Fetal crown-rump length: reevaluation of relation to menstrual age (5-18 weeks) with high-resolution real-time US.

Because of recent challenges in the literature regarding the validity of the older crown-rump length (CRL) data developed with conventional static-image ultrasound scanners, the authors evaluated the relationship between CRL and menstrual age of fetuses in a population of 416 patients with good menstrual dates. By using a variety of commercially available transabdominal and transvaginal real-time ultrasound probes, the authors demonstrated that measurements can be made successfully for CRLs varying in size from 2 mm to 12 cm. Regression analysis of the data resulted in development of a new table for predicting menstrual age of fetuses on the basis of CRL measurements obtained between 5 and 18 weeks gestation. Although the magnitude of the raw residuals increased over time, the variability in predicting menstrual age was demonstrated to be relatively constant at +/- 8% (2 standard deviations) when expressed as a percentage of the predicted value. The accuracy in predicting menstrual age from CRL after 14 weeks was equivalent to but not better than conventional measurements such as biparietal diameter and femur length.

Fetal body ratios in second trimester: a useful tool for identifying chromosomal abnormalities?

This study was carried out to determine if second trimester fetal body ratios are useful in detecting chromosomally abnormal fetuses. As a reference population, normative data for five fetal body ratios (femur length/biparietal diameter, biparietal diameter/fetal length, femur length/head circumference, head circumference/abdominal circumference, and femur length/abdominal circumference) were derived using regression analysis from a population of chromosomally normal fetuses (n = 1770) who underwent genetic amniocentesis at our institution between 14 and 21 menstrual weeks. During the same time period, 37 chromosomally abnormal fetuses were identified by amniocentesis. In comparing the two groups using the 10th and 90th percentiles as cutoffs between normal and abnormal, approximately 25% of chromosomally abnormal fetuses were identified, whereas approximately 20% of the normal fetuses were incorrectly classified as abnormal. Moreover, the use of 1.5 standard deviations above the mean for BPD/FL identified only 19% of Down syndrome fetuses. Our data, and those from a comprehensive review of the literature, suggest that the sensitivity of these ratios in detecting chromosomally abnormal fetuses is too low to recommend them for routine screening.

In utero analysis of fetal growth: a sonographic weight standard.

Regression analysis was used to develop an in utero fetal weight model from a population of 392 predominantly middle-class white patients with certain menstrual histories. There was a gradual increase in fetal weight from 35 g at 10 weeks to 3,619 g at 40 weeks, with uniform variance of +/- 12.7% (1 standard deviation) throughout gestation. When tested against the estimated weights of 1,771 chromosomally normal fetuses between 14 and 21 weeks, the mean percent difference was 0.8% and the average absolute percent error was 3.3%. When compared with actual delivery data for 163 fetuses in the group, the mean percent difference was 0.8% and the average absolute percent error was 1.1%. These data are compared with other prenatal weight curves obtained at ultrasound and with data from several large postnatal weight studies.

How accurate is second trimester fetal dating?

In this study, the Hadlock models for fetal dating using single and multiple parameters were tested retrospectively in 1770 chromosomally normal singleton fetuses in the second trimester (14 to 21 weeks of fetal development). The 95% confidence interval using measurements of the fetal head and femur individually was approximately +/- 1 week, which is comparable to the results of recently published dating models from other centers designed specifically for use during this time frame. The use of multiple-parameter models results in statistically significant improvement in prediction of age, in terms of both random error and maximum observed errors. We conclude that these models, developed for dating between 14 and 42 weeks of fetal development, provide highly accurate estimates of fetal age in the second trimester of pregnancy.

Sonographic fetal growth standards. Are current data applicable to a racially mixed population?

Currently available sonographic growth standards for fetal head size, abdominal size, and limb length are based primarily on studies from white populations. To determine whether these published standards are appropriate for a racially mixed, indigent population, we compared our published data from a middle-class white population with data generated from a black/Hispanic population seen at a county hospital in Houston, Texas. No statistically significant differences were found for any of the following fetal sonographic parameters (20 to 41 weeks): biparietal diameter, head circumference, abdominal circumference, femur length.

Sonographic estimation of fetal age and weight.

Real-time ultrasound is a very effective tool in the estimation of fetal age and weight. The most accurate fetal age estimates are from crown-rump length measurements in early pregnancy. Late pregnancy age estimates are less accurate, but the use of multiple body measurements provides a composite age estimate generally accurate to within 7 per cent of actual age. Estimates of fetal weight are improved with the use of head, body, and femur measurements, and generally are accurate to within 15 per cent of actual birth weight.

Longitudinal studies of thigh circumference growth in normal fetuses.

To characterize the growth of the thigh circumference (ThC) in individual fetuses, longitudinal studies of ThC growth were carried out in 20 fetuses between 19 (+/- 1.8) and 38 (+/- 1.5) weeks, menstrual age. Because of measurement uncertainties, analysis of growth patterns was limited to the data collected after 22 weeks. The Rossavik growth model fit the data well [R2: 95.8(+/- 2.9 SD)%], but considerable coefficient variability was seen, particularly for the coefficient k. Use of the coefficient k-value (1.138) derived from a cross-sectional data set reduced the variability of the coefficient c by 95% and made the results of the longitudinal and cross-sectional studies consistent with each other. This k-value was taken as the appropriate one for ThC growth curves. Studies of ThC growth in individual fetuses revealed considerable variability in growth curve shape, although the majority of curves could be approximated by a straight line. No differences between growth curves for males and females could be detected. The average longitudinal ThC growth curve was found to be very similar to the growth curve obtained in a cross-sectional study of ThC growth. ThC growth after 26.1 weeks could be predicted with an accuracy of +/- 15% from growth models derived from the data obtained before 26.1 weeks. These results indicate that ThC growth after 22 weeks (as with other parameters) can be followed in individual fetuses. However, because of the greater inherent variability in growth patterns, using each fetus as its own control may be more important.

Longitudinal studies of femur growth in normal fetuses.

As an indicator of fetal limb growth, change in the femur diaphysis length (FDL) between 15 and 38 weeks, menstrual age, has been evaluated in a longitudinal study of 20 normal fetuses, as determined by prenatal biparietal diameter, head circumference, and abdominal circumference growth patterns as well as postnatal pediatric assessment. Individual FDL growth curves were uniformly parabolic and followed very similar trajectories. The projected start points [mean: 9.7 (+/- 1.5 SD) weeks] for these curves were in complete agreement with embryologic data, and the curves themselves were well characterized by the Rossavik model [R2 99.5 (+/- 0.5 SD)%]. No differences between males and females were detected, and good agreement was obtained between the average longitudinal growth curve and the cross-sectional growth curve derived from a previously studied data set. Individual growth curve standards, determined from Rossavik models based on data obtained before 26.1 weeks, menstrual age, were within 7% of actual measurements in 94% of the 97 time points studied after 26 weeks. These results indicate that the Rossavik model and its associated individual growth curve standards can be used to evaluate the growth of the femur and thus provide an improved means for detecting skeletal dysplasias and fetal growth retardation.

Estimating fetal age using multiple parameters: a prospective evaluation in a racially mixed population.

This study was designed to evaluate the accuracy of regression models for menstrual age prediction using single versus multiple fetal ultrasonographic measurements (biparietal diameter, head circumference, abdominal circumference, and femur length). The models tested had been previously developed from a study of middle-class white patients in a private hospital. The current study population consisted of 300 indigent black and Hispanic patients seen in a county hospital. This study demonstrated prospectively that the use of multiple parameters in estimating fetal age offers a significant advantage over any single parameter used alone and that the regression equations developed from a middle-class white population appear to be applicable to fetuses from a population with different socioeconomic and racial characteristics.

Mathematical modeling of fetal growth. III. Evaluation of head growth using the head profile area.

Growth of the fetal head has been reexamined in a longitudinal study of 32 normally growing fetuses using the head profile area (HA). The Rossavik growth model (characterized by the coefficients c, k, and s) fit these data very well (R2 [+/- SD] 99.6 [+/- 0.3]%), and regression analysis indicated that optimum results are obtained when the coefficient k is kept constant. Head area growth before 28 weeks was found to be related to the coefficient c, which was strongly correlated (R2 69.2%) with the time of HA growth initiation. These results suggest that c reflects the genetic factors controlling growth in the early part of pregnancy. The coefficient s was negatively correlated with c (R2 82.1%) and had two components, one associated with c (pred. s) and one not associated with c (s-residual). This linkage between c and s, together with the parallel head growth seen after 30 weeks, suggests the possibility of a maternal system in which genetically determined head size (c) can be sensed and modified by other factors (s) to maximize compatibility between the fetal head and maternal pelvis.

Mathematical modeling of fetal growth. IV. Evaluation of trunk growth using the abdominal profile area.

Growth of the fetal abdomen has been reexamined in a longitudinal study of 32 normally growing fetuses using the abdominal profile area (AA) and the Rossavik growth model. The growth of the fetal abdomen was compared to that of the fetal head. In early pregnancy the growth rate was higher for the fetal head than for the fetal abdomen, whereas the opposite was the case in late pregnancy. The coefficient k was lower, and the s was higher, for AA than for the head profile area (HA), giving a less prominent sigmoid shape to the AA growth curve. The best correlation to fetal weight at birth was found for the AA growth curve as a whole. The AA s-residual correlated best of all single parameters, with the coefficient c of HA being next. The s-residual appears to reflect the nutritional supply available in late pregnancy, and we conclude that fetal weight at birth depends more on this supply than on the genetic factors controlling fetal growth.

Mathematic modeling of fetal growth: development of individual growth curve standards.

To provide a more precise means for monitoring individual fetal growth and to improve the detection of growth abnormalities, the Rossavik growth model [P = c(t)k + s(t)] has been used to establish individual growth curve standards. A longitudinal study of the growth of one-dimensional (biparietal diameter, head, and abdominal circumferences), two-dimensional (head and abdominal profile areas), and three-dimensional (head and abdominal volumes) parameters in 18 normal fetuses has revealed that the growth of all parameters can be well-described by the Rossavik model (R2: 97.2 to 99.5%). Strong statistical evidence was obtained indicating that the coefficient k is principally a geometric coefficient having a value determined primarily by the dimension of the parameter being studied. It was also found that the coefficients c and s can be estimated from the data obtained before 28 weeks' conceptual age. Individual growth curve models derived from data obtained before 28 weeks were capable of predicting growth beyond 28 weeks. These models provide growth curve standards for evaluating individual fetal growth beyond 28 weeks.

Fetal thigh circumference: a critical evaluation of its relationship to menstrual age.

To provide standards for evaluating fetal thigh circumference measurements obtained with ultrasound, a study of the growth of this parameter during pregnancy was made. Because of possible sampling problems before 22 weeks (menstrual age), this investigation was limited to the 22-40 week time period. Regression analysis indicated that the optimal model (R2 = 88.9%) for describing thigh circumference (ThC) growth was a power function. ThC variability increased with menstrual age, from 1.3 cm at 22 weeks to 3.1 cm at 40 weeks. These data were used to construct a standard growth curve for the ThC.

Evaluation of fetal thigh circumference measurements: a comparative ultrasound and anatomical study.

This investigation is concerned with establishing the anatomical location of the site proposed for making fetal thigh circumference (ThC) measurements with ultrasound. An evaluation of the accuracy of ultrasonic ThC measurements has also been made. The results of these studies indicate that the change in femur profile (polygonal to oval) used to identify the site of measurement (transition plane) occurs at the junction of the upper and middle thirds of the thigh, at the level of the proximal nutrient foramen of the femur. Ultrasonic ThC measurements are systematically larger (mean: 4.3%; range: 1.3-10.3%) than comparable anatomical measurements. Measurements made within 1-2 cm of the transition plane are quite similar, demonstrating that exact positioning of the plane is not necessary. These results indicate that ThC measurements can be made in a reliable manner and thus could be used to detect changes in soft tissue mass and improve weight estimation procedures.

Use of ultrasound in the detection of macrosomia: a review.

The early detection of the macrosomic fetus is important to the obstetric management of pregnancies in which this growth abnormality occurs. Ultrasound has been used for this purpose but for this approach to be effective, a clear definition of macrosomia, selection of the appropriate parameters for measurement, and specification of the optimal time for carrying out the ultrasound studies are required. In this review we examine the definition of macrosomia and introduce two new terms, mechanical macrosomia and metabolic macrosomia. We also evaluate the effectiveness of the limited number of parameters studied to date and conclude that they are marginally effective, primarily because of their low sensitivity and the limitations of current ultrasound methods for estimating fetal weight. The use of the growth profile, a multiparameter approach to the evaluation of fetal growth, is suggested as a possible means for improving the effectiveness of ultrasound in detecting the macrosomic fetus.

Fetal femur length: a critical reevaluation of the relationship to menstrual age.

This investigation has reexamined the measurement of the femur length and the use of this measurement as a growth and dating parameter. Straight-line measurements of shaft length were found to be smaller than measurements made along the bone curvature. However, the maximum difference did not exceed 2 mm. Thus, because of its simplicity, use of straight-line measurements appears justified. The optimal (R2 = 96.1%) growth curve model was a linear-quadratic function, with variability around the regression line increasing with menstrual age. The optimal (R2 = 96.8%) dating curve model was a linear-quadratic function in which loge menstrual age is a function of femur length. The variability associated with age estimates increased with femur length. These data were used to construct new standard curves for growth assessment and dating of fetuses using femur length measurements.