Diagnostic ultrasound ( PDFDrive )

1090 PART IV Obstetric and Fetal Sonography
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Caliper alignment for NT measurement
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FIG. 31.2 Nuchal Translucency. (A) Gross pathology image of a fetus with trisomy 21 demonstrating the luid collection at the back of the
neck known as the “nuchal translucency” (NT; arrow). (Courtesy of Dr. Eva Pajkrt, University of Amsterdam.) (B) Midsagittal view of a euploid
fetus demonstrating the correct method of measuring the NT. See also Video 31.1. (C) Axial view through the fetal neck showing distended jugular
lymphatic sacs (arrows). (D) Midsagittal view of a irst-trimester fetus showing the correct method of measuring the crown-rump length. (E) Midsagittal
view of a irst-trimester fetus showing a thick NT (8.6 mm). (F) Schematic illustrating the correct method of caliper placement. (Courtesy of Dr.
Bernard Benoit, Princess Grace Hospital, Monaco.)
pregnancies. he maternal serum of women carrying fetuses
with trisomy 21 has a higher concentration of free beta human
chorionic gonadotropin (free β-hCG) and lower concentration
of pregnancy-associated plasma protein A (PAPP-A) compared
with the serum of women carrying euploid fetuses. he use of
these serum markers, in conjunction with maternal age, results
in identiication of 62% of fetuses with trisomy 21 at an FPR
of 5%. 18
Combined First-Trimester Screening
here is no association between NT measurement and serum
levels of free β-hCG or PAPP-A in euploid fetuses or in those
with trisomy 21. his independence allows the combination
of NT screening and biochemical screening, resulting in a
more efective method of risk assessment than either method
individually. 19,20 Wald et al. 21 demonstrated that the combination
of NT measurement with maternal serum PAPP-A and
free β-hCG, known as the combined irst-trimester screening
test, results in the detection of 85% of trisomy 21 fetuses at an
FPR of 5%.
First-trimester screening using NT measurement and serum
biochemical measurement has been further validated in four
large studies. he One-Stop Clinic to Assess Risk (OSCAR)
screening trial in the United Kingdom studied 12,339 women
with singleton pregnancies between 10 and 14 weeks’ gestation.
First-trimester screening was accepted by 97.5% of the women,
and if they screened positive (risk ≥ 1 : 300), 77% underwent
invasive diagnostic testing. here were 25 cases of trisomy 21,
of which 23 (92%) were detected, with an FPR of 5%. he detection
rates of both trisomy 13 and 18 were 100%. 22
A multicenter trial in North America (BUN study) similarly
evaluated 8514 patients with singleton pregnancies between 74
and 97 days’ gestation. A screening result was considered positive
if the risk of trisomy 21 was 1 in 270 or higher or the risk of
trisomy 18 was 1 in 150 or higher. here were 61 cases of trisomy
21; detection rate was 79% with a 5% FPR. he detection rate
of trisomy 18 was 90% with a 2% FPR. 23
he Serum, Urine and Ultrasound Screening Study (SURUSS)
evaluated the eicacy, safety, and cost-efectiveness of irst- and
second-trimester screening for trisomy 21. his prospective study
was conducted primarily in the United Kingdom on 47,053
pregnancies between 9 and 13 weeks’ gestation. In the irst trimester
the combined test had a sensitivity of 85% for the detection
of trisomy 21 with an FPR of 6%. 24
he First and Second Trimester Evaluation of Risk (FASTER)
was the largest trial based in the United States and was designed
to determine how best to screen pregnant women for trisomy
21. his multicenter trial included 36,120 patients with complete
irst-trimester data, of whom 92 fetuses had trisomy 21. he trial
included NT measurements as well as serum biochemistry in
both the irst and the second trimester, revealing the results to
patients only in the second trimester, ater both serum screens.