University of Groningen Challenges in prenatal screening and diagnosis in the Netherlands Bakker, Merel

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University of Groningen

Challenges in prenatal screening and diagnosis in the Netherlands

Bakker, Merel

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Bakker, M. (2017). Challenges in prenatal screening and diagnosis in the Netherlands. Rijksuniversiteit Groningen.

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7

First Trimester Facial Markers for

Abnormal Development – Feasibility,

reproducibility and normal ranges

of the PNT, MNM angle, PL-distance,

PNT/NBL-ratio & PFSR

M. Bakker 1 M. Pace 1 E. de Jong-Pleij 3 E. Birnie 1,2 K. O. Kagan 4 C. M. Bilardo 1

1_{Department of Obstetrics and Gynaecology, Fetal Medicine Unit,}

University Medical Centre, Groningen, the Netherlands.

2_{Department of Genetics, University Medical Centre Groningen,}

University of Groningen, Groningen, the Netherlands.

3_{Department of Obstetrics and Gynecology, St. Antonius Hospital,}

Nieuwegein, Utrecht, the Netherlands.

4_{Department of Obstetrics and Gynecology,}

University of Tuebingen, Germany. Published in Fetal Diagnosis & Therapy, 2016 Nov 18.

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First Trimester Facial Markers for Abnormal

Development – Feasibility, reproducibility

and normal ranges of the PNT, MNM angle,

PL-distance, PNT/NBL-ratio & PFSR

M. Bakker1_{, M. Pace}1_{, E. de Jong-Pleij}3_{, E. Birnie}1,2_{, K.O. Kagan}4_{and C. M. Bilardo}1 1_{Department of Obstetrics and Gynaecology, Fetal Medicine Unit,}

Univer-sity Medical Centre, Groningen, the Netherlands.

2_{Department of Genetics, University Medical Centre Groningen, University} of Groningen, Groningen, the Netherlands.

3_{Department of Obstetrics and Gynecology, St. Antonius Hospital,} Nieuwe-gein, Utrecht, the Netherlands.

4_{Department of Obstetrics and Gynecology, University of Tuebingen, Germany.}

Objective:

To investigate the feasibility and repro-ducibility of the prenasal-thickness-to-nasal-bone-ratio (PNT/NBL ratio), mandibular-nasion-maxilla (MNM) angle, facial profile (FP) line, profile line (PL-)distance and prefrontal-space-ratio (PFSR) in the first trimester of preg-nancy, to develop normal ranges, and to evaluate these markers in abnormal fetuses.

Methods:

In this cross sectional study measure-ments were performed by two opera-tors. Feasibility was tested between op-erators with the X2 test. Inter-operator agreement was quantified descriptively by Bland and Altman’s limits of agree-ment, repeatability coefficient and in-traclass coefficient. Prediction intervals, indicating the interval in which 95% of the individual prediction would lie for a given CRL value, were calculated for all measurements.

Results:

Feasibility within and between op-erators was good. The NBL, PNT, PFSR and MNM angle increased with CRL, whereas the PNT/NBL ratio and the PL-distance decreased. All parameters, except the PL-distance and the PFSR had high inter-operator agreement. The PNT and PNT/NBL ratio were increased in most trisomic cases. The MNM-angle was increased in case of micrognathia and complete bilateral cleft. The PFSR and the PL-distance showed large inter-observer variation.

Conclusion:

The PNT, PNT/NBL ratio and MNM an-gle can already be measured in the first trimester of pregnancy. The PNT and the PNT/NBL ratio are abnormal in more than a third of the trisomic fetuses. The MNM-angle is increased in fetuses with micrognathia and facial clefts.

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Introduction

Assessment of the fetal face in the second trimester of pregnancy has become an im-portant part of fetal evaluation, not only for the detection of facial anomalies but also in screening for trisomies, especially trisomy 21. Established second trimester profile markers for trisomies are the nasal bone length (NBL), the prenasal thickness (PNT), the ratio between the NBL and PNT and more recently the prefrontal space ratio (PFSR).1-4

Other second trimester profile parameters, e.g. the profile line (FP line) and maxilla-nasion-mandible angle (MNM-angle), have been studied as markers for facial anomalies including profile alterations in case of aneuploidies.2,3,5-7_{These are proven reproducible}

markers for the diagnosis of retrognathia, maxillary alveolar ridge interruption, sloping forehead, frontal bossing and flat profile.6,7_{Reference values for these markers are}

avail-able for the second trimester, however normal ranges for the PNT/NBL ratio, PL-distance, MNM-angle and PFSR in the first trimester are still missing.

The frontomaxillary facial angle (FMF-angle) and the NBL have been introduced in the first trimester to improve screening algorithms for trisomies and as a result improve detection rates and decrease false positive rates.8-11_{Measurement of the PNT, PNT/ NBL}

ratio, MNM-angle, FP line and PFSR in the first trimester could possibly further improve the detection of trisomies and/or facial abnormalities early in pregnancy.

Although cell free fetal DNA (cffDNA) is becoming rapidly available worldwide and in some countries has replaced the CT as screening test for trisomies, the CT is still the stan-dard of care in the majority of countries. Further improvement of detection of trisomies is still valuable in case cffDNA is not performed or when it is performed as second tier test and to enhance the first trimester detection of structural anomalies which are not trisomy related.

Aim of this study was to investigate whether the PNT/NBL-ratio, MNM angle, FP line and PFSR can already be measured in the first trimester of pregnancy (feasibility); to study the inter-operator agreement (reproducibility); and to develop normal ranges for these measurements. Furthermore, we preliminarily explored the performance of these new potential first trimester markers in a number of abnormal fetuses.

Methods

DESIGN

A retrospective cross sectional study on the measurement of PNT/NBL-ratio, MNM angle, FP line, PL-distance and PFSR in the first trimester and early second trimester of pregnancy was undertaken at the University Medical Centre Groningen, the Nether-lands. We selected images stored between 2011 and 2013 from singleton pregnancies between 11-14 weeks of gestation, satisfying the FMF guidelines and with known follow-up. Only images with a true midsagittal plane were used. All images were acquired trans-abdominally, using a Voluson E8 equipped with a 4-8 Hz probe (GE Medical Systems). For each image two experienced operators independently measured the PNT/NBL-ratio, MNM angle, FP line, PL-distance and PFSR.

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ABNORMAL CASES

Stored first trimester pictures of fetuses with chromosomal anomalies or other genetic or structural anomalies diagnosed during pregnancy were retrieved from the FMU databas-es of the UMC Groningen and the University of Tuebingen. Only picturdatabas-es with a clear pro-file image where the NBL, the PT and the bony landmarks of the fetal propro-file were clearly discernable were selected and measured following the methodology described above.

PARAMETERS AND MEASUREMENTS

The NBL was measured from the proximal to the distal end of the white ossification line in midsagittal view of the profile. The PNT was measured as the shortest distance be-tween the nasion and the frontal skin. Since the nasion is not yet fully formed, we took the lowest point of the frontal bone. Calipers were placed on the outermost borders of the skin or bone.

The MNM angle objectifies the anteroposterior position of the jaws and is defined as the angle between the lines maxilla-nasion and mandible-nasion in the midsagittal plane (see Figure 1).7

The PFSR was established after drawing a line from the anterior aspect of both the mandibula and maxilla which was extended in front of the fetal forehead (MM-line), as the distance of the leading edge of the frontal bone to prenasal skin (d1) divided by the distance of the prenasal skin to the point where the MM-line is intercepted (d2).1_When

the MM-line crossed behind the edge of the prenasal skin, the d1 measurement was still taken between the frontal bone and skin but the d2 measurement was then taken be-tween the MM-line and skin and multiplied by -1 (see Figure 1).

The FP line was defined as the line that passes through the middle point of the anterior

 Figure 1 — (a) Measurement of the MNM-angle: defined as the angle between the lines maxilla-nasion and mandible-Figure 1 — (a) Measurement of the MNM-angle: defined as the angle between the lines maxilla-nasion in the midsagittal plane.

(b) Measurement of the PFSR: established after drwaing a line from the anterior aspect of both the mandibula and maxilla which was extended in front of the fetal forehead (MM-line), as the distance of the leading edge of the frontal bone to prenasal skin (d1) divided by the distance of the prenasal skin to the point where the MM-line is intercepted (d2). When the MM-line crossed behind the edge of the prenasal skin, the d1 measurement was stilll taken between the frontal bone and skin, but the d2 measurement was then taken between the MM-line and skin and multiplied by -1.

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border of the mandible and the nasion. When the FP line passed the frontal bone ante-riorly its position was called ‘negative’. When it passed lengthwise through the frontal bone this was called ‘zero’, and when it passed the frontal bone posteriorly, its position was called ‘positive’.6_{The PL-distance is the perpendicular distance from the FP line to}

the fetal skin (see Figure 1).

STATISTICAL ANALYSIS

Conventional descriptive statistics were applied to all measured parameters. Feasibil-ity for each measure was expressed as the proportion of images that could be measured relative to the total number of images. The difference in feasibility for each measure be-tween the two operators was tested with the X2_{test (or Fisher’s Exact Test, if appropriate).}

Inter-operator agreement was quantified descriptively with Bland and Altman’s limits of agreement (LOAs) and their 95% confidence intervals, and the bias or mean difference (SD, 95% CI) of operator 1 to that of operator 2. The repeatability coefficient (RC), or the value below which the absolute differences between two measurements would lie with-in 0.95 probability, was calculated. Correlations between the operators’ measurements were expressed using the intraclass correlation coefficient (ICC, two way mixed, single measures). Prediction intervals, indicating the interval in which 95% of the individual prediction would lie for a given CRL value, were calculated for all measures.

Values of the NBL, PNT, MNM-angle, FP line, PL-distance, PNT/NBL-ratio and PFSR of the individual anomaly cases were also expressed as multiples of the expected value for CRL (MoM). A p-value <0.05 (two-sided) was considered a statistically significant difference.

Table 1 — Characteristics

¬ ¬ ¬ ¬ ¬ Normal N=148 ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ Abnormal N=77 ¬ ¬ ¬ ¬ ¬

Mean [CI 95%] Std. deviation [p25-p75]Median Mean [CI 95%] Std. deviation [p25-p75]Median CRL 65.66 [64.17 – 67.15] 9.17 [58.43 – 73.08]66.60 [66.74 – 71.50]69.12 10.42 [61.78 – 76.68]68.90 NT 1.90 [1.80 – 1.99] .59 [1.50 – 2.19]1.80 [3.80 – 4.77]4.29 2.12 [2.80 – 5.30]3.60 Gestational age 12.76 [12.65 – 12.87] .69 [12.14 – 13.29]12.71 [12.95 – 13.33]13.14 .78 [12.60 – 13.71]13.24

CRL = crown rup length, NT = nuchal transluceny

Table 2 — Feasibility and inter-operator differences in obtaining measurements

N=148 Operator 1 (%) Operator 2 (%) Operator 1 and

operator 2 p (operator 1 - operator 2) NBL 110 (74.32) 118 (79.73) 107 (72.30) .269 PNT 98 (66.22) 117 (79.05) 93 (62.84) .013 PNT/NBL ratio 81 (54.73) 105 (70.95) 78 (52.70) .004 MNM angle 112 (75.68) 117 (79.05) 108 (72.97) .487 PL distance 101 (68.24) 109 (73.65) 97 (65.54) .306 PFSR 93 (62.84) 114 (77.03) 91 (61.49) .008

NBL = nasal bone length. PNT = prenasal thickness. PNT/NB ratio = prenasal thickness/nasal bone length ratio. MNM angle = maxilla-nasion-mandible angle. PL distance = profile line distance. PFSR = prefrontal space ratio.

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Results

In total 225 fetal profile images were included in the study; 148 from normal fetuses and 77 from fetuses with a (chromosomal) anomaly (trisomy 21: N=63, trisomy 18: N=4, mi-crognathia: N=5, bilateral cleft: N=2,22q11.21 duplication: N=1, Roberts syndrome: N=1, and Turner syndrome: N=1). Table 1 shows the characteristics of both groups.

Feasibility, expressed as the proportion of how often a marker could be measured, as well as the difference and overlap in feasibility between operators, is shown in Table 2. Feasibility was the highest for the NBL and the MNM angle and the lowest for the PNT/ NBL ratio. Table 3 shows the means and medians of NBL, PNT, PNT/NBL ratio, MNM-angle and PFSR for each operator separately. The mean NBL, PNT and PFSR measured by operator 1 were larger than in operator 2. Operator 2 had a larger mean for the MNM-angle and PL-distance. Both operators had the lowest SD for the PNT/NBL ratio and the

Table 4 — Agreement and correlation between operators

Mean ∆ SD of ∆ ICC (95% CI) SE RC LOA LOA 95% CI NBL

(operator 1-2) .140 .240 (.655 – .910).835 .030 .470 [-.330 – .620] Lower limit (-.400 – -.270)Upper limit (.550 – .680) PNT

(operator 1-2) .080 .220 (.559 – .805).706 .030 .420 [-.340 – .510] Lower limit (-.400 – -.280)Upper limit (.450 – .570) MNM angle

(operator 1-2) -.194 2.111 (.649 – .819).746 .301 4.138 [-4.333 – 3.944] Lower limit (-4.922 – -3.743)Upper limit (3.355 – 4.533) PL distance

(operator 1-2) -.090 .400 (.450 – .711).596 .060 .790 [-.880 – .700] Lower limit (-.990 – -.770)Upper limit (.590 – .810) D1 (part of PFSR

(operator 1-2)) .080 .210 (.548 – .801).700 .030 .417 [-.340 – .500] Lower limit (-.400 – -.280)Upper limit (.440 – .560) D2 (part of PFSR

(operator 1-2)) .030 .790 [.519 – .758].655 .110 1.550 [-1.579 – 1.520] Lower limit (-1.800 – -1.360)Upper limit (1.300 – 1.730)

NBL = nasal bone length. PNT = prenasal thickness. PNT/NB ratio = prenasal thickness/nasal bone length ratio. MNM angle = maxilla-nasion-mandible angle. PL distance = profile line distance. PFSR = prefrontal space ratio. d1 = distance of the leading edge of the frontal bone to prenasal skin. d2 = distance of the prenasal skin to the point where the mm-line is intercepted (d2).

Table 3 — Mean, medians and SD per operator

¬ ¬ ¬ ¬ ¬ Operator 1 ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ Operator 2 ¬ ¬ ¬ ¬ ¬

Mean [CI 95%] Std. deviation [p25-p75]Median Mean [CI 95%] Std. deviation [p25-p75]Median NBL 1.91 [1.81 – 2.00] .500 [1.60 – 2.20]1.80 [1.69 – 1.85]1.77 .440 [1.50 – 2.00]1.70 PNT 1.44 [1.38 – 1.50] .290 [1.20 – 1.63]1.40 [1.26 – 1.38]1.32 .330 [1.10 – 1.50]1.30 PNT/NBL ratio .770 [.724 – .816] .210 [.625 – .914].750 [.711 – .138].756 .232 [.590 – .865].750 MNM angle 4.566 [4.006 – 5.126] 2.990 [2.375 – 5.983]4.300 [4.243 – 5.286]4.760 2.849 [2.815 – 5.955]4.300 PL distance 2.08 [1.99 – 2.17] .463 [1.70 – 2.30]2.00 [2.08 – 2.26]2.17 .490 [1.80 – 2.50]2.20 PFSR .067 [-.069 – .202] .656 [-.335 – .512]0 [-.213 – .084]-.065 .802 [-0.555 – 0.374]0

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Table 5 — Description abnormal cases other than trisomy 21

N=63 Measured % Mean MoM Range MoM >p95 % (N) NBL 49.2* .97 .27 – 1.75 -PNT 95.2 1.43 .76 – 2.52 33.3 (20/60) PNT/NBL ratio 46.0 1.63 .55 – 7.13 37.9 (11/29) MNM angle 96.8 0.98 .13 – 3.33 9.7 (6/61) FP line 95.2 - - -PL distance 93.7 1.16 .66 – 2.61 16.9 (10/59) PFSR** 88.9 - -

-* In 50.8% (32/63) of the trisomy 21 cases the nasal bone was absent and as a consequence could not be measured. ** No MoMs could be calculated.

Table 6 — Description abnormal cases other than trisomy 21

N=63 NBL

(MoM) (MoM)PNT ratio (MoM)PNT/NBL MNM angle(MoM) PL distance(MoM) PFSR* FP line 1 Duplication 22q11.21

with palatoschisis and micrognathia

- 1.4 (1.07) - 5.66 (0.96) 2.0 (.95) .43 0 2 Roberts syndrome with

micrognathia, club feet, and short upper arms

3.3 (1.75) 1.7 (1.30) .52 (.66) 15.90 (2.69) 1.7 (.81) 1.47 0 3 Bilateral complete cleft,

normal karyotype 2.7 (1.43) 1.8 (1.38) .67 (.85) 21.74 (3.68) 1.8 (.85) 0 4 Bilateral cleft, normal

karyotype - 2.0 (1.53) - 30.40 (5.15) 2.0 (.95) 3.50 0 5 Micrognathia 3.6 (1.91) 2.8 (2.14) .78 (.99) 18.60 (3.15) 3.0 (1.42) .65 Neg 6 Micrognathia 2.5 (1.33) 2.4 (1.83) .96 (1.22) 19.80 (3.35) 2.5 (1.18) 1.72 0 7 Micrognathia - .17 (1.30) - 13.50 (2.29) .30 (1.42) .88 Pos 8 Micrognathia .17 (.90) .16 (1.22) .94 (1.20) 14.80 (2.51) .16 (.76) .53 0 9 Micrognathia .16 (.85) .15 (1.15) .94 (1.20) 16.60 (2.81) .17 (.81) 1.00 0 10 Trisomie 18 - 1.8 (1.38) - 6.80 (1.15) 2.3 (1.09) 1.00 Pos 11 Trisomie 18 1.5 (.80) 1.9 (1.45) 1.27 (1.62) 9.40 (1.59) 2.4 (1.14) .39 0 12 Trisomie 18 1.1 (.58) 2.1 (1.60) 1.91 (2.43) 4.30 (0.73) 2.3 (1.09) 1.16 0 13 Trisomie 18 - 1.7 (1.30) - 4.78 (0.81) 1.8 (.85) -.50 0 14 Turner 2.0 (1.06) - - 10.20 (1.73) 2.9 (1.37) -.12

-* No MoMs could be calculated.

NBL = nasal bone length. PNT = prenasal thickness. PNT/NB ratio = prenasal thickness/nasal bone length ratio. MNM angle = maxilla-nasion-mandible angle. PL distance = profile line distance. PFSR = prefrontal space ratio. highest for the MNM-angle.

Agreement and correlation between operators for each measurement are shown in Table 4. The mean difference between operators was the lowest for the measurement of d2 (.030 mm) and the highest for the MNM angle (-.194). The lowest SD was found for the measurements of d1 and the PNT (.210 and .220). The LOAs were the smallest for d1 [-.340 – .500] and the PNT [-.340 – .510].

Correlation between operator 1 and operator 2 was high for the measurement of the

Table 3 — Mean, medians and SD per operator

¬ ¬ ¬ ¬ ¬ Operator 1 ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ Operator 2 ¬ ¬ ¬ ¬ ¬

Mean [CI 95%] Std. deviation [p25-p75]Median Mean [CI 95%] Std. deviation [p25-p75]Median NBL 1.91 [1.81 – 2.00] .500 [1.60 – 2.20]1.80 [1.69 – 1.85]1.77 .440 [1.50 – 2.00]1.70 PNT 1.44 [1.38 – 1.50] .290 [1.20 – 1.63]1.40 [1.26 – 1.38]1.32 .330 [1.10 – 1.50]1.30 PNT/NBL ratio .770 [.724 – .816] .210 [.625 – .914].750 [.711 – .138].756 .232 [.590 – .865].750 MNM angle 4.566 [4.006 – 5.126] 2.990 [2.375 – 5.983]4.300 [4.243 – 5.286]4.760 2.849 [2.815 – 5.955]4.300 PL distance 2.08 [1.99 – 2.17] .463 [1.70 – 2.30]2.00 [2.08 – 2.26]2.17 .490 [1.80 – 2.50]2.20 PFSR .067 [-.069 – .202] .656 [-.335 – .512]0 [-.213 – .084]-.065 .802 [-0.555 – 0.374]0

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Figure 2(a-f) — Shows the reference ranges for the NBL, PNT, PNT/NBL ratio, MNM-angle, PL-distance and PFSR in relation to CRL (from 45 - 84 mm for all measurements) in normal fetuses and in a cohort abnormal fetuses. (c) Measurement of the PL-distance: perpendicular distance from the FP line to the fetal skin.

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NBL (ICC .84), MNM-angle (ICC .746), PNT (ICC .71) and d1 (ICC .70) (Table 4). Correla-tion for d2 and the PL-distance was moderate, with an ICC of .66 and .60 respectively. Figures 2a-f show the reference ranges for NBL, PNT, PNT/NBL ratio, MNM-angle, PL-distance and PFSR in relation to CRL (from 45 to 84 mm for all measurements) in nor-mal fetuses. The mean NBL increased with CRL from 1.3 mm to 2.6 mm. Mean PNT also increased with CRL, from 1.2 mm to 2.0 mm. The mean PNT/NBL ratio decreased slightly with CRL, from .69 to .44.

The mean MNM-angle increased with CRL, from 0.98 degrees to 7.76 degrees. The mean PFSR increased with CRL, from -.97 to .74. The PL was positive in 13/148 fetuses (0.088, 95%CI [.048 – .146]).

Figures 2a-f also show the measurements of the markers in relation to CRL in the ab-normal cases in comparison to the ab-normal ones.

Table 5 shows the characteristics of the trisomy 21 cases. Absence of the nasal bone, an increased MoM of the PNT and PNT/NB ratio were most common in trisomy 21 cases. Table 6 shows the characteristics of the abnormal cases other than trisomy 21. In the cases of bilateral cleft and micrognathia the MNM-angle MoMs were all increased above the p95.

Discussion

This study demonstrates that second trimester profile parameters can be measured reli-ably in the first trimester with a high inter-operator agreement, especially for the NBL, the PNT and the MNM-angle.

Success in obtaining a measurement was operator and marker-dependent. A possible explanation for this is that the selected images were not taken with the purpose of mea-suring the fetal facial markers, but rather the NT. Furthermore, operators may interpret certain landmarks of a marker differently, implying that adherence to strict criteria would be necessary in order to minimize these differences.

In normal fetuses the NBL and the PNT increase with CRL2-4,12-15_{, whereas the PNT/NBL}

ratio decreases, suggesting that the growth of the NBL in early pregnancy exceeds that of the PNT. Conversely, in the second and third trimester of pregnancy NBL and PNT seem to grow at a similar rate, resulting in a constant ratio in both normal and abnormal fetus-es.2_{Although operators had a different success rate in obtaining the PNT measurement,}

the PNT/NBL ratio had the lowest SD between operators, meaning that it can be already reliably measured in the first trimester of pregnancy.

The MNM-angle and PFSR increase with CRL, whereas in the second and third trimes-ter the MNM-angle and PFSR remain constant.1,57

The increase of the MNM-angle in the first trimester can be explained by a more distinct growth and forward displacement of the maxilla compared to the mandibula, which is only connected to the cranium through the temporomandibular joint and facial muscles and as a consequence the forces producing the development of the mid face with forward displacement of the maxilla could have a lesser effect on the mandibular growth.16,17

The increase of the PFSR can be explained by forward displacement and growth of the maxilla, with a growth rate that seems to exceed that of the prenatal thickness. The

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mea-surement of PFSR proved to be challenging: incorrect placement of the MM-line of only a few tenths of a millimeter has a large impact on the measured angle and consequently on the measurement of d2. In spite of a recent adaptation of the PFSR to facilitate its mea-surement in the first trimester, its detection rate for trisomies remained still about half of that in the second and third trimester.18

The PL-distance decreases with increasing CRL; this seems to be the result of forward movement of the maxilla and a decrease of convexity of the forehead in the first trimes-ter.16_{Inter-operator agreement was low, possibly because a slightly different start of the}

line at the mandible or a different interpretation of the nasion has a major impact on the measured PL distance.

We speculate that due to the small dimension of the fetal face in the first trimester and the high susceptibility to inaccuracies in placing lines with different landmarks, the PFSR and the PL-distance cannot be measured with sufficient reliability.

In abnormal fetuses the evaluation of the facial markers revealed that prenasal edema can already be present in the first trimester. The PNT was increased and the NB was ab-sent in the majority of the trisomy 21 and 18 cases in which it could be measured, which is in line with the literature.10,19_{The PNT/NBL ratio can therefore also be a marker for}

trisomies in the first trimester.14

Also in abnormal cases the PFSR, the FP and the PL distance line showed limitations, showing inconsistent trends. An explanation for the heterogenous results might be that in some fetuses with trisomy 21 the PNT is increased, whereas the growth and forward displacement of the maxilla lags behind, resulting in a smaller d2 and decreased PFSR. In cases where the PNT is normal or mildly increased, but d2 markedly increased because of severe midfacial hypoplasia, the PFSR will be increased.

The FP line showed no clear trends in trisomic fetuses, although a negative FP line seems indicative of pathology.6

At variance with Yazdi1_{et al, who found an abnormal PL-distance in 41.1% of the}

tri-somy 21 cases, we found an abnormal PL-distance in 18.6% (11/59), with a mean MoM of 1.16. This could be explained by the different measurement of the PL-distance where Yazdi et al use the maxilla instead of the nasion as second landmark. In view of the high inter-operator variability and different causes of an increased PL-distance (e.g. skin ede-ma, smaller jaw, or measurement error) we do not envisage a role of the PL-distance in the first trimester of pregnancy.

As in the second trimester, the MNM-angle was increased in the bilateral cleft, micro/ retrognathia, 22q11.21 duplication and Roberts syndrome cases and in one case of triso-my 18 with a suspicion of micrognathia suggesting that this could be used as an accurate and reproducible early marker for micrognathia and facial clefts.

Limitation of the study is that measurements were performed on first trimester pic-tures originally taken with the purpose of measuring the NT and not specifically to visu-alize facial landmarks. As a consequence feasibility only relates to the success in obtain-ing measurements in the preselected images. At this moment, where the future of first trimester ultrasound screening for chromosomal anomalies is uncertain, it is difficult to speculate on the additional role of the facial markers. Their most important role may be found in the setting of early ultrasound screening for fetal anomalies, as objective tool to assist in the detection of abnormal facial feature isolated or as part of genetic syndromes. Due to the limited number of abnormal cases and to the retrospective design of the

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study, it is still premature to draw firm conclusions on the possible role of the facial markers in first trimester screening.

Conclusion

The PNT and PNT/NBL ratio are already abnormal in a great proportion of first trimester trisomic fetuses, however, as other strong first trimester ultrasound markers for aneu-ploidies are available, it is unlikely these will be of use in improving existing first trimes-ter screening algorithms. The MNM angle seems to be an easy and reproducible marker to assist in first trimester detection of micrognathia and facial clefts. Facial markers may especially play a role as adjunct in the early diagnosis of structural anomalies and genetic syndromes other than trisomies.

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18. Yazdi B., Riefler P., Fischmuller K., Sonek J., Hoopmann M., Kagan K. O. The frontal space measurement in euploid and aneu-ploid pregnancies at 11-13 weeks' gestation. Prenat Diagn 2013 Dec; 33(12): 1124-1130. 19. Orlandi F., Bilardo C. M., Campogrande M.,

Krantz D., Hallahan T., Rossi C., Viora E. Measurement of nasal bone length at 11-14 weeks of pregnancy and its potential role in Down syndrome risk assessment. Ultra-sound Obstet Gynecol 2003 Jul; 22(1): 36-39. 20. Cicero S., Longo D., Rembouskos G.,

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