First trimester screening is designed to identify whether you may be at increased risk of having a baby with Down syndrome or trisomy 18. These conditions are types of chromosomal abnormalities. The screening involves a maternal blood test and an ultrasound measurement called “nuchal translucency.”
First trimester screening can be done between 11 and 13 weeks gestation (sooner than other screening tests), and results take about one week. First trimester screening must be performed by an ultrasonographer or physician who is certified in nuchal translucency measurement.
It’s important to remember that first trimester screening is an indicator, not a definitive answer, about a baby’s health or condition. Even with an “abnormal” screen, most of the time the baby is healthy. Likewise, not every baby with Down syndrome or trisomy 18 will be identified through first trimester screening. Once a first trimester screening is performed, additional tests can give a definite answer about these and other conditions, if desired.
When a pregnant woman sees her doctor in the first trimester of pregnancy she may be offered a screening test for Down syndrome and trisomy 18. This recent advancement in prenatal care is called first trimester screening. First trimester screening is designed to identify women who may be at increased risk of having a baby with Down syndrome or trisomy 18. It is important to remember that this is a screening test and that even when it is “screen positive”, most often the baby is healthy. It is also important to remember that not every baby with Down syndrome or trisomy 18 will be identified with this screen.
First trimester screening involves a specific ultrasound measurement called “nuchal translucency” and a maternal blood draw. First trimester screening assesses the chance of having a baby with Down syndrome and trisomy 18. It is typically drawn between 11 and 13 weeks 6 days gestation. Results take approximately 6-7 days.
For women that will be 35 years or older at delivery, have an abnormal fetal ultrasound, or an abnormal first trimester screening, there is a newer type of chromosome screening that is available. Noninvasive prenatal screening (cell free DNA testing) is a maternal blood test that detects common chromosome abnormalities in pregnancies at 10 or more weeks based on directed analysis of fetal DNA in maternal blood. They can also detect the presence of X and Y chromosomes. The detection rate for common trisomies such as 18, 21 and gender chromosomes is estimated to be greater than 99% and detection rate for trisomy 13 is ~ 90%.
If a high risk patient is interested in screening, we also recommend undergoing the first trimester ultrasound at the same appointment. The first trimester ultrasound is an early assessment of your baby; A normal scan would be reassuring, however, it is much too early in pregnancy to rule out all possible complications. An abnormal scan may be an early red flag that baby has an underlying birth defect or chromosome condition.
Humans typically have 46 chromosomes in every cell. These chromosomes carry genes, which are the blueprints for human growth and development. If a child is born with more or less than 46 chromosomes in each cell, he or she will have a problem with mental and physical development. One example of a chromosome abnormality is Down syndrome (trisomy 21) and another is trisomy 18. These two chromosome conditions are screened for with first trimester screening.
Down syndrome is a chromosome problem that occurs when a baby inherits an extra chromosome #21 at conception. This extra chromosome causes the features of Down syndrome, which can include heart defects, mental retardation and other physical disabilities. Down syndrome occurs in 1:800 pregnancies in the general population. The risk of having a baby with Down syndrome increases with increasing maternal age. However, 80% of babies with Down syndrome are born to women less than 35 years of age. Most couples who have a child with Down syndrome have no family history of the condition.
Trisomy 18 is a chromosome problem that occurs when a baby inherits an extra chromosome #18 at conception. It is medically more severe than Down syndrome and babies with trisomy 18 typically will be stillborn or will not live past 1 year of age. Babies that do survive will have severe mental retardation and can have a variety of other birth defects.
In first trimester screening two chemicals are measured from a pregnant woman’s blood: free beta-human chorionic gonadotrophin (free β-hCG) and pregnancy-associated plasma protein A (PAPP-A). In addition, an ultrasound measurement of the baby’s neck is performed to determine the amount of fluid accumulation behind the neck of the baby. This measurement is called “nuchal translucency.” The amounts and patterns of the two chemicals are used, along with the nuchal translucency measurement and the mother’s age, to assess the likelihood that a baby would have Down syndrome or trisomy 18. A positive screen means that the calculated chance of having a baby with Down syndrome is greater than or equal to a 35-year old woman’s risk. It is important to note that the levels of these chemicals are not usually affected by anything mom may or may not have done during the pregnancy.
In all women, first trimester screening identifies approximately 90% of babies who have Down syndrome and about 97% of babies with trisomy 18. In women under the age of 35, first trimester screening will be positive approximately 5% of the time. In women 35 and over, first trimester screening will come back positive in a larger percentage of women taking the test (approximately 25%). However, it will detect almost 97% of babies with Down syndrome. Again, it is important to remember that most of the time, even when the screen is positive, the baby does not have a problem. It is also important to remember that this is a screening tool and it will not detect every baby with Down syndrome or trisomy 18.
There can be several different explanations for a positive result. A positive result does not mean Down syndrome or trisomy 18 has been diagnosed. You should talk to your doctor or genetic counselor in order to get more information regarding your particular situation.
Again, when the first trimester screen is positive, it does not mean that your baby has been diagnosed with a problem. It does tell us that we should offer you additional prenatal tests, such as chorionic villus sampling (CVS) or amniocentesis, to give you more information about your pregnancy. CVS is the removal of a small sample of tissue from the placenta, which allows us to perform chromosome analysis. CVS cannot tell us everything about the baby and it associated with a risk of miscarriage (1:150 – 1:180). Amniocentesis is a procedure that involves withdrawing a small sample of amniotic fluid; this allows us to look at the baby’s chromosomes and to test the AFP levels. However, amniocentesis cannot tell us everything about the baby and is associated with a risk of miscarriage (1:200). A discussion with your doctor or genetic counselor can help you understand these procedures and the benefits, risk and limitations of these options. Alternatively, some patients are not interested in undergoing a diagnostic procedure immediately. In these cases, there is a different type of genetic screening that is available called non-invasive prenatal screening (NIPT). This type of maternal blood screen can better assess (compared to the first trimester screen) whether the baby is at high or low risk of having the condition. This option would be further discussed upon meeting with a genetic counselor after an abnormal first trimester screen.
It is your decision whether or not to have first trimester screening. Some women find having the test to be reassuring. Some face an anxious time between a “positive” screen and the follow-up testing. Others would rather not have the information. The results can help some women make decisions about their options.
At the Maternal Fetal Care Center, our philosophy is to help you through all aspects of pregnancy, delivery and afterwards to the best of our ability. No matter what you decide, we will continue to work with your primary care provider to ensure that your pregnancy has the best possible outcome. Please feel free to talk with your doctor or call us with any questions or concerns at (414) 805-6624.
An alpha-fetoprotein (AFP) test between 15 – 206 weeks is recommended to screen for 98% of open spine and skull defects, and about 60% of abdominal wall defects. These birth defects are not screened for with first trimester screening and usually occur without a family history. Ultrasound examination in the second trimester is another way to screen for these types of defects.
No. A second trimester chromosomal screen is not recommended. This is because any second trimester screening is unlikely to improve the detection rate and there is a greater chance of a false positive result.