A statistical examination of the accuracy of combining femoral length and biparietal diameter as an index of fetal gestational age

Summary. The use of femur length (FL) and biparietal diameter (BPD) for estimation of gestational age (GA) was investigated using 1123 ultrasound measurements between the 14th and 41st week of gestation. Femur length was found to provide a more accurate index than BPD for the entire range of gestational age considered. If an interval of approximately ±2·5 weeks is considered 'satisfactory', the accuracy of FL past the 31st week is not sufficient to justify its use. Combining the FL and BPD measurements via the formula: GA = 9·565 + (0·0414) FL + (0·0024573) FL²+ (0·15209) BPD – (0·00015827) BPD² provides an index of gestational age accurate to approximately ± 2·5 weeks up to the 32nd week of gestation and accurate to approximately ± 2·75 weeks up to the 34th week of gestation. This is the first time that combining FL and BPD in a statistically rigorous manner has been proposed as an index of gestational age, and we suggest that this combination be used as a routine tool in its assessment.     

Femur length versus biparietal diameter for estimating gestational age in the third trimester

It is well established that ultrasound measurement of femur length and biparietal diameter are comparably accurate estimators of gestational age when obtained in the first half of pregnancy. Both estimators, however, become less accurate later in pregnancy. The present study compares the relative accuracy of these estimators when obtained in the third trimester. It is concluded from linear regression analysis that the correlation between gestational age estimated from femur length and the actual gestational age is stronger than that between gestational age estimated from biparietal diameter and actual gestational age. Furthermore, gestational age calculated from femur length is significantly more likely to be within 2 weeks (P less than or equal to .01) and 3 weeks (P less than or equal to .05) of actual gestational age than is gestational age calculated from biparietal diameter. Estimating gestational age from the mean of the gestational age based on biparietal diameter and that based on femur length is less accurate than estimating gestational age from femur length alone.     

Assessment of gestational age of the Asian fetus by the sonar measurement of crown-rump length and biparietal diameter

Summary. Two populations of pregnant Asian and European women were scanned using linear array real-time ultrasound machines. Measurements were made of fetal crown-rump length (CRL) up to 14 weeks gestational age and of biparietal diameter (BPD) up to 20 weeks gestational age. Polynomial growth curves relating CRL and BPD measurements to their gestational ages were fitted to the data by the method of unweighted least squares. There were no significant differences between the Asian and European parameters for the CRL and BPD curves. European nomograms may be used to establish Asian fetal gestational age up to 20 weeks.     

A discrepancy between gestational age estimated by last menstrual period and biparietal diameter may indicate an increased risk of fetal death and adverse pregnancy outcome

Objective To determine if the discrepancy between gestational age estimated by last menstrual period and by biparietal diameter (GA_LMP− GA_BPD) is associated with adverse pregnancy outcome.
Design Population-based follow up study.
Population Singleton pregnancies were studied when a reliable date of last menstrual period and biparietal diameter measured between 12 and 22 weeks of gestation was available (   n = 16,469  ).
Methods Logistic regression analysis and Kaplan-Meier survival analysis were used to analyse the association between GA_LMP− GA_BPD and adverse pregnancy outcome.
Main outcome measures Adverse outcome was defined as abortion after 12 weeks of gestation, stillbirth or postnatal death within one year of birth, delivery < 37 weeks of gestation, a birthweight < 2500 g or a sex-specific birthweight lower than 22% below the expected.
Results The risk of death was more than doubled if GA_LMP− GA_BPD of ≥ 8 days was compared with GA_LMP− GA_BPD of < 8 days (OR 2.2; 95% CI 1.6–3.1). The risk of death was a factor of 6.1 higher if GA_LMP− GA_BPD of ≥ 8 days was combined with increased (> 2 × multiple of median) maternal alphafetoprotein measured in the 2nd trimester.
Conclusions A discrepancy between GA_LMP and GA_BPD generally reflects the precision of the two methods used to predict term pregnancy. However, a positive discrepancy of more than seven days, particularly with high maternal alpha-fetoprotein, might indicate intrauterine growth retardation and an increased risk of adverse perinatal outcome.     

The accuracy of estimated gestational age based on ultrasound measurement of biparietal diameter in preterm premature rupture of the membranes

The management of patients with preterm premature rupture of the membranes is often influenced by the ultrasound measurement of biparietal diameter. The cephalic index has been used in pregnancies with intact membranes to predict the presence of an abnormal biparietal diameter that may be unreliable in estimating gestational age. This study analyzed 100 patients with rupture of the membranes between the beginning of gestation week 28 and the end of week 34. The cephalic index was found to be abnormal 45% of the time; all were in the brachiocephalic range. The biparietal diameter was affected by presentation and presence or absence of amniotic fluid (p less than 0.05). Use of the biparietal diameter was found to be unreliable in estimating gestational age with preterm premature rupture of the membranes. Use of the cephalic index did not enable prediction of which biparietal diameters would be abnormal; therefore use of the biparietal diameter or the cephalic index in preterm premature rupture of the membranes is not recommended. The use of the head circumference, abdominal circumference, and femur measurements may be a better way of predicting gestational age in this select population.     

Comparison of the accuracy of gestational age estimation from fetal heart rate and crown-rump length

Objective: To compare the accuracy of gestational age (GA) estimation predicted by fetal heart rate (FHR) versus crown-rump length (CRL) measurements in early pregnancies.Methods: A total of 145 infertile women with singleton intrauterine pregnancies conceived through in vitro fertilization underwent vaginal ultrasonographic examination in early stage of pregnancy. We performed 197 FHR and 398 CRL measurements. The relationship between GA (counted with the day of oocyte retrieval used as day 14) and the FHR or CRL was explored with regression analysis by means of least squares. Polynomials up to the third order were analyzed, and the coefficient of determination (R²) was calculated.Results: Fetal cardiac activity could be detected as early as 38 days, gradually increased to 188 beats per minute at 62 days of gestation. Regression analysis with GA as dependent variable resulted in R² values of 0.906, 0.907, and 0.908 for the first, second, and third order polynomials, respectively. The standard deviations around the regression line were 2.90, 2.81, and 2.84 days, respectively. The second order polynomial to describe GA (days) estimated by FHR (beats per min): GA = 3.02E-41FHR² + 1.60E-11FHR + 2.22E+1. On the other hand, the CRL was measured at 38 days to 62 days of gestation. Regression analysis resulted in R² values of 0.827, 0.839, and 0.841, respectively. The deviations were 4.11, 3.83, and 3.89 days, respectively. The second order polynomial to describe GA estimated by CRL (mm): GA = −2.10E-21CRL² + 1.49E+01CRL + 3.86E+1. There was no difference between the R² values and no difference between the standard deviations around the regression lines.Conclusion: Our results indicate no statistically significant difference in the accuracy of GA estimation predicted by FHR versus CRL measurements. But better mathematical fits were obtained when the data of FHR were used for GA estimation. These findings suggest that the FHR measurement may be a new method for early ultrasound dating. This study mentions for the first time the possibility of “FHR dating” with functional data.     

The relation between fetal nasal bone length and biparietal diameter in the Korean population

OBJECTIVE: To evaluate the relation between fetal nasal bone length (NBL) and biparietal diameter (BPD) at 15-19.9 (20) weeks of gestation by ultrasonography in the Korean population. METHODS: The study population included 1268 Korean women (aged between 19 and 45 years) with a singleton pregnancy who registered at the Maternal Fetal Medicine Unit of Samsung Cheil Hospital between September 2003 and February 2005. Ultrasound measurements of NBL were performed using a strict sagittal plan of the fetal head. Other fetal biometry profiles were conducted before amniocentesis for fetal karyotyping. RESULTS: NBL and fetal biometry profiles were measured successfully in 77.9% (988/1268) of the fetuses. NBL was found to increase linearly as a function of BPD (P < 0.001) with a median NBL of 4.4 mm (range 1.9-7.9). NBL increases through 15-19.9 (20) weeks of gestation were given by the equation NBL (mm) = 0.0836 x BPD (mm) + 1.368 (R2 = 0.1, P < 0.001). CONCLUSIONS: Fetal NBL and BPD are linearly related in the second trimester. Fetal NBL in the Korean population is likely to be shorter than that reported for Caucasians and African-Americans.     

Third-trimester biparietal diameter as a predictor of fetal lung maturity

The major fetal risk associated with elective delivery is unexpected fetal lung immaturity and the development of hyaline membrane disease soon after birth. Prior to elective vaginal or abdominal delivery it has become standard obstetric practice to predict fetal lung maturity by the analysis of amniotic fluid obtained by amniocentesis or vaginal pool sample following preterm rupture of membranes. A correlation between third-trimester fetal biparietal diameter and the lecithin/sphingomyelin (L/S) ratio has been established by several investigators. In order to determine if a threshold BPD could be consistently correlated with fetal lung maturity, we retrospectively examined the hospital and laboratory records of a group of 115 nondiabetic parturients in whom BPD measurements and amniotic fluid analysis for L/S ratio had been performed for various clinical indications. A threshold BPD of greater than or equal to 9.2 cm in all parturients who underwent elective repeat cesarean delivery was associated with no hyaline membrane disease (HMD). Two of the three neonates who developed HMD had mature L/S ratios but were products of pregnancies complicated by third-trimester hemorrhage. A review of our present data suggests that about one-third of clinically-indicated amniocenteses in the absence of maternal diabetes or third-trimester hemorrhage could potentially be avoided without adverse neonatal impact. Possible therapeutic application of this finding requires further prospective study.     

Normal growth of the fetal biparietal diameter in an African population

The fetal biparietal diameter (BPD) was measured by ultrasound during normal pregnancy in 558 Nigerian women. The range of BPD values for each week of pregnancy from 12 weeks until term (713 measurements) was computed from cross-sectional data. The data were used to illustrate the variation in weekly growth rate according to fetal maturity and head size. The results revealed a curve similar to those reported from Caucasian populations but with much less flexion in the later weeks of pregnancy. The BPD values per week were also less in this study group. The reasons for these differences are discussed.     

Fetal thyroid in two-dimensional ultrasonography: nomograms according to gestational age and biparietal diameter

Objective

To establish fetal thyroid nomograms based on gestational age and biparietal diameter and to compare obtained results with previously published data.

Study design

A cross-sectional study of 241 healthy pregnant women at 14–38 week of gestation was undertaken. Exclusion criteria were: known maternal thyroid or systemic disease, unknown date of last menstrual period, multiple pregnancy and fetal malformations. Fetal thyroid diameter (FTD), circumference (FTC) and area (FTA) were measured by two-dimensional ultrasonography and plotted against gestational age (GA) and biparietal diameter (BPD).

Results

FTD, FTC and FTA increased logarithmically to GA and BPD. Fetal thyroid measurements as a function of GA were expressed by logarithmic formulas: ln(FTD) = 3.6025–23.0315/GA, ln(FTC) = 4.6227–22.8003/GA, ln(FTA) = 6.6303–45.0831/GA. The following logarithmic formulas were obtained for fetal thyroid measurements according to BPD: ln(FTD) = 3.4068–45.4271/BPD, ln(FTC) = 4.4271–44.8359/BPD, ln(FTA) = 6.2390–88.4408/BPD. There were highly significant correlations between thyroid measurements and GA or BPD: r = 0.87–0.90, p < 0.00001.

Conclusions

We have established age-dependent and age-independent nomograms of fetal thyroid. These nomograms will enable prenatal diagnosis in fetuses at risk of thyroid disorders.     

Intrauterine growth of twins as estimated from birthweight and the fetal biparietal diameter

From 1560 twin pregnancies the birthweights were considered in relation to menstrual age. Compared to singletons a retardation of growth from about the 33rd wk of pregnancy was found. 407 measurements of the fetal biparietal diameter were performed in 69 twin pregnancies. Compared to singletons a retardation of growth of the fetal biparietal diameter was found from about the 30th wk of pregnancy. The suggestion is made that in singletons the birthweight values of prematurely born children are below fetal weight values at the same duration of pregnancy of children who will be born at term.     

Accuracy of prediction of gestational age by ultrasound measurement of biparietal diameter in Nigerian women

The accuracy of assessment of gestational age by ultrasound measurement of biparietal diameter (BPD) was evaluated in 84 Nigerian women. Ultrasound and menstrual delivery dates were compared in these women who had no complications of pregnancy and delivered infants whose birthweights were appropriate for 40 weeks. The results showed that ultrasound dating was more accurate than menstrual dating as evident from the number of women who delivered on and within 1 or 2 weeks of predicted delivery dates. It is concluded that (1) our locally produced BPD normogram is as accurate as those from elsewhere and (2) the normogram is recommended for use in Nigerian women for accurate pregnancy dating.     

Improving the accuracy of fetal foot length to confirm gestational duration

OBJECTIVE: To establish normative fetal foot length ranges using last menstrual period (LMP) and ultrasound dating by biparietal diameter and to examine variations in these ranges by ethnicity. METHODS: A consecutive series of 1,099 eligible subjects receiving abortions had fetal foot lengths measured directly. Models of fetal foot length were developed by using assessment of gestational duration by LMP alone, ultrasonography alone, and "best estimate" (LMP confirmed by ultrasonography). RESULTS: The full sample model using ultrasound dating (n = 1,099) yielded the following equation: foot length = -30.3 + days of gestation x 0.458 (R(2) of 0.92). Regression by LMP-determined gestational duration by using the "best estimate" sample (n = 491) provided an almost identical equation (foot length = -29.8 + days of gestation x 0.45) and a similar R(2) value of 0.87, although the standard errors were larger. Gestational duration by ultrasonography alone produced a better model fit than duration by LMP alone. Regressions by ethnicity were not significantly different compared with the simple regression, regardless of method used to determine gestational duration. CONCLUSION: A reconsideration of fetal foot length measurements to confirm gestational duration is important. More accurate tables of these measurements allow for greater precision in correlating gestational duration and foot length. Fetal foot length tables using ultrasonographically confirmed gestational duration and current statistical standards should replace tables currently used. Biparietal diameter as a single measurement provides adequate estimation of gestational duration in the second trimester for pregnancy termination, proving more reliable than LMP dating. LEVEL OF EVIDENCE: II-2.     

Normal range growth curves for fetal biparietal diameter, occipito frontal diameter, mean abdominal diameters and femur length

Normal range curves for the growth of fetal biparietal diameter (BPD), occipitofrontal diameter (OFD), mean abdominal diameter (AD), and femur length (FL) were obtained in a longitudinal series of ultrasound measurements in 19 normal pregnancies. Regression analysis was used to fit the data to equations. The best equations for BPD and AD on gestational are close to similar longitudinally obtained regression curves from other Scandinavian countries.     

Ultrasound assessment of fetal biparietal diameter and femur length during normal pregnancy in Iranian women.

OBJECTIVES: To determine the pattern of intrauterine growth and predicted biparietal diameter (BPD) and femur length (FL) at point in gestation of Iranian fetuses. METHODS: In an extensive and long-standing prospective study in Tehran, Iran 15693 BPD and 15594 FL measurements were obtained from the fetuses of 1324 normal pregnant women. Weekly mean values and the standard deviations (S.D. ) were calculated for both BPD and FL from 12 to 40 weeks of pregnancy. Comparison was also made between our results and previous Western studies using t-test analysis. RESULTS: Iranian fetuses had smaller BPD and shorter FL measurements in comparison with Western studies (P<0.05). There is a lag in BPD growth of our fetuses. The lag in FL growth is even more than BPD. Growth of the BPD and FL showed an asymptotic curve like that of Western studies but both of our values were lower. CONCLUSION: Ethnicity may influence ultrasonic fetal biometric measurements.     

Increment of the fetal biparietal diameter during normal pregnancy,as determined by ultrasound

The fetal biparietal diameter was measured by ultrasound during normal pregnancy. 1076 measurements were performed. For each week of pregnancy, from 23 until 43 weeks, the mean, percentiles and smoothed percentiles were determined. From statistical analysis no influence of fetal sex and maternal parity on the fetal biparietal diameter was found. From the 50th percentile the weekly increment of the fetal biparietal diameter was calculated.