Between 11 and 14 weeks gestation, sonography remains very accurate for pregnancy dating, and can also be used to evaluate for abnormalities of the ovaries and cervix. At this stage in the pregnancy, we can also see the placenta, get a first look at baby’s anatomy, and perform nuchal translucency screening, a very important early means of assessing the risk of birth defects.
During the development of the embryo, there is always a clear space beneath the skin at the back of the neck. Between 11 and 14 weeks into the pregnancy (when the embryo measures between 4.5 and 8.5 mm), the size of this space can be measured by ultrasound, and it turns out that the size of the clear space bears a relationship to certain chromosome problems, including Down syndrome and trisomy 18, and also to certain structural abnormalities of the baby.
Risk for Down Syndrome and Trisomy 18
In fact, the combination of ultrasound measurements of the developing embryo, together with mom’s age and blood test results, can be used to calculate the chance that a pregnancy is affected by Down syndrome or trisomy 18. In order to be used in these calculations, we need to obtain a nuchal translucency measurement with the baby looking either straight toward or straight away from the ultrasound machine, in a profile view. The measurement is done with remarkable precision. We measure the nuchal translucency to the 10th of a millimeter. We are able to measure the nuchal translucency in 85 to 95 percent of pregnancies, but sometimes measurement with the precision needed will not be possible. Not every baby will cooperate during the scan, and in some cases, there will be irregularities of the uterine wall, scarring from prior abdominal surgeries, or other technical factors that prevent us from obtaining a clear measurement. This happens between 5 and 15 percent of the time.
If we can measure the nuchal translucency accurately, then a computer program can be used to calculate the risk for Down syndrome and trisomy 18 by using a combination of the mother’s age and laboratory measurements of beta hCG and pregnancy-associated protein A (PAPP-A), which are obtained via a blood sample from mom. The risk calculation can be refined by including results of a blood sample drawn between 15 and 20 weeks for the quad screen (alpha-fetoprotein, hCG, inhibin and estriol).
In the State of California, there is a Prenatal Screening Program which is used for risk calculations. The State requires that all first trimester and quad screen specimens are performed within the State laboratory system. The State system runs a little differently from other screening systems. More information about the State Program is available at www.cdph.ca.gov/programs/PNS/Pages/default.aspx.
If we are able to obtain a nuchal translucency measurement and results from the beta hCG and PAPP-A between 10 and 13½ weeks, plus the quad screen, the risk calculation will flag about 5 percent of women as being at increased risk for Down syndrome or trisomy 18. If all of these women undergo amniocentesis testing or CVS testing, we will identify between 85 and 90 percent of cases of Down syndrome, and between 80 and 85 percent of cases of trisomy 18, and miss the remaining 10 to 15 percent of Down syndrome cases and 15 to 20 percent of trisomy 18 cases.
If we are not able to obtain a nuchal translucency measurement, then the calculation of risk will be a bit less precise, and screening will identify about 5 percent less cases of both Down syndrome and trisomy 18.
In contrast to other programs, the State of California does not provide risk calculations for other chromosome conditions, such as trisomy 13.
It is important to realize that the only way to be certain that the baby does or does not have a numeric defect, such as Down syndrome, is by means of amniocentesis or CVS testing, which are covered in separate sections on our website.
Risk for Other Birth Defects
The nuchal translucency measurement is more than just a screening for Down syndrome. A very small nuchal translucency measurement – less than 2.5 mm – places the pregnancy in a low-risk group for problems, such as fetal heart abnormalities. Pregnancies where the nuchal translucency measurement is between about 2.5 and 3.5 mm are in an intermediate-risk group, and for these women, a genetic sonogram at about 18 weeks into the pregnancy plus a detailed sonogram of the heart (called a fetal echocardiogram) at about 22 weeks are recommended. Women with a nuchal translucency measurement of 3.5 mm or more have pregnancies at increased risk for chromosome problems, for abnormalities of the fetal heart, and for other birth defects, ranging from abnormalities of the skeletal system to the baby’s diaphragm. For these pregnancies, genetic counseling and a genetic sonogram at 18 weeks, and a fetal echocardiogram and a detailed scan at about 22 weeks, are recommended.
In the past, sonography at 11 to 14 weeks was not an effective way to screen for birth defects of the baby. The embryo at that stage was just too small to evaluate well. As time has gone on, the ultrasound equipment and our ability to interpret the images have both improved. Now, at the 11- to 14-week scan, we are able to identify many major central nervous system, cardiac, abdominal, and limb abnormalities. Our center’s goal is to identify 30 percent of major structural malformations at the time of the nuchal translucency scan.
When is the best time to schedule a nuchal translucency scan? The accuracy is highest when the lab work is done between 10 and 11 weeks into the pregnancy, and we recommend scheduling the nuchal scan between 12 and 13 weeks. As long as the lab test has been done more than a week before, we should be able to provide you with the Down syndrome and trisomy 18 risk calculations at the time of your ultrasound scan.