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Biometria 11 14

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AYUDA PARA DIAGNOSTICO PRENATAL.

AYUDA PARA DIAGNOSTICO PRENATAL.

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  • 1. Ultrasound Obstet Gynecol 2002; 20: 564–574Fetal transabdominal biometry at 11–14 weeks of gestationBlackwell Science, LtdC.S. VON KAISENBERG*, E. FRITZER*, H. KÜHLING* and W. JONAT**Department of Obstetrics and Gynecology, University of Kiel, Kiel, GermanyK E Y W O R D S: Biometry, First trimester, Growth charts, Nuchal translucency, Skeletal dysplasiaA BSTRACT INTRODUCTIONObjective To establish comprehensive transabdominal ultra- Previous studies during the first trimester have predomin-sonographic reference ranges for viable normal singleton antly used transvaginal ultrasound to assess parametershuman fetuses at 11–14 weeks’ gestation. such as the gestational sac, amniotic sac and yolk sac, the embryonic or fetal crown−rump length (CRL), the biparietalMethods Single transabdominal ultrasound measurements parameter and the femur length. Their main aim was towere taken once per pregnancy at a gestational age of determine the gestation from ultrasound examination andbetween 11+0 and 14+0 weeks (crown−rump length, 45– to predict the estimated date of delivery or to search for84 mm), in viable singleton fetuses with nuchal translucency ominous signs of pregnancy failure1–9. Further studies≤ 3 mm and without detectable structural anomalies, using established first-trimester transvaginal growth charts forfour standard planes: (i) biparietal diameter (BPD) and the biparietal diameter (BPD), head circumference (HC),fronto-occipital diameter (FOD) resulting in head circumfer- transverse cerebellar diameter (TCD), abdominal diametersence (HC), anterior horn (Va), posterior horn (Vp), and hemi- and abdominal circumference, femur and humerus length,sphere (HEM); (ii) transcerebellar diameter (TCD) and and foot length8.cisterna magna (CM); (iii) abdominal anteroposterior (AAP) The objective of this study was to create comprehensiveand abdominal transverse diameter (ATD) resulting in growth charts using transabdominal ultrasound at 11–14abdominal circumference (AC); and (iv) femur length (FL). weeks, the gestational age at which nuchal translucency (NT)The respective ratios Va/HEM, Vp/HEM, HC/AC, BPD/FL, is measured. Such charts may facilitate the interpretation ofBPD/FOD, FL/CRL, FL/BPD and FL/AC and the estimated first-trimester biometry regarding early onset symmetricalweight were derived. Reference ranges were constructed or asymmetrical growth restriction, the interpretation ofand the mean and 5th and 95th centiles were plotted against abnormal measurements associated with aneuploidies, andgestation. the detection of severe forms of skeletal dysplasias.Results There was a general increase in biometric parame-ters with gestation. The ratios for the ventricles vs. hemi- M ET H O D Ssphere and BPD/FL ratio decreased while the BPD/FOD andHC/AC ratios remained constant. Analysis of the reference Only viable singleton pregnancies were included in the study.range for BPD/FL was performed in both 167 and 664 Each fetus was measured once during the period 11–fetuses and the results showed almost the identical type of 14 weeks (CRL, 45–84 mm). In all cases, there was an unre-equation, indicating a high degree of accuracy for the growth markable maternal history including a regular menstrualcharts. period and absence of oral contraception or pregnancy dur- ing the previous 3 months. No correction of dates using theConclusions We have established comprehensive reference CRL was performed. In all cases, a CRL was taken and NTranges for first-trimester fetal biometry by transabdom- was 0.7–3 mm and there were no obvious structural fetalinal sonography. These charts may have a role in the anomalies. Fetuses which subsequently miscarried and thosediagnosis of early onset symmetrical or asymmetrical with chromosomal abnormalities were excluded from thegrowth restriction and in the interpretation of measure- calculation of normal reference ranges. All measurementsments in chromosomally abnormal fetuses, and they were performed by one experienced sonographer usingmay help in the detection of skeletal dysplasias or acrania/ transabdominal ultrasound (ATL SonoCT and tissue har-anencephaly. monic imaging software, ATL (Philips), Solingen, Germany).Correspondence: Dr C.S. von Kaisenberg, Department of Obstetrics and Gynecology, University of Kiel, Michaelisstr 16, 24105 Kiel, Germany(e-mail: vkaisenberg@email.uni-kiel.de)Accepted 28-08-02564 ORIGINAL PAPER
  • 2. First-trimester biometry von Kaisenberg et al.This sonographer was a holder of The Fetal Medicine anterior and posterior cerebral ventricle diameter (Va andFoundation’s Certificate of Competence for first-trimester Vp), and hemisphere (HEM) were obtained from a transversescanning and the unit had been a first-trimester training center axial plane of the fetal head showing a central midline echofor several years. broken in the anterior third by the cavum septi pellucidi, if No reproducibility studies were carried out, but previous already present, and demonstrating the anterior and pos-studies investigating variability for NT measurements found terior horns of the lateral ventricles. BPD and FOD werethat in 95% of cases the intraobserver, interobserver and cal- measured from the outer border of the skull and HC wasiper placement repeatability of measuring fetal NT were less calculated (3.14 × (BPD + FOD)/2). Va was the distancethan 0.54 mm, 0.62 mm and 0.58 mm, respectively10. The between the lateral wall of the anterior horn to the midlinemeasurements under investigation in this study were larger and Vp was the distance between the medial and lateral wallsthan normal NT measurements and therefore it could be rea- of the posterior horn. The hemisphere was measured fromsonably assumed that, with the same standard of scanning as the midline to the inner border of the skull. TCD and cisternafor NT, the potential relative variability would be much magna diameter (CM) were measured in the suboccipito-lower in this study than for the NT study10. bregmatic plane of the head. The femur length (FL) was meas- Four planes were used as previously described for the ured from the greater trochanter to the lateral condyle. Forsecond trimester11 and examples are shown in Figures 1–4. abdominal circumference (AC), a transverse section of theMeasurements of BPD, fronto-occipital diameter (FOD), fetal abdomen was obtained at the level of the stomach andFigure 1 Axial standard view of the fetal skull and brain showing the falx cerebri, cavum septi pellucidi and lateral ventricles with the choroid plexusat 20 weeks (left) and at 13+3 weeks (right) and the respective measurements biparietal diameter, fronto-occipital diameter, anterior horn, posteriorhorn and hemisphere.Figure 2 Axial standard view of the skull, showing the cerebellum and cisterna magna at 20 weeks (left) and at 13+3 weeks (right) and the respectivemeasurements transcerebellar diameter and cisterna magna.Ultrasound in Obstetrics and Gynecology 565
  • 3. First-trimester biometry von Kaisenberg et al.Figure 3 Transverse standard view through the fetal abdomen, showing the spine, ribs, stomach, and bifurcation of the portal vein at 20 weeks (left)and at 13+3 weeks (right) and the respective measurements abdominal anteroposterior diameter and abdominal transverse diameter.Figure 4 Longitudinal standard view of the fetal femur at 20 weeks (left) and at 13+3 weeks (right) and the measurement of femur length.the bifurcation of the main portal vein into its right and left a Gaussian distribution with a mean and standard deviationbranches. The anteroposterior (AAP) and transverse (ATD) and that, in general, both vary smoothly with gestational age.diameters were measured and AC was calculated (3.14 × The estimation of functions for mean and standard deviation(AAP + ATD)/2). The following ratios and the estimated fetal is based on regression analysis. If the deviation increasesweight12 (BPD,AC) were calculated: HC/AC, BPD/FL, BPD/ strongly with gestational age, it is easier to model the loga-FOD, Va/HEM, Vp/HEM11. Subsequently, FL/CRL, FL/BPD rithm of the measurements than the original values. In thisand FL/AC were derived. Measurements were stored in the study this had to be done seven of 21 times. After the invest-Fetal Database (Viewpoint, General Electrics, Wesling, igation of the deviation the function for the mean is calcu-Germany). Statistical analysis was carried out and reference lated by using a least-squares regression analysis. First a cubicranges were calculated using a mathematical model described polynomial is fitted. If the cubic coefficient is not significantlyby Royston and Wright13. We obtained the mean and 5th and different from zero, then a quadratic polynomial is fitted and95th centiles for 11+0, 12+0, 13+0, and 14+0 weeks and the same assessment is made of the quadratic coefficient. Thismeasurements were plotted against gestation. process is repeated until no further removal of terms is pos- sible. If it is necessary to fit polynomials of degrees higher than cubic, fractional polynomials should be used; however,Details of statistical methods in this study this was not necessary. When the measurementThe calculation of reference ranges for the biometry meas- has approximately a Gaussian distribution, the fitted valuesurements of the 11–14-week scan followed closely the pro- following regression of scaled absolute residuals on age arecedure described by Royston and Wright13. This method an estimation of the standard deviation curve. If there isassumes that, at each gestational age, the measurements have no trend with gestational age, the standard deviation is566 Ultrasound in Obstetrics and Gynecology
  • 4. First-trimester biometry von Kaisenberg et al.estimated as the standard deviation of the unscaled residuals. identical type of equation, indicating a high degree of accur-If there is a trend, then polynomial regression analysis is acy for the growth charts. Measurements for TCD (n = 120) andneeded in the same way as for the mean. The assumption of a CM (n = 117) could not be taken in all cases from 11+0Gaussian distribution is tested by using Z-scores. A normal weeks, but at 14 weeks the average score for the cerebellumprobability plot of the Z-scores gives an impression of the was 3.2 (scores: 1, not seen; 2, uncertain seen; 3, accept-degree to which the values follow a Gaussian distribution. able seen; 4, well seen; 5, very well seen), which is compatibleThis can be verified formally by the Shapiro−Francia W′ test14 with the observation that the cerebellum can be measuredand by the Shapiro−Wilk test15,16 and all P-values using both transabdominally in nearly all cases at 14 weeks.tests were > 0.05. A plot of the Z-scores against gestational All fetal biometric parameters showed an increase withage should be randomly scattered about zero. If there appears gestation. The ratios for the ventricles and the BPD/FL ratioa trend or there is a deviation from a Gaussian distribution, decreased while the BPD/ FOD and HC/AC ratios remainedthen the whole computation must be done again after a suit- constant with increasing gestation. The distribution of normalable transformation of the measurements. However, in this ranges and numbers per week are provided in Tables 1–22.study this was not the case. Once a satisfactory model has been Equations for calculation of the mean and centiles for eachdetermined, the centile curves for the reference interval are parameter are given in Table 23.calculated by substituting the expressions for the mean andstandard deviation into the following equation: centile = mean DISCUSSION+ K × standard deviation, where K is the corresponding cen-tile of the standard Gaussian distribution. If the estimation All fetal biometric parameters showed an increase with ges-of the curves is done on a log scale, this must be considered. tation while the ratios for the ventricles and the BPD/FL ratio decreased and the BPD/ FOD and HC/ AC ratios remained constant with increasing gestation. Previous studies at 14–RE SUL TS 40 weeks’ gestation have established likelihood ratios forMeasurements for FOD (n = 167), Va (n = 162), Vp (n = 162), fetal chromosomal defects using specific alterations of fetalHEM (n = 162) and AAP and ATD (n = 164) were made. biometry such as brachycephaly (BPD/ FOD > 95th centile),Measurements for BPD (n = 675) and FL (n = 664) were taken ventriculomegaly (Va/ HEM or Vp/ HEM > 95th centile),more frequently as part of the routine for the first-trimester microcephaly (HC < 5th centile), enlarged cisterna magnascan, which also included CRL (n = 660) and NT (n = 660). (CM > 95th centile), short femur (FL < 5th centile or BPD/An analysis of the reference range for BPD/FL was performed FL > 95th centile), and small-for-gestational age (AC < 5thin both 167 and 664 fetuses and the results showed an almost centile)17. The CRL of trisomy 18 fetuses is known to beTable 1 Distribution of measurements for the crown−rump length (mm)against gestation (weeks), 90% confidence interval (n = 660)Weeks 5th centile Mean 95th centile11+0 40.5 46.8 53.212+0 49.6 58.6 67.613+0 60.9 70.4 79.814+0 74.6 82.1 89.6Table 2 Distribution of measurements for the biparietal diameter (mm)against gestation (weeks), 90% confidence interval (n = 675)Weeks 5th centile Mean 95th centile11+0 14.1 17.3 20.512+0 17.7 20.9 24.113+0 21.4 24.5 27.714+0 25.0 28.2 31.3Ultrasound in Obstetrics and Gynecology 567
  • 5. First-trimester biometry von Kaisenberg et al.Table 3 Distribution of measurements for the fronto-occipital diameter(mm) against gestation (weeks), 90% confidence interval (n = 167)Weeks 5th centile Mean 95th centile11+0 15.9 19.8 23.712+0 20.5 24.4 28.313+0 25.1 29.0 32.914+0 29.7 33.6 37.5Table 4 Distribution of measurements for the head circumference (mm)against gestation (weeks), 90% confidence interval (n = 167)Weeks 5th centile Mean 95th centile11+0 47.1 57.2 67.312+0 60.5 70.6 80.713+0 73.9 84.0 94.114+0 87.3 97.4 107.5Table 5 Distribution of measurements for the cisterna magna (mm)against gestation (weeks), 90% confidence interval (n = 117)Weeks 5th centile Mean 95th centile11+0 0.9 1.4 2.012+0 1.1 1.6 2.413+0 1.3 2.0 2.914+0 1.6 2.3 3.4Table 6 Distribution of measurements for the transverse cerebellar dia-meter (mm) against gestation (weeks), 90% confidence interval(n = 120)Weeks 5th centile Mean 95th centile11+0 5.2 6.7 8.512+0 7.1 9.1 11.613+0 8.6 11.0 14.014+0 9.1 11.7 15.0568 Ultrasound in Obstetrics and Gynecology
  • 6. First-trimester biometry von Kaisenberg et al.Table 7 Distribution of measurements for the anterior cerebral ven-tricle (mm) against gestation (weeks), 90% confidence interval (n = 162)Weeks 5th centile Mean 95th centile11+0 2.2 3.3 4.412+0 3.0 4.1 5.213+0 3.8 4.9 6.014+0 4.6 5.7 6.7Table 8 Distribution of measurements for the posterior cerebral ven-tricle (mm) against gestation (weeks), 90% confidence interval (n = 162)Weeks 5th centile Mean 95th centile11+0 2.2 3.1 4.012+0 2.9 4.0 5.113+0 3.7 5.0 6.314+0 4.4 5.9 7.4Table 9 Distribution of measurements for the hemisphere diameter(mm) against gestation (weeks), 90% confidence interval (n = 162)Weeks 5th centile Mean 95th centile11+0 4.8 6.4 8.012+0 6.4 8.0 9.613+0 8.0 9.6 11.214+0 9.6 11.2 12.8Table 10 Distribution of measurements for the abdominal transversediameter (mm) against gestation (weeks), 90% confidence interval(n = 164)Weeks 5th centile Mean 95th centile11+0 12.8 15.0 17.612+0 15.4 18.1 21.313+0 18.6 21.8 25.714+0 22.4 26.3 30.9Ultrasound in Obstetrics and Gynecology 569
  • 7. First-trimester biometry von Kaisenberg et al.Table 11 Distribution of measurements for the abdominalanteroposterior diameter (mm) against gestation (weeks), 90%confidence interval (n = 164)Weeks 5th centile Mean 95th centile11+0 12.2 14.3 16.412+0 15.2 18.1 21.113+0 18.1 21.9 25.714+0 21.1 25.7 30.4Table 12 Distribution of measurements for the abdominal circumference(mm) against gestation (weeks), 90% confidence interval (n = 164)Weeks 5th centile Mean 95th centile11+0 35.7 45.4 55.112+0 47.5 57.2 66.913+0 59.4 69.1 78.814+0 71.2 80.9 90.6Table 13 Distribution of measurements for the femur length (mm)against gestation (weeks), 90% confidence interval (n = 664)Weeks 5th centile Mean 95th centile11+0 3.2 4.8 7.012+0 4.7 6.6 9.213+0 6.8 9.1 12.214+0 9.9 12.6 16.1Table 14 Distribution of the ratio of anterior cerebral ventricle dia-meter (Va) to hemisphere (HEM) against gestation (weeks), 90%confidence interval (n = 162)Weeks 5th centile Mean 95th centile11+0 0.349 0.513 0.67812+0 0.379 0.513 0.64713+0 0.410 0.513 0.61614+0 0.441 0.513 0.585570 Ultrasound in Obstetrics and Gynecology
  • 8. First-trimester biometry von Kaisenberg et al.Table 15 Distribution of the ratio of posterior cerebral ventricle dia-meter (Vp) to hemisphere (HEM) against gestation (weeks), 90%confidence interval (n = 162)Weeks 5th centile Mean 95th centile11+0 0.356 0.504 0.71412+0 0.377 0.504 0.67513+0 0.398 0.504 0.63914+0 0.421 0.504 0.604Table 16 Distribution of the ratio of head circumference (HC) toabdominal circumference (AC) against gestation (weeks), 90%confidence interval (n = 164)Weeks 5th centile Mean 95th centile11+0 1.085 1.231 1.37612+0 1.085 1.231 1.37613+0 1.085 1.231 1.37614+0 1.085 1.231 1.376Table 17 Distribution of the ratio of biparietal diameter (BPD) to femurlength (FL) against gestation (weeks), 90% confidence interval (n = 664)Weeks 5th centile Mean 95th centile11+0 2.626 3.709 5.23912+0 2.347 3.152 4.23213+0 2.097 2.678 3.42014+0 1.873 2.275 2.764Table 18 Distribution of the ratio of biparietal diameter (BPD) tofronto-occipital diameter (FOD) against gestation (weeks), 90%confidence interval (n = 167)Weeks 5th centile Mean 95th centile11+0 0.766 0.846 0.92612+0 0.766 0.846 0.92613+0 0.766 0.846 0.92614+0 0.766 0.846 0.926Ultrasound in Obstetrics and Gynecology 571
  • 9. First-trimester biometry von Kaisenberg et al.Table 19 Distribution of measurements for the ratio of femur length(FL) to crown−rump length (CRL) against gestation (weeks), 90%confidence interval (n = 660)Weeks 5th centile Mean 95th centile11+0 0.069 0.104 0.13912+0 0.081 0.115 0.14913+0 0.098 0.132 0.16614+0 0.120 0.155 0.189Table 20 Distribution of measurements for the ratio of femur length(FL) to biparietal diameter (BPD) against gestation (weeks), 90%confidence interval (n = 663)Weeks 5th centile Mean 95th centile11+0 0.188 0.283 0.37812+0 0.228 0.323 0.41813+0 0.285 0.380 0.47514+0 0.358 0.453 0.548Table 21 Distribution of measurements for the ratio of femur length(FL) to abdominal circumference (AC) against gestation (weeks), 90%confidence interval (n = 164)Weeks 5th centile Mean 95th centile11+0 0.062 0.093 0.12312+0 0.083 0.114 0.14413+0 0.104 0.134 0.16514+0 0.125 0.155 0.186Table 22 Distribution of the estimated fetal weight (BPD,AC)12 againstgestation (weeks), 90% confidence interval (n = 158)Weeks 5th centile Mean 95th centile11+0 52 61 7112+0 65 75 8813+0 80 93 10814+0 99 115 134572 Ultrasound in Obstetrics and Gynecology
  • 10. First-trimester biometry von Kaisenberg et al.Table 23 Equations for calculation of mean and centiles Table 23 ContinuedCrown–rump length (CRL) BPD/FOD Mean −82.4827 + 11.7565·wks Mean 0.8461 5th centile 30.8275 − 2.7528·wks + 0.0300·wks3 5th centile 0.7661 95th centile −195.7929 + 26.2658·wks − 0.0300·wks3 95th centile 0.9260Biparietal diameter (BPD) FL/CRL Mean −22.5100 + 3.6190·wks Mean 0.3696 − 0.0565·wks + 0.0029·wks2 5th centile −25.6892 + 3.6190·wks 5th centile 0.3354 − 0.0565·wks + 0.0029·wks2 95th centile −19.3309 + 3.6190·wks 95th centile 0.4038 − 0.0565·wks + 0.0029·wks2Fronto-occipital diameter (FOD) FL/BPD Mean −30.9727 + 4.6122·wks Mean 0.9399 − 0.1510·wks + 0.0083·wks2 5th centile −34.8603 + 4.6122·wks 5th centile 0.8450 − 0.1510·wks + 0.0083·wks2 95th centile −27.0850 + 4.6122·wks 95th centile 1.0348 − 0.1510·wks + 0.0083·wks2Head circumference (HC) FL/AC Mean −90.3529 + 13.4114·wks Mean −0.1360 + 0.0208·wks 5th centile −100.4502 − 13.4114·wks 5th centile −0.1666 + 0.0208·wks 95th centile −80.2556 + 13.4114·wks 95th centile −0.1054 + 0.0208·wksCisterna magna (CM) Weight (BPD,AC) Mean exp(−1.5985 + 0.1749·wks) Mean exp(1.7757 + 0.2122·wks) 5th centile exp(−1.9801 + 0.1749·wks) 5th centile exp(1.6256 + 0.2122·wks) 95th centile exp(−1.2168 + 0.1749·wks) 95th centile exp(1.9259 + 0.2122·wks)Transcerebellar diameter (TCD) Mean exp(−9.5418 + 1.7103·wks − 0.0609·wks2) 5th centile exp(−9.7859 + 1.7103·wks – 0.0609·wks2) 95th centile exp(−9.2977 + 1.7103·wks − 0.0609·wks2) below the 5th centile in first-trimester fetuses18. CertainAnterior cerebral ventricle (Va) transvaginally obtained ratios between BPD and femur and Mean −5.3349 + 0.7851·wks humerus lengths, HC and AC and foot length have been 5th centile −6.4276 + 0.7851·wks 95th centile −4.2421 + 0.7851·wks investigated in the first trimester with respect to their useful-Posterior cerebral ventricle (Vp) ness as markers of aneuploidies19 but only trisomy 13 fetuses Mean −7.2059 + 0.9360·wks showed abnormal values for HC/ AC, BPD/ HL and BPD/FL, 5th centile −5.8386 + 0.7303·wks and other studies have shown relatively short femur to be of 95th centile −8.5731 + 1.1416·wks limited use for the detection of trisomy 21 before 18 weeks20.Hemisphere (HEM) Mean −11.3596 + 1.6124·wks Early measurements of BPD were also used to predict long 5th centile −12.9822 + 1.6124·wks bone growth in later pregnancy21, and early transvaginal 95th centile −9.7369 + 1.6124·wks diagnosis of skeletal dysplasias has also been attempted usingAbdominal transverse diameter (ATD) FL against CRL and FL against BPD ratios with good predic- Mean exp(0.6448 + 0.1875·wks) tion in severe cases22. Our data for FL vs. gestation, FL/CRL, 5th centile exp(0.4833 + 0.1875·wks) 95th centile exp(0.8064 + 0.1875·wks) and FL/ BPD correspond well with those of Gabrielli et al.22.Abdominal anteroposterior diameter (AAP) The FL/ AC ratio in later pregnancy has also been used in the Mean −27.5424 + 3.8053·wks prediction of the severity of skeletal dysplasia and a ratio 5th centile −20.0957 + 2.9392·wks < 0.16 resulted in a lethal outcome in all nine cases studied, 95th centile −34.9890 + 4.6714·wks whereas a ratio ≥ 0.16 resulted in a non-lethal form of skeletalAbdominal circumference (AC) Mean −85.0484 + 11.8552·wks dysplasia or in a healthy child23. 5th centile −94.7459 + 11.8552·wks Studies on the crown−chin length have shown that detailed 95th centile −75.3510 + 11.8552·wks first-trimester biometry may help in detecting specific mal-Femur length (FL) formations such as acrania/anencephaly24. Mean exp(−2.0220 + 0.3255·wks) In conclusion, the establishment of comprehensive refer- 5th centile exp(−2.9381 + 0.3737·wks) 95th centile exp(−1.1058 + 0.2772·wks) ence ranges for first-trimester transabdominal fetal biometryVa/HEM may allow the diagnosis of early onset symmetrical or asym- Mean 0.5131 metrical intrauterine growth restriction and the interpreta- 5th centile 0.0102 + 0.0307·wks tion of measurements in chromosomally abnormal fetuses 95th centile 1.0159 − 0.0307·wks and may help in detecting skeletal dysplasias or acrania/anen-Vp/HEM Mean exp(−0.6845) cephaly. The use of these biometric measurements in addition 5th centile exp(−1.6418 + 0.0554·wks) to measurement of NT, first-trimester biochemistry, first- 95th centile exp(0.2728 − 0.0554·wks) trimester ductus venosus Doppler sonography and first-Head circumference/abdominal circumference trimester anomaly scanning may contribute to a total shift Mean 1.2305 towards first-trimester screening. 5th centile 1.0849 95th centile 1.3760BPD/FL R EF ER EN C ES Mean exp(3.1027 − 0.1629·wks) 5th centile exp(2.2044 − 0.1126·wks) 1 Hadlock FP, Harrist RB, Deter RL, Park SK. Fetal femur length as a 95th centile exp(4.0010 − 0.2131·wks) predictor of menstrual age: sonographically measured. AJR Am J Roentgenol 1982; 138: 875–8Ultrasound in Obstetrics and Gynecology 573
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