Screening for intrauterine growth retardation using the ultrasound biparietal diameter

Fifteen hundred patients were scanned to predict fetal age and weight by biparietal diameter measurement. Three hundred eleven patients had more than one scan. Fortythree small for gestational age (SGA) babies were ultimately delivered in this population. Prenatal ultrasound screening criteria for grouping into appropriate for gestational age (AGA) or small for gestationa1age (SGA) categories were: a only an absolute biparietal diameter below the third percentile for the reference Rochester region population; b only ΔBPD calculated as less than 50 percent of the mean growth rate for the reference population; and c a combination of these two factors. Intrauterine growth retardation was most accurately determined when an absolutely small biparietal diameter was found (at the time of the last ultrasound examination) in a woman with accurate gestational age assessment. Least accurate was the prediction based on one biparietal diameter measurement in a woman with poor clinical dates. A 50 percent false positive detection rate and poor sensitivity to intrauterine growth retardation were found using ultrasound biparietal diameter measurements as a screening test in this manner.     

Coronal biparietal diameter. A reliable alternative to the traditional transverse biparietal diameter

The traditional ultrasonically derived fetal biparietal diameter (BPD) is important in the determination of fetal age, weight, and growth. This measure cannot be obtained, however, with direct occiput anterior or posterior or with a head in the pelvis. This study looked at the BPD utilizing coronal (anatomically at right angles to the traditional BPD through the thalami, third ventricle, and cavum septum pellucidum) rather than transverse section through the fetal head. In 265 patients at 14 to 41 weeks' gestation, simultaneous measurement of the coronal and transverse BPDs revealed that they were interchangeable (r = 0.99). The major drawback of the coronal BPD is the inability to obtain a head circumference because of the absence of the occipitofrontal diameter.     

Gray scale measurement of biparietal diameter.

Fetal biparietal diameter measurements were obtained simultaneously by gray scale and bistable ultrasonography. The bistable presentation was measured from leading edge to leading edge, while the gray scale presentation was measured from leading point to leading point and middle to middle of the band of echoes defining the fetal head. Statistical analysis of 50 patients demonstrated good reproducibility of the biparietal diameter measurement by each method and intraobserver variation fell well within the acceptable range. In all but one patient differences in measurements between observers and measurement methods were trivial and of no clinical significance. In the patients in whom there was a statistically significant difference, the variability was from 1 to 2 mm with the exception of one patient in whom the variability was about 4 mm.     

Ultrasonography of the fetal thyroid: nomograms based on biparietal diameter and gestational age.

OBJECTIVE: To describe gestational age-dependent and -independent nomograms for fetal thyroid size. METHODS: Two hundred fetuses were evaluated between 16 and 37 weeks' gestation in this cross-sectional study. RESULTS: Nomograms of fetal thyroid size were created by using the 5th, 10th, 50th, 90th, and 95th percentiles based on biparietal diameter and gestational age. A second-order polynomial fit for biparietal diameter and a linear fit for gestational age best described thyroid circumference measurements. Variations in thyroid circumference measurements increased with both larger biparietal diameter and advancing gestational age. There was no intraobserver or interobserver variability in thyroid circumference measurements (P > .20). CONCLUSIONS: Both biparietal diameter and gestational age serve as good predictors of fetal thyroid circumference. When the biparietal diameter is difficult to measure, gestational age can be used to assess thyroid size.     

A standardized plane for biparietal diameter measurement

Biparietal diameter measurement is the most widely accepted means of determining gestational age. However, a standard anatomic plane for performing this measurement has not been established. Three hundred forty-four measurements on 256 patients who gave birth to normal infants were obtained and compared at various cranial levels based on brain anatomy. The results document that use of a standardized plane increases measurement reliability but does not significantly alter variability. We postulate that variability is based predominantly on biologic variation in fetal growth rates rather than on technical errors.     

Assessment of differences in sonar biparietal diameter values regarding fetal age and weight

Various investigators have reported different mean BPD measurements to represent the attainment of a specific gestational week or of a predicted mean weight. These differences are too large to be attributed solely to inherent population variabilities. Nonuniformity in methodology and in standardization of equipment are additional factors contributing to this observed variance. Furthermore in ascertaining the accuracy of sonar BPD measuring methods, comparison of in utero results can only be made with postnatal caliper readings obtained from fetuses delivered by repeat cesarean section. This study demonstrates that the closest correlation between prenatal sonar and postnatal caliper BPD values is obtained when nonpersistent image scanning (NPIS) and electronic calipers are integrated into the procedure of ultrasonic cephalometry. However since the calibration of electronic markers is dependent upon the speed of ultrasound through the skull and brain tissue per unit of time, it is mandatory that a universally agreed upon velocity be used by all manufacturers of ultrasonic equipment. Biparietal diameter differences resulting from inherent population variabilities alone, can only be assessed after uniform methodology is adopted. Until such time, each center utilizing diagnostic sonography must establish and depend on its own intrauterine BPD scale, for the clinical management of patients.     

The relationship of fetal transverse body diameter and biparietal diameter in the diagnosis of intrauterine growth retardation

This study evaluates the relationship of fetal transverse body diameter and biparietal diameter in the diagnosis of intrauterine growth retardation (IUGR). Criteria were formulated which result in 68 per cent sensitivity and 69 per cent specificity for diagnosing IUGR. The sensitivity is comparable to that achieved with other sonographic modalities, and there are important advantages as well. This method does not depend on accurate identification of the last menstrual period, and does not require serial studies. The anatomy for the necessary measurements is readily obtained and reproducible. Subtraction is the only calculation involved. Determination of the relationship of fetal transverse body diameter and biparietal diameter is recommended as a simple screening test for IUGR.     

Estimation of gestational age during the first trimester by real-time measurement of fetal crown-rump length and biparietal diameter

A series of 237 patients in the first trimester of pregnancy were examined to derive normal growth curves for fetal crown-rump length (CRL) and biparietal diameter (BPD) using sector scanner real-time ultrasound equipment. Conventional B-scan CRL measurements were obtained from 50 of these patients: In 41 (82%), the values fell within a range ± 5 mm of those of real-time. Another group of 97 patients between 7 and 13 wk of gestation was then examined in a blind trial to confirm the accuracy of real-time measurements in estimating gestational age. An estimate was made in 95% of cases to within 4.6 days with CRL and to within 5 days with BPD. A similar comparison was made with both of these dimensions using a comparative nomogram, giving a prediction within 3.9 days in 957percnt; of cases. This method is now used routinely.     

Fetal biparietal diameter: a critical re-evaluation of the relation to menstrual age by means of real-time ultrasound

The relationship between fetal biparietal diameter (BPD) and menstrual age was determined by cross-sectional analysis of 533 fetuses (12 to 40 weeks) examined with a linear array real-time (dynamic image) scanner using specifically defined methodology. Mathematical modeling of the data demonstrated that the optimal fit was the linear cubic function (r2 = 99 per cent); predicted BPD values calculated from the function were most comparable with composite data from cross-sectional studies performed with static scanners after 1974 (average difference, 0.22 mm) and least comparable with composite data from cross-sectional studies performed with static scanners before 1974 (average difference, 2.0 mm). The variability associated with predicting menstrual age from the BPD increased progressively throughout gestation; the maximal variability was noted between 36 and 42 weeks (+/- 3.6 weeks). Comparison with our longitudinal study of BPD growth indicates that the cross-sectional data represent a valid estimate of the true longitudinal BPD growth curve of the population.     

Three-dimensional multiplanar ultrasound for fetal gender assignment: value of the mid-sagittal plane.

OBJECTIVE: To evaluate the feasibility and accuracy of fetal gender assignment from three-dimensional ultrasound (3D US) data at 10-24 weeks' gestation. METHODS: Three-dimensional ultrasound volume data on 47 fetuses were reviewed and divided into groups: 10-14, 15-18, and 19-24 weeks. Fetal genitalia were studied in axial and sagittal planes for gender assignment, using published criteria. The most valuable plane for assignment was noted. Gender assignment was compared with gender at birth. RESULTS: Gender assignment was possible in 44 of 47 fetuses; 25 assigned male and 19 female. Between 10 and 14 weeks the mid-sagittal plane alone was diagnostic for all fetuses. Accuracy of assignment between 11 and 14 weeks was 100%. Between 15 and 18 and 19 to 24 weeks, male assignment was 100% accurate. Female assignment was 100% accurate between 15 and 18 weeks. Accuracy decreased to 60% in the 19 to 24 week group, however, in two of the five cases in which gender was wrongly assigned to be male, the assignment was prospectively considered doubtful due to poor resolution of volume data. Excluding these two cases, accuracy for female assignment in the 19-24 week group was 100%. CONCLUSION: Using 3D US, gender assignment was possible in 44 of 47 of fetuses, as early as 11 weeks' gestation. In early gestation, 11-14 weeks, accuracy of male and female gender assignment was 100%.     

A coded cephalometer for interactive multiple measurements of biparietal diameter

A coded cephalometer is described which allows multiple interactive independent measurements of the fetal biparietal diameter at one examination and comparative multiple measurement trials on the same patient using different equipments. The principles of the system are described, and in particular the methods of coding the displayed values and randomising the marker positions. The cephalometer may be used with either a real time scanner or a conventional B scanner for both the assessment of performance and the training of operators.     

Fetal gender assignment by first-trimester ultrasound.

OBJECTIVE: Ultrasound determination of fetal sex can benefit decision-making regarding invasive prenatal testing in pregnancies at risk of sex-linked genetic abnormalities. The aim of this study was to assess the accuracy of fetal sex determination by ultrasound at 12-14 weeks of gestation in a large cohort. METHODS: Fetal gender assessment by transabdominal ultrasound was performed in 656 singleton pregnancies at 12-14 weeks of gestation. The genital region was examined in the mid-sagittal plane. The angle of the genital tubercle to a horizontal line through the lumbosacral skin surface was measured. The fetus was assigned male gender if the angle was > 30 degrees , and female gender if the genital tubercle was parallel or convergent (<10 degrees ) to the horizontal line. At an intermediate angle of 10-30 degrees the gender was not determined. Crown-rump length (CRL) was measured in all cases. RESULTS: Gender assignment was possible in 613 of the 656 (93%) fetuses. Gender identification according to CRL was feasible in 85%, 96% and 97% of the fetuses at gestational ages of 12 to 12 + 3, 12 + 4 to 12 + 6 and 13 to 13 + 6 weeks, respectively. Phenotypic sex was confirmed in 555 newborns. The accuracy of male gender assignment in this group was 99-100% at all ages, and that of female gender assignment was 91.5% at 12 to 12 + 3 weeks, 99% at 12 + 4 to 12 + 6 weeks and 100% at 13 to 13 + 6 weeks. CONCLUSION: Prenatal gender assignment by ultrasound has a high accuracy rate at 12-14 weeks. These results indicate that invasive testing can probably be carried out in fetuses identified as males at this gestational age. However, in fetuses identified as female at a CRL of <62.6 mm, despite the relatively high 91.5% accuracy rate, the decision regarding invasive testing should be postponed until a higher CRL is achieved.     

Sex-specific fetal weight prediction by ultrasound.

OBJECTIVE: To improve sonographic birth-weight prediction by developing fetal gender-specific formulae. METHODS: This was a retrospective cross-sectional study. Two gender-specific formulae were produced from the data of 527 patients and the data of a further 349 patients were used to evaluate the formulae. Inclusion criteria were a singleton live fetus, gestational age above 25 weeks, birth weight between 1000 g and 4500 g and fetal biometry within 8 days of delivery. Data retrieval was specifically for the purpose of this study. RESULTS: To yield the best-fit weight formula for each fetal gender we employed step-wise regression analysis based on fractional polynomials with the biometric parameters biparietal diameter (BPD), head circumference (HC), transverse abdominal diameter (TAD), abdominal circumference (AC) and femur length (FL): estimated fetal weight for girls (g) = - 4035.275 + 1.143 x BPD3 + 1159.878 x AC1/2 + 10.079 x FL3 - 81.277 x FL2 [in cm]; estimated fetal weight for boys (g) = 43576.579 + 1913.853 x log10BPD + 0.01323 x HC3 + 55.532 x AC2 - 13602.664 x AC1/2 - 0.721 x AC3 + 2.31 x FL3 [in cm]. These formulae showed superior results compared with those of conventional weight formulae. CONCLUSION: Gender-related fetal weight calculation allows optimized prediction of fetal weight at birth.     

Analysis of biparietal diameter as an accurate indicator of gestational age

To date, there have been 25 studies attempting to correlate biparietal diameter and gestational age; of these, 17 meet specific criteria and thus are included in this evaluation. Although all studies have shown an increase in biparietal diameter with advancing gestational age, between 17 wk and term there is a discrepancy of approximately 3 wk. In this evaluation the charts are compared and the variations analyzed. An ideal table is constructed by combining the mean gestational age, calculated from a statistical analysis, and the variation at each millimeter point, determined by comparison of all the studies. It is hoped that this new 90% composite table, which consists of a range of gestational ages in weeks for each biparietal diameter, will not only provide more realistic indications of fetal age but also will serve as a reminder to sonographers, obstetricians, and patients of the relative accuracy and variations of biparietal diameter measurements.     

Validation of diameter measurements by ultrasound: intraobserver and interobserver variations assessed in vitro and in fetal sheep.

OBJECTIVES: Blood flow evaluation in the fetus is hampered by inaccuracy in the measurement of vessel diameter. The impact of biological variation on reproducibility studies is unknown. The present study evaluated diameter measurements carried out with modern ultrasound equipment by assessing inter- and intraobserver variations. DESIGN: Repeated measurements analyzed by a general factorial model of analysis of variance. METHODS: Three observers measured repeatedly (six or more times for each tube and session) silicone tube implants (0.6, 2.2 and 6 mm), applying ultrasound imaging in eight fetal sheep, and the same silicone tubes in vitro. Analysis of variance was carried out on 199 observations made in vitro and 537 in vivo. RESULTS: The upper 95% confidence limit for the intra- and interobserver variations was higher for measurements made in vivo than in vitro. It was highest for the largest diameter and varied between 0.10 and 0.38 mm. When the diameter was determined as an average of repeated measurements, the error was reduced: six measurements in vivo had upper 95% confidence limits for intraobserver variation of 0.04 and 0.09 mm for diameters of 0.6 and 6 mm, respectively. CONCLUSIONS: The results show that ultrasound diameter measurements have a high reproducibility even for vessels of small dimensions when repeated measurements are taken with high-frequency ultrasound under favorable conditions. The low variation described in the present investigation of silicone tubes, compared to previous studies, suggests that diameter changes of living vessels represent a separate source of measurement variation.