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Chromosomal Screening


Early Screening: First Trimester

At your initial prenatal visit, you were given a substantial amount of information regarding options for care during your pregnancy. One important choice is whether or not to have screening for chromosomal abnormalities such as Down syndrome.

Down syndrome is the result of an extra chromosome 21. The main concern with Down syndrome is mental retardation and an increased risk of birth defects, especially heart defects. There is nothing that prospective parents can do to cause or prevent Down syndrome. About 1 in 800 babies are born with Down syndrome. The risk of having a baby with Down syndrome increases as maternal age increases but can occur at any age.

Trisomy 18 and Trisomy 13 result from an extra chromosome 18 and 13, respectively. These chromosomal abnormalities cause serious physical birth defects and severe mental retardation. Most babies with Trisomy 18 and Trisomy 13 are lost before delivery or shortly after birth. As with Down syndrome, the risk of Trisomy 18 or Trisomy 13 increases as maternal age increases.

First Trimester Screening

This test combines a blood test done between 9-13 weeks with an ultrasound done between 11-14 weeks. The ultrasound measures the thickness of a fluid layer behind the baby’s neck (nuchal translucency). This test detects roughly 85 percent of pregnancies with Down syndrome and 90 percent of pregnancies with Trisomy 18 & Trisomy 13 at a 5 percent “screen positive” rate. In other words, 5 percent of women who have the First Trimester Screening Test will have a “screen positive” result. The vast majority of these babies are normal, but in this 5 percent of women are 85 percent of pregnancies with Down syndrome and 90 percent of pregnancies with Trisomy 18 and Trisomy 13. These women are then offered diagnostic testing (CVS or amniocentesis). If the blood test is done at least several days before the ultrasound, results will be available at the time of the ultrasound.

This test offers a high detection rate of chromosomal abnormalities with the information being available early in pregnancy. This allows for the option of CVS, as well as amniocentesis, if diagnostic testing is desired. However, the detection rate for chromosomal abnormalities is not as high as with Sequential Screening (see below). Also this option does not include a blood test at 15 to 16 weeks, which also screens for certain birth defects such as spina bifida (also called Neural Tube Defects). Therefore, this test usually is followed by a second ultrasound at 18 to 22 weeks. This second ultrasound is a more accurate way to assess the baby’s development than the 15 to 16 week blood test.

Sequential Screening

The sequential approach to screening for chromosomal abnormalities combines the First Trimester Screening with a second trimester blood test done as early as 15 weeks. The results of each of the tests are given out as soon as available. This test detects about 95 percent of pregnancies with Down syndrome at a “screen positive” rate that is slightly higher than the First Trimester Screening Test. As with First Trimester Screening, most women with a “screen positive” Sequential Test will have a normal baby.

This option provides the highest detection rate. Those patients at higher risk after First Trimester Screening will have the option of early diagnostic testing (CVS), while those at lower risk can still have the second trimester blood test and higher detection rate. However, it is not known at what First Trimester Screening result  (1 in 50,  1 in 100, etc.) diagnostic testing should be offered. Since this approach has a slightly higher “screen positive” rate, it could lead to more diagnostic tests being performed compared to First Trimester Screening.

Diagnostic Tests versus Screening

Screening tests identify a population of women who are at risk of having a baby with a chromosomal abnormality. Screening tests do not provide a diagnosis. Instead they predict the likelihood of a chromosomal abnormality being present. Screening tests carry no risk to the pregnancy. Therefore, most women with a “screen positive” result have a normal baby, but the risk is increased for a chromosomal abnormality being present. These women are then offered diagnostic testing. A “screen negative” result does not guarantee a normal baby, but the risk for the baby having a chromosomal abnormality would be very low.

Although currently available screening tests have relatively high detection rates at low screen positive rates, the main disadvantage of screening tests is that they do not detect all chromosomal abnormalities, but instead provide an individual risk assessment for Down syndrome and Trisomy 18 & Trisomy 13. Diagnostic tests are highly accurate in detecting chromosomal abnormalities, but they are associated with a small risk of miscarriage. Diagnostic tests include Chorionic Villus Sampling (CVS) and amniocentesis.

CVS can be performed at 10 to 12.5 weeks and involves removal of a small amount of placental tissue. The risk of miscarriage is about 1 in 100. Amniocentesis is performed as early as 15 weeks and involves removal of a small amount of amniotic fluid. The risk of miscarriage is usually quoted as 1 in 300. However, more recent studies suggest the risk may be less.

Your doctor will offer you the option of screening for chromosomal abnormalities, but you are not required to have it. The choice is yours. The same is true for diagnostic testing. If a test is “screen positive”, the results will be explained to you in detail and the option of diagnostic testing will be reviewed.