Increased Nuchal Translucency with Normal Karyotype
Abstract & Commentary
By John C. Hobbins, MD, Professor and Chief of Obstetrics, University of Colorado Health Sciences Center, Denver, is Associate Editor for OB/GYN Clinical Alert
Synopsis: If the fetus survives until midgestation, and if a targeted ultrasound at 20 to 22 weeks fails to reveal any abnormalities, the risk of an adverse perinatal outcome and postnatal developmental delay is not statistically increased.
Source: Souka AP, et al. Increased nuchal translucency with normal karyotype. Am J Obstet Gynecol. 2005;192:1005-1021.
Souka and colleagues have recently published a review that should be very helpful to the clinician confronted with the dilemma of counseling patients whose fetuses have increased nuchal translucencies (NTs) in the first trimester.
The concept of first trimester screening with ultrasound and maternal biochemistry is taking off in a big way in the United States, and, not surprisingly it has triggered fallout involving standardization of examinations, training of sonographers and sonologists, and the usual politics of who should be overseeing these activities.
The simple reason for the belated but, now, genuine enthusiasm for the first trimester screening process is that it works. The sensitivity of the method to screen for Down syndrome (DS) is around 85%. When combined with second trimester biochemistry and a genetic sonogram, the sensitivity, accordingly to FASTER trial data, is almost 100%. However, until now only sparse information has been available to clinicians regarding what to tell their patients whose fetuses have increased NTs with normal karyotypes.
In the above review, which surfaced in April, 2005, data were combined from Souka et al’s extensive experience and from other published studies to enlighten us on the meaning of an increased NT based on size alone.
First, in data from all pregnancies (karyotypically normal and abnormal) if the NT was between 11 and 13 weeks was between the 95th and 99th percentile, the chance of a karyotypic abnormality was 3.7%. Also, the risk of fetal death was 1.3% and risk of a major fetal anomaly was 2.5%. Nevertheless, in the same group there was a 93% chance of being alive and well after birth. However, if the NT was above 3.5 mm, the risk of fetal death, anomaly, and abnormal karyotype rose exponentially to a point where only 15% would be alive and well when the NT exceed 6.5 mm (see Table).
| Table | ||||
| Relation between nuchal translucency thickness and prevalence of chromosomal defects, miscarriage, or fetal death and major fetal abnormalities. | ||||
| Nuchal translucency |
Chromosomal defects
|
Fetal death
|
Major fetal abnormalities
|
Alive and well
|
| < 95th percentile |
0.2%
|
1.3%
|
1.6%
|
97%
|
| 95th-99th percentile |
3.7%
|
1.3%
|
2.5%
|
93%
|
| 3.5-4.4 mm |
21.1%
|
2.7%
|
10.0%
|
70%
|
| 4.5-5.4 mm |
33.3%
|
3.4%
|
18.5%
|
50%
|
| 5.5-6.4 mm |
50.5%
|
10.1%
|
24.2%
|
30%
|
| > 6.5 mm |
64.5%
|
19.0%
|
46.2%
|
15%
|
| Source: Souka AP, et al. Am J Obstet Gynecol. 2005;192:1005-1021. | ||||
Now if one were to concentrate only on those fetuses with normal karyotypes, the data from 27 studies involving 6153 fetuses (in which varying cutoffs for increased NT were used) show that the overall risk of fetal anomalies was 7.3%.
The most common anomalies encountered were a cardiac abnormality. For example, data accumulated from 4 studies involving 3448 patients showed that if the NT was 2.5-3.4 mm, cardiac anomalies occurred in 17/1000 fetuses. If the NT was > 3.4 mm, the risk of cardiac anomalies rose to 78/1000. Most birth statistic centers quote a prevalence of congenital heart disease in the overall population to be about 5-8/1000.
Other common anomalies associated with increased NTs, with or without a cardiac component, are diaphragmatic hernia, exomphalos, body stalk abnormalities, skeletal defects, and various syndromes such as fetal akinesia, Noonan syndrome, Smith-Lemli-Opitz syndrome, and spinal muscular atrophy. Those not seemingly associated with increased NT were neural tube defects, holoprosencephaly, gastroschisis, and renal anomalies.
Last, the few studies that have addressed the relationship between increased fetal NT and developmental delay in the overtly normal infant fortunately have shown no correlation.
Commentary
As indicated in the Souka et al paper, the risk of a chromosome abnormalities when the NT is above the 95th percentile (in relationship to the crown-rump length) exceeds 3.7% and because this risk is easily greater than the risk of invasive sampling either by CVS or amniocentesis, many patients will choose to determine their fetus’ karyotype. However, until now, it has been unclear what risk to quote to these patients for an adverse outcome once they are informed of the good news regarding the normal karyotype.
Here are some follow-up suggestions for these patients:
- Since many fetal anomalies can be identified by an early transvaginal ultrasound exam at 12-15 weeks, most patients would benefit from a fetal survey at this time.
- At 16-18 weeks, a majority of the anomaly syndromes mentioned above can be suspected with a comprehensive transabdominal examination.
- Since cardiac anomalies are so prevalent in this fetal population, it is strongly suggested that a fetal echocardiogram be accomplished at 22-24 weeks.
If these are all normal, the authors have shown that the risk of fetal anomalies, fetal death, or neonatal problems is not statistically increased.
Souka and colleagues have recently published a review that should be very helpful to the clinician confronted with the dilemma of counseling patients whose fetuses have increased nuchal translucencies (NTs) in the first trimester.You have reached your article limit for the month. Subscribe now to access this article plus other member-only content.
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