1st and 2nd Trimester Screening for Down Syndrome FAQ

A screening test is a safe, noninvasive way to learn about the risk of having a baby with certain birth defects such as Down syndrome (trisomy 21), trisomy 18 and 13, open neural tube defects and cardiac defects. A screening test is meant to identify pregnancies at high risk, not to provide a definitive diagnosis.

Amniocentesis and CVS are diagnostic tests that provide a definitive diagnosis of Down syndrome and other chromosome abnormalities. These invasive tests are associated with a risk of complications to the pregnancy.

Trisomy 21 is due to an extra copy of chromosome 21 and is the most common trisomy at the time of birth. Trisomy 21 causes Down syndrome, which is associated with mild to moderate intellectual disabilities and may also lead to digestive issues and congenital heart defects. It is estimated that Down syndrome is present in one out of every 740 newborns.

The risk of having a child with Down syndrome gradually increases with the age of the mother but can occur at any age.

Trisomy 18 is due to an extra copy of chromosome 18. Trisomy 18 causes Edwards syndrome and is associated with a high rate of miscarriage. Infants born with Edwards syndrome may have various medical conditions and a shortened lifespan. It is estimated that Edwards syndrome is present in approximately one out of every 5,000 newborns.

Besides trisomy 21 and trisomy 18, there are other trisomies which are less common at birth or associated with milder medical conditions.

The neural tube, which forms early in pregnancy, eventually develops into the baby’s brain and spinal cord. If this tube does not close completely, an opening can remain along any part of the baby’s spinal column or head, resulting in an open neural tube defect, such as spina bifida or anencephaly. Open neural tube defects occur in 1 out of every 1,000 births. The risk of having an open neural tube defect does not increase with the age of the mother. Maternal health complications including uncontrolled maternal diabetes can increase risk for open neural tube defects.

70% of babies with Down syndrome are born to women under the age of 35. These tests are noninvasive and are the most sensitive way to assess the risk for Down syndrome, trisomy 18, open neural tube defects, cardiac and other abnormalities. The American College of Gynecology (ACOG) recommends that Down syndrome screening with nuchal translucency (NT) and maternal blood testing be offered to all women regardless of age.

Nuchal translucency (NT) is the fluid that accumulates at the back of the neck of the developing fetus underlying the skin. The measurement of the nuchal translucency (NT) correlates with the risk for Down syndrome, fetal cardiac defects and other fetal defects. This measurement is obtained between 11 weeks 2 days and 14 weeks 2 days.

An increased nuchal translucency can indicate an increased risk for chromosome defects, such as trisomy 21 or 18, cardiac or other defects. If the nuchal translucency (NT) is significantly thickened, further evaluation with 1st and 2nd trimester maternal blood tests, including NIPT (noninvasive prenatal testing) or consideration of prenatal diagnosis including CVS or amniocentesis.

A detailed ultrasound of the fetal heart and fetal anatomy are recommended between 19 and 22 menstrual weeks to exclude fetal cardiac and other anomalies which may be associated with a thickened NT.

Full Integrated Screening is the State of California’s screening program, which involves a nuchal translucency ultrasound and two maternal blood tests. This screen can provide you with a preliminary early report in the first trimester and the final result in the second trimester about your risk of Down syndrome, trisomy 18, and open neural tube defects. The sensitivity of Full Integrated Screening for the detection of Down syndrome is 90%. This test requires an ultrasound (between 11 and 13 weeks 6 days) with special measurements of the nuchal translucency (NT),and a blood test performed in the first trimester (between 10 weeks and 13 weeks 6 days), combined with another blood sample obtained in the second trimester (Quad Marker Screening or Afp4) between 15 and 20 weeks.

Full Integrated Screening provides the most sensitive information from the California Prenatal State Screening Program (pdf) about the risk of Down syndrome, trisomy 18, and open neural tube defects. Based upon these results, you may decide to pursue or forgo diagnostic testing.

Full Integrated Screening involves a first trimester blood test, which needs to be obtained between 10 and 13 weeks 6 days of pregnancy, and a nuchal translucency (NT) ultrasound, which must be done between 11 weeks 2 days and 14 weeks 2 days of pregnancy. The first trimester blood test can be scheduled one week prior to or at the same time as the NT ultrasound, as long as this is obtained between 10 weeks and 13 weeks 6 days of pregnancy. The NT ultrasound can be done between 11 weeks 2 days and 14 weeks 2 days of pregnancy. These tests cannot be obtained after 14 weeks 2 days of pregnancy.

The second blood test (Quad Marker Screening or Afp4) for Full Integrated Screening must be drawn between 15 and 20 menstrual weeks to be sent to the California State Prenatal (pdf).

The preliminary results for Full Integrated Screening during first trimester are available one week after the first trimester blood specimen (between 10 and 13 weeks 6 days) is sent to the California Prenatal Screening Program (pdf). The NT ultrasound is completed between 11 weeks 2 days and 13 weeks 6 days of pregnancy. The first trimester blood test and the NT can be performed at the same time (between 11 weeks 2 days and 13 weeks 6 days of pregnancy) and the results will be available one week later.

Alternatively, the first trimester blood test can be drawn one week prior to the NT ultrasound (between 10 weeks and 13 weeks 2 days of pregnancy) and the preliminary results of the NT and first trimester blood test will be available at the time of the NT study.

Final results for Full Integrated Screening are available one week after the second blood test (Quad Marker Screening or Afp4) is drawn, between 15 and 20 menstrual weeks.

The Quad Marker Screening or Afp4 is sensitive in detecting 80% of fetuses at risk for spina bifida or other open neural tube defects and abdominal wall defects as well as trisomy 18. This test may also identify pregnancies at higher risk for fetal growth abnormalities and maternal hypertension (pre-eclampsia), as well as Smith-Lemli-Opitz Syndrome (SLOS), which is a rare genetic syndrome associated with mental deficiency.

Combining the Quad Marker Screening Test or Afp4 test with a detailed second trimester ultrasound between 19 and 22 menstrual weeks when an evaluation of the brain and spine can be performed is approximately 95% sensitive in detecting open neural tube defects.

Genetic counseling is the process of helping people understand and adapt to the medical, psychological and familial implications of genetic contributions to disease. This process integrates the following:

• Interpretation of family and medical histories to assess the chance of disease occurrence or recurrence.
• Education about inheritance, testing, management, prevention, resources and research.
• Counseling to promote informed choices and adaptation to the risk or condition.

In addition to the genetic consultation service, we offer genetic testing for numerous conditions including ethnicity specific carrier screening, cystic fibrosis and hemoglobinopathies. Ask your health care provider about a referral for a genetic counseling consultation at Peninsula Prenatal Services.

If you have questions about making an appointment, please feel free to contact us at: (650) 343 1655.