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Combined First-Trimester Nuchal Translucency Screening
Combined first-trimester nuchal translucency screening is a noninvasive screening test for fetal Down syndrome and trisomies 13 and 18. Prenatal genetic counseling services are also provided in conjunction with first-trimester screening, if requested.
If you are a health care provider referring your patient to Johns Hopkins for first-trimester screening, learn more here.
- Learn about common tests during pregnancy in the Johns Hopkins Health Library.
Frequently Asked Questions
What are Down syndrome, trisomy 13 and trisomy 18?
Down syndrome and trisomies 13 and 18 are chromosomal disorders that cause intellectual disability and birth defects. Babies with Down syndrome have an extra 21st chromosome (trisomy 21), which causes intellectual disability and various medical problems involving the heart, digestive tract and/or other organ systems. Trisomy 18 (having an extra 18th chromosome) and trisomy 13 (having an extra 13th chromosome) are more severe disorders that cause profound intellectual disability and severe birth defects in many organ systems. Few babies with trisomies 13 or 18 survive more than a few months. Anyone can have a baby with these chromosomal abnormalities, but the chance increases with the mother’s age.
What can combined first-trimester screening tell me about my pregnancy?
Combined first-trimester screening is not a diagnostic test, which means it cannot tell you whether your baby has Down syndrome, trisomy 13 or trisomy 18. Instead, the screening provides a probability that the baby might have Down syndrome, trisomy 13 or trisomy 18. This probability, or chance, is based on three criteria: your age, information obtained on an ultrasound and bloodwork. The screening results can either alert you and your doctor that your baby is at an increased risk for one of these chromosomal disorders or be reassuring that your baby is at a lower risk for these conditions.
Who will perform the combined first-trimester screening ultrasound?
When will combined first-trimester screening be performed?
This test is available to women between 11 4/7 and 13 6/7 weeks of pregnancy.
What can I expect during the combined first-trimester screening?
This screening includes an ultrasound and maternal bloodwork. The ultrasound will confirm how far along your pregnancy is. In addition, a measurement of the fluid underneath the skin along the back of your baby’s neck, called the nuchal translucency (NT), will be taken. The ultrasound will also try to determine if your baby’s nasal bone is present. A maternal blood sample is used to analyze three chemicals called free beta-human chorionic gonadotropin (hCG), pregnancy-associated plasma protein-A (PAPP-A) and alpa-fetoprotein (AFP), which is found in the blood of all pregnant women. In some pregnancies, when the baby has Down syndrome, trisomy 13 or trisomy 18, there is extra fluid behind the baby’s neck, and/or the hCG, AFP and PAPP-A results are higher or lower than average. While a baby’s nasal bone may be absent in some pregnancies with a chromosomal abnormality, most babies with this finding are normal. Combining your age-related risk with the NT measurement, nasal bone data and bloodwork provides you with one risk figure for Down syndrome and one risk figure for trisomy 13 or trisomy 18. Your obstetrician will receive your screening results from NTD Laboratories in approximately one week.
How accurate is the combined first-trimester screening?
Because this is a screening test, a positive result (showing an increased risk) does not mean that your baby has a problem, only that further diagnostic tests are options for you to consider. Also, a negative or normal result (one that shows a decreased risk) does not mean that the baby will not have a chromosomal abnormality. The first-trimester screening’s detection rate is approximately 96 percent for pregnancies in which the baby has Down syndrome and is somewhat higher for pregnancies with trisomy 13 or trisomy 18. A nuchal translucency ultrasound can be performed without the bloodwork, but the detection rate is reduced to about 70 percent. This screen is not designed to provide information about the possibility of other chromosomal conditions but does have limited utility for screening for some other genetic syndromes, genetic disorders and birth defects.
Should I have second-trimester screening performed?
The second-trimester maternal serum screening test, also known as the quad screen, is performed between 16 and 20 weeks, and measures chemicals in the mother’s blood. Like the first-trimester screening, results from a second-trimester quad screen can be used to statistically adjust a woman’s age-related risk for Down syndrome and trisomy 18 (not trisomy 13). In addition, the AFP portion of the screen in the second trimester can identify pregnancies at an increased risk for open neural tube defects, such as spina bifida. It is not recommended that quad screening be performed if combined first-trimester screening has already been performed. However, AFP can be drawn as an independent test to screen for spina bifida.
How is first-trimester screening billed?
The screening involves a charge for the ultrasound examination and a charge for the bloodwork. Both the bloodwork and the ultrasound examination will be billed by Johns Hopkins; these charges vary over time. As with any service, if your insurance company requires preauthorization, you are responsible for obtaining it from your obstetrician or primary care physician prior to the appointment.
What if the screening shows an increased risk for one of the conditions?
If the screening results indicate that your baby is at an increased risk for Down syndrome or trisomy 13 or 18, this does NOT mean that your baby necessarily has one of these conditions. A genetic counselor is available to go over your results and to discuss additional screening and testing options, such as chorionic villus sampling (CVS) and amniocentesis. CVS and amniocentesis are diagnostic procedures that can tell you with greater than 99 percent accuracy whether or not a baby has a chromosomal abnormality. Also, extra fluid behind the baby’s neck (a larger-than-expected nuchal translucency) is known to be associated with other birth defects, like congenital heart defects and skeletal problems.
How do I schedule an appointment for first-trimester screening?
You can call the Prenatal Genetics Service at 410-955-3091 to arrange an appointment, or if you have questions regarding your screening or testing options.