Second-trimester sonographic soft markers: what can we learn from the experience of first-trimester nuchal translucency screening?

Genetics: update on prenatal screening and diagnosis

There have been tremendous advances in the ability to screen for the "odds" of having a genetic disorder (both mendelian and chromosomal). With microarray analyses on fetal tissue now showing a minimum risk for any pregnancy being at least 1 in 150 and ultimately greater than 1%, it is thought that all patients, regardless of age, should be offered chorionic villus sampling/amniocentesis and microarray analysis. As sequencing techniques replace other laboratory methods, the only question will be whether these tests are performed on villi, amniotic fluid cells, or maternal blood.

Second-trimester soft markers: relation to first-trimester nuchal translucency in unaffected pregnancies

OBJECTIVE

Genetic sonography following first-trimester combined screening appears to increase substantially detection rates for Down syndrome but it relies on the unproved assumption of independence between these tests. In this study we aimed to investigate the relationship between first-trimester nuchal translucency (NT) and a series of second-trimester soft markers and structural defects in unaffected pregnancies.

METHODS

NT measurement in the first trimester was followed by second-trimester scan (18 to 23 + 6 weeks) including examination for three categorical markers (intracardiac echogenic foci, hyperechogenic bowel and structural defects) and measurement of nasal bone length, nuchal-fold thickness, femur length, humerus length, renal pelvis diameter and prenasal thickness. All continuous variables were expressed in multiples of the median (MoM) for gestation and correlation coefficients between log-transformed NT and second-trimester variables were calculated. In addition, frequencies of soft markers and structural defects in cases with increased NT were compared to those with normal NT, using MoM cut-offs.

RESULTS

In a dataset of 1970 cases, NT was significantly correlated (P < 0.05) with all second-trimester continuous variables, the correlation being strongest for nuchal-fold thickness (r = 0.10). There was a higher frequency of cases with second-trimester nuchal-fold thickness above the 97.5(th) centile (10.7 vs. 2.2%) and hyperechogenic bowel (2.4 vs. 0.1%) in cases with increased NT.

CONCLUSIONS

Straightforward reassessment of risk using likelihood ratios derived from the second-trimester genetic sonogram might lead to inaccurate estimates. Multivariate models using continuous second-trimester variables might be preferable in sequential screening strategies.

Pränatale Detektion fetaler chromosomaler Aberrationen im 1. und 2. Trimenon

Die modernen nichtinvasiven Verfahren der pränatalen Medizin, vertreten vor allem durch das Ersttrimesterscreening, ermöglichen eine frühe Risikoeinschätzung bzgl. der häufigsten Aneuploidien. Durch die flächendeckende Anwendung dieser Methode mit derzeit weit über 4000 zertifizierten Gynäkologen bundesweit stellt dies heute einen weit verbreiteten Standard dar. Die klassische „genetische“ Sonographie, d. h. die Detektierung von Softmarkern als Hinweiszeichen für eine Aneuploidie im 2. Trimenon, hat an Bedeutung verloren. Allerdings ist der Fehlbildungsultraschall im 2. Trimenon nach wie vor von sehr großer Bedeutung und stellt den „Goldstandard“ für die Entdeckung angeborener Fehlbildungen dar. Der Pränatalmediziner muss in der Lage sein, während dieser Untersuchung Softmarker zu erkennen, um ggf. eine Reevalution des mütterlichen Risikos bzgl. Aneuploidien vorzunehmen.

Isolated absent or hypoplastic nasal bone in the second trimester fetus: is amniocentesis necessary?

OBJECTIVE

To elucidate the significance of isolated absent or hypoplastic nasal bone in the second trimester ultrasound scan.

METHODS

All cases of absent or hypoplastic nasal bone (length < 5th percentile) encountered during 2007-2009 were retrieved from database and all the ultrasound findings including structural abnormalities and soft markers for Down syndrome and fetal karyotype were reviewed. The cases were categorized into a study group with isolated absent or hypoplastic nasal bone and a comparison group with additional ultrasound findings. The incidence of Down syndrome confirmed by karyotyping was compared between the two groups.

RESULTS

Among 14 fetuses with absent or hypoplastic nasal bone identified, six (42.9%) had Down syndrome and eight (57.1%) were normal. All (100%) of the six fetuses with isolated absent or hypoplastic nasal bone (Study Group) had normal karyotype, while six (75%) of the other eight fetuses with additional ultrasound findings (Comparison Group) had Down syndrome (p = 0.010).

CONCLUSIONS

The use of isolated absent or hypoplastic nasal bone in the second trimester ultrasound scan for Down syndrome screening may not be effective. Amniocentesis, however, is indicated for fetuses with structural abnormality or additional soft marker which should be carefully searched by an experienced ultrasonographer.

Model-predicted performance of second-trimester Down syndrome screening with sonographic prenasal thickness

OBJECTIVE

The purpose of this study was to estimate the Down syndrome detection and false-positive rates for second-trimester sonographic prenasal thickness (PT) measurement alone and in combination with other markers.

METHODS

Multivariate log Gaussian modeling was performed using numerical integration. Parameters for the PT distribution, in multiples of the normal gestation-specific median (MoM), were derived from 105 Down syndrome and 1385 unaffected pregnancies scanned at 14 to 27 weeks. The data included a new series of 25 cases and 535 controls combined with 4 previously published series. The means were estimated by the median and the SDs by the 10th to 90th range divided by 2.563. Parameters for other markers were obtained from the literature.

RESULTS

A log Gaussian model fitted the distribution of PT values well in Down syndrome and unaffected pregnancies. The distribution parameters were as follows: Down syndrome, mean, 1.334 MoM; log(10) SD, 0.0772; unaffected pregnancies, 0.995 and 0.0752, respectively. The model-predicted detection rates for 1%, 3%, and 5% false-positive rates for PT alone were 35%, 51%, and 60%, respectively. The addition of PT to a 4-serum marker protocol increased detection by 14% to 18% compared with serum alone. The simultaneous sonographic measurement of PT and nasal bone length increased detection by 19% to 26%, and with a third sonographic marker, nuchal skin fold, performance was comparable with first-trimester protocols.

CONCLUSIONS

Second-trimester screening with sonographic PT and serum markers is predicted to have a high detection rate, and further sonographic markers could perform comparably with first-trimester screening protocols.