Ultrasonic fetal weight prediction: role of head circumference and femur length

The accurate sonographic estimate of fetal weight is helpful in those instances when the fetal weight estimate might alter clinical management. Most sonographic weight predicting formulas have been based predominantly on measurements from the term fetus and then applied to the preterm fetus. Yet, the morphology of the preterm and term fetus differs considerably. The authors have examined the predictive accuracy of three published sonographic formulas in 69 preterm fetuses scanned within 48 hours of delivery. The mean birth weight was 1396 g. Thirty-nine of the infants were less than 1500 g. Sixty-two percent were products of pregnancies complicated by premature rupture of membranes. The results were compared with new equations derived from combinations of head and abdominal circumferences, biparietal diameter, and femur length obtained from the first 33 fetuses and then tested on the remaining 36. Whereas each formula correlated highly with birth weight, the selected new formula was more accurate than the published formulas by each criteria examined. In contrast to the latter, the mean error (actual minus predicted weight) of most new equations did not significantly differ from zero when tested prospectively. In addition, it appeared that the accuracy of two new formulas not incorporating femur length could be further enhanced in the group of fetuses whose femur length differed from the mean by at least 2 standard deviations by multiplying the predicted weight by the ratio of actual to mean femur length. The authors conclude that the use of head circumference and femur length coupled with a population restricted to the preterm fetus enhances the accuracy of sonographic weight predictions.     

Fetal weight estimation formulas with head, abdominal, femur, and thigh circumference measurements

Most current sonographic techniques for estimating fetal weight involve measurements of the head, abdomen, and femur length both alone and in combination. The value of the fetal thigh circumference measurement in addition to the head, abdominal, and femur length measurements has not been investigated previously. Eighty-nine patients were scanned within 72 hours of delivery, and the following ultrasonic fetal parameters were obtained in all fetuses: biparietal diameter, head circumference, abdominal circumference, femur length, and thigh circumference. With multiple stepwise regression analysis, the best-fit formulas were developed with one, two, three, four, and five parameters, respectively. The best results were obtained by combining measurements of all five parameters: biparietal diameter, head circumference, abdominal circumference, femur length, and thigh circumference. The mean error of this formula was 6%, the mean deviation 0.3%, and the SD 7.8%. These data suggest that the addition of thigh circumference to measurements of the head, abdomen, and femur length improves the accuracy of fetal weight estimates.     

Longitudinal reference charts for growth of the fetal head, abdomen and femur

OBJECTIVES: The aims of the present study were to establish improved reference charts for growth of the fetal head, abdomen and femur, and to determine the effect of fetal and maternal factors. STUDY DESIGN: This prospective longitudinal study included 650 low-risk pregnancies. Outer-outer biparietal diameter (BPD), head circumference (HC), mean abdominal diameter (MAD), abdominal circumference (AC) and femur length (FL) were measured by ultrasound, and the statistical analysis was based on regression analysis and multilevel modeling. RESULTS: Reference percentiles for the growth of MAD, AC and FL showed continuous growth in gestational week 10-40, while BPD and HC showed a slightly blunted growth toward the end of pregnancy. FL was the only variable that was not influenced by gender. There was a significant negative association between breech presentation and all five biometrical variables, while maternal weight was positively associated with all five variables. Cephalic index significantly influenced BPD and HC. Maternal height had a positive effect on BPD, HC, AC and FL, and parity had a positive effect on MAD and AC, while smoking influenced negatively HC, MAD, and FL. Terms for calculating conditional reference values and customisation for individualised growth assessment are presented. CONCLUSIONS: New reference charts for the growth of fetal head, abdomen and femur are suggested for assessing fetal size and growth, and can be adjusted for maternal and fetal factors to suite individual pregnancies.     

Estimation of fetal weight]

Four equations for fetal weight estimation are analyzed. All of them are improved, adding a factor that corrects the fetal somatotype 90.3 ((FL/AD)-0.68)). The differences between the means are not significant.     

Estimation of fetal weight during labor

Fetal weight was estimated clinically and by two ultrasonographic methods in 62 patients in labor at term. Maternal obesity precluded clinical estimation of fetal weight in eight patients (12.9%) and a combination of maternal obesity, anterior placentation, and oligohydramnios prevented ultrasonographic fetal measurement in four patients (6.5%). In 43 patients, in whom all three methods of estimate were obtainable, a regression equation using ultrasonically measured fetal abdominal circumference provided the greatest accuracy. However, significant interpatient variability affecting the accuracy of all three methods of estimating fetal weight may limit the clinical application of these measurement techniques.     

Comparison of fetal weight estimation formulas with and without head measurements

Most published sonographic techniques for estimating fetal weight use measurements of the biparietal diameter (BPD) and abdominal circumference. In many clinical situations, however, accurate head measurements cannot be obtained. One hundred one patients were scanned within 72 hours of delivery. Using multiple stepwise regression analysis, a best-fit formula was developed using abdominal circumference and femur length, or femur length alone. The mean error was 109 g/kg fetal weight for abdominal circumference and femur length that was comparable with BPD and abdominal circumference formulas for estimated fetal weight. The mean error when femur length was used alone was 129 g/kg fetal weight. When these models were tested prospectively on 67 patients and compared with published formulas, they yielded results that were similar in accuracy. Among these additional patients, the model using one parameter (femur length) had an average error of 114 g/kg, whereas the femur length and abdominal circumference showed a mean error of 125 g/kg.     

Amniotic fluid index and estimated fetal weight for prediction of fetal macrosomia: a prospective observational study

Objective: The purpose of this study was to assess the value of combining the estimated fetal weight (EFW) and amniotic fluid index (AFI) measured in term patients early in labor with intact membranes for prediction of macrosomia.

Methods: In a single center, prospective observational study, 600 patients in the first stage of labor before rupture of membranes in whom ultrasonography was performed to measure AFI and EFW, and these data were analyzed statistically to evaluate prediction of fetal macrosomia.

Results: Macrosomia occurred in 64 cases (10.6%). The AFI was significantly higher in the macrosomic group (p?=?0.001). It was noted that the area under receiver operating characteristic (ROC) curves for EFW was 0.93 and that of AFI was 0.67. Based on suggested combined EFW and AFI cutoffs of 4000?g and 164?mm, respectively, the positive predictive value (PPV) for combined parameters (92.3%) was higher than that of EFW (75%) and that of AFI (27%) and the likelihood ratio for combination (93.7%) was higher than that of EFW (24.7%) and that of AFI (21%).

Conclusion: Combined use of EFW and AFI improves prediction of macrosomia at birth rather than the EFW alone.     

Estimation of fetal weight from ultrasonic measurements.

Ultrasonic measurements were made on 65 fetuses within 48 hours of delivery. Multiple regression analysis of birth weight and the natural logarithm of birth weight against several measured variables were obtained. The formula giving the best correlation was a polynomial regression of the natural logarithm of birth weight vs. trunk circumference, circumference, and a long axis measurement. The correlation was improved by excluding the first 15 patients but was not improved further by excluding the next 15. The best correlation was 0.944, giving a predicted birth weight error of +/- 103 Gm. (1 S.D.).     

Estimation of fetal weight in the third trimester by ultrasound

A method to estimate the intrauterine fetal weight by use of ultrasound measurements of the fetal biparietal diameter (BPD) and the abdominal diameter (AD) is presented. From a consecutive series of single pregnancies the 238 pregnancies (3% of the hospital population) with ultrasound measurements obtained within 4 days before delivery were used in the estimation of birth weight. In addition, the estimated formula was applied on a test material consisting of 100 similarly selected pregnancies from the same hospital. To evaluate the expected selection effects, the birth weight for gestational age in the study group was compared with birth weight for gestational age in the total population. The weight could be estimated as 0.0351 X AD1.65 X BPD0.69 X exp(0.00196 X gestational age). Thus, the actual birth weight was within 83-120% of the estimated weight (95% prediction limits), with a residual coefficient of variation of about 9%. The gestational age could be omitted without major influence on the weight prediction. When applying the formula on the test material, 70% of the actual weights deviated less than 10% from the estimated fetal weight, but a tendency towards a slight overestimation of the weight for light for gestational age infants was found.     

Repeated fetal betamethasone treatment and birth weight and head circumference

OBJECTIVE: To assess the effect of repeated courses of betamethasone on birth weight and head circumference. METHODS: We conducted a historical cohort study of inpatients receiving betamethasone therapy over 5 years. We compared birth weights and head circumferences of infants whose mothers received one course of betamethasone with those of infants whose mothers received multiple courses. Multiple regression analysis was used to adjust for potential confounding variables. Sufficient power (80%) existed to detect a 20% difference between the groups (alpha = 0.05). RESULTS: Mean birth weights (+/-SD) were 1717 +/- 707 g in the single-course group (n = 107) and 1783 +/- 647 g in the multiple-course group (n = 45) (P =.59, Student t-test). Mean head circumference was 28.2 +/- 3.6 cm in the single-course group and 29.2 +/- 2.9 cm in the multiple-course group (P =.15, Student t-test). In regression analysis, birth weights (1757 g and 1752 g) and head circumferences (28.5 cm and 29.0 cm) did not differ significantly different between the single-course and multiple-course groups. CONCLUSION: Multiple courses of betamethasone do not reduce birth weight or head circumference in neonates compared with single-course therapy.     

Estimated fetal weight in the evaluation of growth in twin gestations: a prospective longitudinal study

A prospective longitudinal study was conducted in order to determine by sonographically estimated fetal weight the patterns of fetal growth in twins. Thirty-five healthy women with normal twin pregnancies were examined every three weeks from the 15th week of gestation to delivery. Among the measurements obtained were the biparietal diameter (BPD), the abdominal circumference, and the calculated fetal weight. From 15-28 weeks, the growth velocity of the BPD and abdominal circumference remained fairly constant, with a steady increase in incremental growth. Beyond this age, we observed a slowing in growth of the BPD, while the abdominal circumference continued at a constant rate. The growth velocity of the weight steadily increased throughout pregnancy. Although greater biologic variability in weight between twin A and B was observed as gestational age progressed, the overall mean weights of twin A and B were not statistically different. We have generated a nomogram of fetal weight gain throughout pregnancy.     

Candidate gene study for smoking,alcohol use,and body weight in a sample of pregnant women

Objective: Prenatal smoking, alcohol use, and obesity have significant effects on maternal and fetal health. However, not much is known about the genetic contributions to these risk factors among pregnant women. We evaluate the associations between several candidate genes and smoking, alcohol use, pre-pregnancy body weight, and weight gain during pregnancy in a sample of pregnant women.

Methods: The study analyzes a sample of about 1900 mothers from the Danish National Birth Cohort. We test the association between 1450 SNPs in/near 117 genes/loci and various risk factor measures.

Results: Only a few SNPs in FTO were significantly associated with pre-pregnancy obesity and body mass index (4 and 2 SNPs, respectively) after SNP-level correction for multiple testing. A few loci were significantly related to various smoking measures (any smoking, quitting and cigarette number) with gene/locus-level correction for multiple testing, but not after SNP-level correction. Similarly, some loci were significant for the alcohol measures at the gene/locus-level but not at SNP-level correction.

Conclusion: The study suggests that the majority of the evaluated candidate genes may not play an important role in influencing these risk factors among pregnant women, highlighting the importance of other genetic factors and non-genetic contributors to their etiology.     

Assessment of fetal weight based on ultrasonic femur length after the second trimester.

OBJECTIVE: The ultrasonic measurement of the fetal femur length is a sensitive and precise variable for estimation of fetal growth and development. The objective of this study is to predict fetal weight in fetuses of more than 24 weeks gestation by ultrasound measurement of the femur length. METHOD: In this study, pregnant mothers were identified by the criteria of normalities, such as: well-known LMP, regular menstrual cycles, no use of OCP for the last 3 months, no use of alcohol or cigarettes, no drug abuse, no history of diabetes or chronic HTN. Multiple gestations, congenital anomalies and still-births were excluded. Birth-weight measurements (adjusted for maternal age, baby's sex, parity and week of gestation) were taken immediately after birth. RESULT: The relation between fetal weight and fetal femur length has been determined by cross-sectional analysis of 900 normal fetuses (> or = 25 weeks gestation) using real time ultrasonography. Mathematical modeling of the data has demonstrated that the femur growth curve is non-linear beyond 24 weeks gestation. With the aid of a scientific calculator the data were analyzed and a simple second-grade equation has been derived: EFW (kg) = 0.042FL(2) (cm)+0.32FL-1.36, S.D. approximately +/-235 g (Honarvar's Formula 2). With the use of this data, the error in estimation of EFW given FL is +/-235 g. CONCLUSION: This simple, new and accurate equation appears to be clinically reliable and easy to use and suggests that previous normal ultrasonic fetal femur length curves for another population may underestimate or overestimate normal fetal weight for the Iranian population. Thus, our formula is an excellent means to estimate true fetal weight.     

Maternal estimation of fetal weight as a complementary method of fetal weight assessment: a prospective clinical trial

OBJECTIVE: To assess the accuracy of fetal weight estimations performed by parturients vs. clinical and sonographic estimates. graphic estimates. STUDY DESIGN: Term parous women were included in this prospective trial. Each woman estimated the weight of her fetus. Fetal weight was also estimated sonographically and clinically. The clinical evaluations were performed by an attending gynecologist and a resident. Estimate accuracy for extreme fetal weights was evaluated separately. RESULTS: We enrolled 128 women. Maternal and resident estimations' accuracy was identical. The attending physician estimates were superior to the residents' and parturients', with a mean absolute error of 318 g and 67% of the evaluations within the 10% boundaries of the actual birth weight. Sonographic evaluation was the most accurate method, with a mean absolute error of 244 g and 76% of evaluations within 10% of the actual birth weight. When partitioned to extreme fetal weight groups (upper and lower 10th percentiles), the maternal evaluation was at least as accurate as the clinical evaluations of both the attending and resident. CONCLUSION: Maternal evaluation of the fetus's weight is as accurate as physicians' clinical estimations, and is advised as a complementary method of assessment. This estimation may greater relevance when intrauterine growth restriction or macrosomia is suspected.