incidence of Down syndrome is estimated at 1/1000 births for women under 30 years of age, 1/400 for women who are 35 years old, 1/100 for 40 year old women, and 1/30 for women at age 45. Trisomy 18 is much rarer with incidence rates of 1/3000 live births. The incidence of ONTDS is 1/500-1000 live births.
How is the screening performed?
The screening test requires blood to be drawn from the mother. It examines four levels in the mother's blood; alpha-fetoprotein (AFP), human chorionic gonadotropin (hCG), estriol, and inhibin-A. Together with factors which include maternal age and ethnicity, an assessment of the probability that the baby has a potential genetic disorder, is made.
When is the screening performed?
The quad screen is performed between 16-20 weeks of gestation.
How accurate are the results?
The quad screen detects 85% of babies with spina bifida, 70% of babies with Trisomy 18, and 70% of babies with Down syndrome. There is a false positive rate of 5%. (A false positive rate means that the test indicates a problem when the baby is actually fine.)
How would the screening results affect my obstetric care?
Should your screening come back positive, you would be offered an amniocentesis to confirm the screening results. Should those results come back confirming the abnormality, you may need to undergo further testing, be delivered at a tertiary care center, or be enrolled in our perinatal hospice program.
Where would the screening take place?
The blood for the quad screen can be drawn here at Tepeyac Family Center.
Go to top…Chorionic Villus Sampling (CVS)
What is CVS?
CVS is a diagnostic test performed to detect genetic disorders between 10-12 weeks of gestation. It is done as a follow-up to an abnormal first trimester screen and based on maternal risk factors. CVS is not able to detect open neural tube defects.
How is CVS performed?
Guided by ultrasound, the doctor places either a needle through your abdomen or a thin catheter through your vagina to take a placental biopsy for evaluation.
Are there risks or contraindications to having the procedure performed?
Miscarriage is the primary risk of CVS, occurring in 1 out of every 100 procedures. CVS is not recommended for women who have an active infection (i.e. STD), those pregnant with twins, or those who have experienced vaginal bleeding in their pregnancy. Additionally, some studies suggest that there is a small risk of the CVS causing defects in the fingers and toes of developing babies, however other studies indicate that this increased risk may occur primarily when the CVS is done earlier, i.e. before 10 weeks.
What are the side effects of the procedure?
Bleeding, cramping and pain at the puncture site, leaking amniotic fluid from the vagina, and infection may occur after CVS.
How accurate are the results?
The accuracy of chorionic villus sampling is 98-99%.
How would the results of the CVS affect my obstetric care?
If the test shows that your baby has a disorder, your general obstetric care will likely remain unchanged. However, depending on the nature of the diagnosis, you may be asked to undergo an amniocentesis, referred to a specialist and/or enrolled in our perinatal hospice program.
Where would the procedure take place and who would perform it?
CVS is performed at a number of specialty centers in the area.
Go to top…Amniocentesis
What is amniocentesis?
Amniocentesis is a diagnostic test performed to detect genetic disorders and open neural tube defects in pregnancy. It is performed as a follow-up to an abnormal screening test and based upon maternal risk factors.
How is amniocentesis performed?
Guided by ultrasound, the doctor places a needle through your abdomen into the amniotic sac. A small amount of amniotic fluid is withdrawn and sent to the laboratory for analysis.
When is the test performed?
Amniocentesis is performed between 16-20 weeks of gestation.
Are there risks to having the procedure performed?
Miscarriage is the primary risk of amniocentesis, occurring in 1 out of every 500 procedures. (Some studies suggest much lower risk). Another risk is that your water may break prematurely which could lead to infection or premature labor. (Leaking of amniotic fluid is not uncommon after an amniocentesis, but it usually is only a small amount that stops by itself after a week.) If your water did break, you would be monitored carefully for the rest of your pregnancy to assess for complications. Infection in the amniotic fluid after amniocentesis is also a very rare, (1/1000 procedures), but possible occurrence. A fourth but extremely rare complication is that it is possible for the needle to come in contact with the baby. Ultrasound guidance helps ensure that this doesn’t happen by allowing the doctor to steer the needle away from the baby.
What are the possible complications of the procedure?
Cramping and pain at the puncture site and the leaking of amniotic fluid from the vagina are possible complications of amniocentesis. There is also the risk that the puncture site would not re-close, resulting in chronic leaking, infection, or even fetal death.
How accurate are the results?
The accuracy of amniocentesis for detecting abnormalities is 99%.
How would the results of the amniocentesis affect my obstetric care?
If the test shows that your baby has a disorder, your general obstetric care will likely remain unchanged. However, depending on the nature of the diagnosis, you may be referred to a specialist, delivered at a tertiary care center, or enrolled in our perinatal hospice program.
Where would the procedure take place and who would perform it?
Amniocentesis is performed here at Tepeyac Family Center by our physicians.
Go to top…Cystic Fibrosis Carrier Testing
What is Cystic Fibrosis (CF)?
Cystic Fibrosis is a genetic disorder that severely affects a person's respiratory and digestive systems. It is caused by an abnormal gene that makes the body produce thick mucous in the lungs. The mucous causes infections which can be life threatening. This mucous can also affect the pancreas causing serious problems with food absorption. It is most common in Caucasian and Jewish populations, but is also common in Latino populations. In general, people with CF have a shortened life span. Though some will die in childhood, others are living active lives into their 40s or even longer. While there is no cure for the disease, research is ongoing to discover treatments that can lengthen, and improve the quality of life of, those people affected by CF. It should be noted that treatments have improved greatly in recent years.
Should I be tested?
CF is a recessive gene disorder, which means that in order for your child to have a chance of developing the disease, both you and your husband/partner would have to be carriers of the abnormal gene. About 1/26 caucasians are carriers of CF. We offer CF carrier testing so that you can determine whether or not you are a carrier. If you have been tested once, you do not need to be tested again.
How am I tested?
A blood sample is taken to determine your carrier status.
What will the test results indicate?
The test results will indicate whether or not you are a carrier for the most common mutations of the gene. If you are not a carrier, you can feel reassured that your baby (and future babies) will likely not be born with CF. If you are a carrier, we would recommend that your husband/partner be tested. If he is not a carrier, then you can feel reassured that your baby (and future babies) will likely not be born with the disorder. If both you and your husband/partner are carriers, then there is a 25% chance that your child will be born with CF.
How will I know if my child will have CF or not?
If both you and your husband/partner are carriers, an amniocentesis can be performed to detect whether or not your child will be born with CF. This testing is optional. You may wish to wait until the baby is born to pursue additional testing.
What happens if it is determined that my child has CF?
CF cannot be treated before birth. The purpose of having this information is so that you can prepare to welcome a child with special health care needs into your family.
Go to top…