Overview Prenatal screening is offered to all pregnant women. Prenatal screening may include ultrasound and blood testing in order to determine if the pregnancy is at a higher risk for a particular birth defect. If a screening test is positive, then the patient may elect to pursue diagnostic testing. [top]
Genetic Counseling Genetic counseling is offered to patients who are considering prenatal screening. The counselor reviews the basis of population screening, but also conducts an interview of the family history to determine if the pregnancy may be at increased risk for a specific birth defect or genetic condition. This process helps the couple decide which, if any, prenatal tests they wish to pursue.
Prenatal Screening A variety of tests can be used to detect birth defects, including Down syndrome (trisomy 21) and trisomy 18. Down syndrome and trisomy 18 are chromosome abnormalities that cause birth defects and mental retardation. In any pregnancy there is a small chance that the baby has one of these abnormalities. This risk increases as the woman gets older and women over 35 are usually offered amniocentesis, which is the test that can tell definitively whether the baby has a chromosome abnormality.
Women under 35 usually do not have an amniocentesis because the chance that the baby has a chromosome abnormality is lower. These women are offered a screening test to determine if they should be offered amniocentesis. The screening test does not tell for sure if the baby has a chromosome abnormality or not. It is used to identify pregnancies that are at high risk [and therefore, may benefit from diagnostic testing.] for Down syndrome or trisomy 18. Screening for open neural tube defects is also part of routine pregnancy care, but the chance of a neural tube defect does not change with age as does the risk of chromosome abnormalities.
No screening evaluation can detect all fetal concerns, i.e. they miss approximately 40% of all chromosome abnormalities as well as other birth defects.
Who Should Be Screened Any woman can have screening. Maternal serum screening is intended for low risk pregnancies. However, in high risk patients such as those 35 years or older, or those who have a family history of a chromosome disorder or spinal defect, it is recommended that they consider more specific prenatal diagnostic tests such as CVS, early amniocentesis, or standard amniocentesis. Screening may not pick up chromosome abnormalities other than Down syndrome and trisomy 18. Approximately 40% of all chromosome abnormalities are different diagnoses. Therefore, screening in the high-risk group can give false reassurance if the tests are negative. It is important to understand that no screening program is diagnostic. Both the American College of Obstetricians and Gynecologists and the American College of Medical Geneticists recommend that women 35 years of age and over consider amniocentesis and not rely solely on screening programs. [top]
First Trimester Screening A screening test is not a definitive test; it is meant to measure the risk for Down syndrome or trisomy 18 so that women with a high risk can be offered amniocentesis. Most high-risk pregnancies turn out to be normal.
First trimester screening, performed between 11.5 and 13.5 weeks of pregnancy involves two parts; a specialized ultrasound scan (a nuchal translucency scan) and a blood test. The tests are not harmful. The combination of nuchal translucency measurement and the first trimester blood test detects about 85% of fetuses with Down syndrome or trisomy 18. Although the nuchal translucency measurement would be available at the time of ultrasound, results of first trimester screening are available approximately one week after the appointment.
First trimester screening does not address the risk for neural tube defects.
To detect open neural tube defects (i.e. spinal defects) in a low risk population, it is still necessary to do AFP (alpha fetoprotein) testing in the second trimester (between 15 to 20 weeks). Again this is a screening test and will not guarantee that the fetus does not have a neural tube defect. However, a combination of the AFP test and a detailed ultrasound scan will detect most spinal defects.
I-Nuchal Translucency Scan
A nuchal translucency scan measures the fluid filled area at the back of the neck between 11.5 to 13.5 weeks. The measurement can be used to calculate the chance that the fetus has Down syndrome or trisomy 18.
One of the pioneers in developing standards for this measurement is Dr. Nicolaides of King's College Hospital in London. His group has data on over 100,000 patients, giving a detection rate of approximately 80% for Down syndrome and even higher for trisomy 18. To ensure consistent interpretation, King's College Hospital has developed a computer program to calculate risk. The program is only available to certified centers, such as ours.
II-Maternal Protein Measurement First trimester biochemical measurement of free beta hCG (human chorionic gonadotrophin) and PAPP-A (pregnancy associated plasma protein A) maximizes the detection of Down syndrome and trisomy 18 fetuses. The blood sample is collected from a fingertip.
If the early screen is positive, this does not mean that the fetus has a chromosome problem. Approximately 5-7% of all patients having these tests receive an increased risk and will be offered further testing such as early (earlier than 14 weeks) or standard amniocentesis. Of those who have a positive combined test, approximately 1 in 20 patients will have a chromosome abnormality. Of the fetuses with normal chromosomes and slightly increased nuchal translucency, most (80-90%) will be healthy at birth.
Our office notifies patients directly if the result is positive, but also informs the referring obstetrician. Patients may elect to return for further consultation which would include ultrasound, but may also include diagnostic amniocentesis.
Patients who have an increased nuchal translucency and normal amniocentesis results are advised to have a level II ultrasound scan which includes a focused heart scan at about 20 to 22 weeks. An increased nuchal translucency increases the risk of a heart defect to 1 in 70. Other birth defects including mild hydrocephalus, clubfoot, and diaphragmatic hernia have also been associated with an increased nuchal translucency. These can be discussed in detail with the genetic counselor.
The majority of patients with an abnormal serum screen do have
a normal fetus.
Patients with abnormal values are offered genetic counseling,
ultrasound and amniocentesis. These tests can usually determine
whether or not the fetus has an open neural tube defect or a
Because the "Miss-rate" is high for high-risk
patients, i.e. those with a family history of spinal defects
or patients who will be
35 or older at the time of birth, amniocentesis is considered
of First Trimester Screening The first trimester screen results in a high detection rate (85%) for Down syndrome and trisomy 18. This detection rate is better than second trimester triple marker screening (MSAFP Triple Screen), especially for women under 35.
Early screening provides the option for early testing.
A nuchal translucency test is the only screening test for possible
fetal heart defects.
The screening does not present a risk to the fetus.
Second Trimester Maternal Serum Screening
The MSAFP blood test is called the Expanded AFP screen, is performed at 15 to 20 weeks and measures three substances; AFP, hCG, and estriol. This is a blood test that the patient's obstetrician orders.
This test is used to detect pregnancies with an increased risk for Down syndrome, trisomy 18, or spinal defects.
We are a California State approved prenatal diagnostic center for the follow up of abnormal maternal serum screening. If your doctor's office receives notification from the State that your blood test is positive, you are referred for genetic consultation, ultrasound and consideration of amniocentesis.
Positive Second Trimester Screen Results
The majority of patients with an abnormal serum screen do have a normal fetus.
Patients with abnormal values are offered genetic counseling, ultrasound and amniocentesis. These tests can usually determine whether or not the fetus has an open neural tube defect or a chromosome abnormality.
Because the "Miss-rate" is high for high-risk
patients, i.e. those with a family history of spinal defects or patients
who will be 35 or
older at the time of birth, amniocentesis is considered a much more definitive
Fetal Anatomy Ultrasound Ultrasound is the use of sound waves to make pictures of the fetus. It is used before and during amniocentesis and CVS to assist the doctor in performing those procedures. We also evaluate the fetal anatomy; however, in at least 30% of pregnancies, not all organ systems can be clearly seen at the time of the amniocentesis. The best time to perform a detailed anatomical evaluation of the fetus is around 20 weeks, which is three to five weeks after the usual time for amniocentesis. Typically, the purpose of ultrasound is to evaluate the brain and heart anatomy movements and fetal face.
Physical birth defects such as neural tube defects (spina bifida), heart defects, cleft lip and club feet can be diagnosed by ultrasound; however, chromosome abnormalities may be suspected and only diagnosed by CVS or amniocentesis.
Approximately 3-4% of babies have a birth defect, which can range from
mild to severe. Ultrasound can potentially detect 30-50% of these birth defects
if the organs can be seen.
The majority of all birth defects occur with no family history, i.e. in
low risk pregnancies.
In some high-risk situations, early evaluation may be indicated, with a
repeat ultrasound at a later gestational age. Your genetic counselor or Dr.
McCallum or Dr. Liner can advise you on timing.
The best time for cardiac evaluation is 20-22 weeks. [top]
This is one of the most frequent disorders in Caucasians and can be screened for with a simple blood test. Each parent must carry a cystic fibrosis gene mutation before the fetus can be affected. The disease causes chronic lung problems and significantly reduces life expectancy. The American College of Obstetrics and Gynecology suggests that screening test be offered to all pregnant couples.
Fragile X Syndrome
Fragile X syndrome is the most common inherited cause of mental retardation. If you have a relative with fragile X syndrome, or simply a family history of mental retardation, you may consider screening.
Thalassemia is an inherited anemia (blood disease). It is seen in Asians, Asian Indians, Africans, and people of Mediterranean descent. Patients seen for counseling are routinely screened to see if they are carriers of thalassemia.
Ashkenazi Jewish Ancestry
A panel of tests is offered to patients who are Ashkenazi Jewish. The standard panel includes Tay-Sachs disease, Canavan disease and cystic fibrosis. The American College of Medical Genetics (ACMG) recommends testing for at-risk couples. Testing for familial dysautonomia has recently been made more commercially available, and the carrier risk is similar to Tay-Sachs. The recommendation for testing is anticipated from the ACMG in 2005. [top]