Correlation between fetal gastric size and amniotic fluid volume

PURPOSE: Since abnormal conditions of the fetal digestive tract may alter both amniotic fluid volume and fetal gastric volume, we sought to determine whether amniotic fluid volume is correlated with fetal gastric volume in normal pregnancy. METHODS: A total of 280 fetal gastric size measurements were made prospectively from routine sonographic examinations of women with normal singleton pregnancies between 16 and 42 weeks of gestation. The fetal stomach was defined as the largest area including the pyloric site on transverse or oblique real-time sonographic scans. Gastric volume was calculated according to the formula for a prolate ellipsoid. The amniotic fluid index (AFI) was used for the evaluation of amniotic fluid volume. RESULTS: Both fetal gastric volume and AFI were significantly correlated with gestational age (R2= 0.422 and R2= 0.128, respectively). Only a weak correlation was found between gastric volume and AFI (R2= 0.036, p <0.001). On multivariate linear regression analysis adjusting for gestational age and fetal biometric measurements, gastric volume was not an independent and significant predictor of AFI. CONCLUSIONS: Although sonographically determined fetal gastric volume measurements appear to be useful in the assessment of fetal digestive tract anomalies, fetal gastric volume has no clinically significant effect on the amniotic fluid volume in normal pregnancy.     

Is there a relationship between estimated fetal weight and umbilical artery Doppler impedance indices?

OBJECTIVE: To establish whether there is a relationship between estimated fetal weight (EFW) and umbilical artery Doppler waveform impedance indices in the third trimester. METHODS: The pulsatility index (PI) and S/D (systolic/diastolic) ratio were obtained together with the EFW from 274 low-risk pregnancies. Measurements were made at fortnightly intervals from 30 weeks' gestation until delivery. A relationship between the two impedance indices and EFW was sought at gestational age ranges of 30-32, 33-35, 36-38 and 39-41 weeks. RESULTS: There were 918 pairs of PI (with S/D) and EFW available for analysis. The mean of the impedance indices decreased with advancing gestational age as expected. There was no clinically relevant correlation between impedance indices and EFW within any of the gestational age ranges. CONCLUSION: Impedance indices from the umbilical artery Doppler waveform decrease with advancing gestational age due, at least partially, to expansion of the placental vascular tree. Within narrow gestational age ranges in the third trimester, there is no meaningful correlation between fetal weight and impedance indices. It is therefore not necessary to adjust umbilical artery Doppler impedance indices to account for fetal size.     

Sonographic detection of trisomy 13 in the first and second trimesters of pregnancy.

OBJECTIVE: The purpose of this study was to examine sonographic findings in fetuses with trisomy 13. METHODS: A retrospective review of the cytogenetic laboratory databases at 6 tertiary referral centers identified all cases of trisomy 13. The prenatal sonographic studies in fetuses of less than 22 weeks' gestation, done before invasive testing for karyotype, were reviewed for anatomic and biometric findings. We defined abnormal fetal biometric findings as a biometric measurement (biparietal diameter, abdominal circumference, or femur length) below the fifth percentile in the second trimester. RESULTS: There were 8 cases of trisomy 13 found in the first trimester and 54 cases found in the second trimester, a total of 62 in all. In the first trimester, 6 of 8 had an anomaly identified (4 with cystic hygroma). In the second trimester, 49 of 54 were identified by sonography; 45 had an anomaly, and 4 had an abnormal fetal biometric measurement without an anomaly. The 5 missed diagnoses had early gestational age (<17 weeks; n = 3) or an inadequate survey secondary to poor visualization. Overall, 22 of 54 fetuses with trisomy 13 had an abnormal biometric measurement. The most common anomalies detected in the second trimester were heart defects (n = 34), central nervous system anomalies (n = 30), facial clefts (n = 19), abnormal hands (n = 13), and genitourinary anomalies (n = 9). CONCLUSIONS: Targeted sonography identified abnormal fetal anatomy or abnormal biometric measurements in 95% of fetuses with trisomy 13 in the second trimester after 17 weeks' gestation. A biometric measurement below the fifth percentile was noted in nearly half of cases in the second trimester.     

Detection rate of early fetal echocardiography and in utero development of congenital heart defects.

OBJECTIVE: The purpose of this study was to evaluate the detection rate of early fetal echocardiography and the in utero development of congenital heart defects (CHD). METHODS: Cases were selected from all singleton pregnancies between 1997 and 2003 in which detailed fetal 2-dimensional and color-coded Doppler echocardiography was performed in our prenatal unit between 11 weeks' and 13 weeks 6 days' gestation; 2165 cases with complete outcome parameters were analyzed. RESULTS: During this study period, CHD were diagnosed in 46 fetuses. Between 11 and 13 weeks' gestation, 29 CHD were diagnosed (11 weeks, 9 cases; 12 weeks, 8 cases; and 13 weeks, 12 cases); 9 CHD were found in the second trimester and 2 in the third trimester. The in utero detection rate of fetal echocardiography was 86.96% (n = 40). Six additional CHD (13.04%) were detected postnatally. The spectrum of detected CHD changed with advancing gestational age and was different from the postnatal detected heart defects. CONCLUSIONS: Early fetal echocardiography is feasible and allows the detection of most CHD. Congenital heart defects vary in appearance at different stages of pregnancy and may evolve in utero with advancing gestational age. Therefore, early fetal echocardiography should always be followed by echocardiography at mid gestation.     

Fetal nasal bone length in euploid and aneuploid fetuses between 11 and 20 weeks' gestation: a prospective study.

OBJECTIVE: To develop normative data for nasal bone length between 11 and 20 weeks' gestation and to assess the utility of nasal bone hypoplasia in the detection of fetal aneuploidy in the second trimester. METHODS: Well-dated, nonanomalous fetuses were examined between 11 and 20.9 weeks' gestation. The nasal bone was assessed and measured, and normative data from 11 to 20 weeks' gestation were determined. The nasal bone lengths in fetuses with confirmed aneuploidy were compared with the normative data. RESULTS: The fetal nasal bone length increased linearly with advancing gestational age. Nomograms including the 10th, 50th, and 90th percentiles were created. Nasal bone hypoplasia was seen in 6 of 6 cases of fetal trisomy in the second trimester. CONCLUSIONS: Nasal bone hypoplasia in the early second trimester identifies a cohort of fetuses at high risk for aneuploidy.     

Cervical length assessment in women with idiopathic polyhydramnios.

OBJECTIVE: The aims of the study were to determine cervical length among patients with polyhydramnios and to assess the relationship between the severity of polyhydramnios, cervical length and gestational age at delivery. PATIENTS AND METHODS: A prospective study was designed including 92 consecutive singleton pregnancies with polyhydramnios between 24 and 40 weeks' gestation. Cervical length was measured using transvaginal sonography. Polyhydramnios was defined when amniotic fluid index (AFI) was equal to or greater than 20 cm. A single sonologist performed all the examinations of the cervical length and the AFI. RESULTS: The median cervical length and AFI were 37.5 (range, 7-52) mm and 28.8 (range, 20-43) cm, respectively. A significant gradual shortening of the cervical length was observed with advancing gestational age (P=0.027). No significant association was found between AFI and cervical length (P=0.24). A cut-off of 15 mm (n=5) was associated with a significantly lower gestational age at delivery (30+/-2.6 weeks vs. 37.2+/-4.2 weeks, respectively, P<0.001). CONCLUSIONS: Women with polyhydramnios have a gradual shortening of cervical length with advancing gestational age. However, this finding is not related to the severity of polyhydramnios.     

Detection of sonographic markers of fetal aneuploidy depends on maternal and fetal characteristics.

OBJECTIVES: The purpose of this study was to determine factors that influence the detection rate of sonographic markers of fetal aneuploidy (SMFA). METHODS: We reviewed the sonographic images of 160 consecutive second-trimester trisomic fetuses for the presence of SMFA, either structural anomalies or sonographic soft markers. RESULTS: One hundred forty-nine (93.1%) records were complete and analyzed; 78 cases (52.3%) were identified with 1 or more SMFA. Sonographic markers of fetal aneuploidy were detected in 42.7%, 75.0%, and 90.9% of trisomies 21, 18, and 13, respectively (P<.005). The detection rate of SMFA had a positive linear correlation with gestational age (adjusted R(2)=0.64; P<.002). Sonographic markers of fetal aneuploidy were detected in 43.7% of fetuses of less than 18.0 weeks' gestation and 64.5% of fetuses of 18.0 weeks' gestation or greater (likelihood ratio=6.4; P<.01). Sonographic markers of fetal aneuploidy were detected in 23.5% of patients with suboptimal image quality versus 58.3% of the others (likelihood ratio=7.5; P<.05). The rate of structural malformation was similar between the male and female fetuses, whereas that of soft markers was 49.4% in male and 30.0% in female fetuses (odds ratio=2.3; range, 1.2-4.5; P<.02). Factor analysis showed that some soft markers and some structural anomalies tended to appear together. CONCLUSIONS: The type of fetal trisomy, gestational age, sex, and quality of images influence the detection rate of SMFA. The highest detection rate for SMFA in the second trimester is at or above 18 weeks' gestational age. Certain markers are detected in clusters. These findings may explain, in part, the variability in reported rates of detection of SMFA among trisomic fetuses. These findings need to be prospectively tested in the general population of pregnancies for applicability to sonographic risk calculations for fetal trisomies.     

Pulmonary artery diameters in healthy fetuses from 19 to 40 weeks' gestation.

OBJECTIVE: The purpose of this study was to construct nomograms for diameters of the fetal main pulmonary artery (MPA), right pulmonary artery (RPA), and left pulmonary artery (LPA) according to gestational age (GA) and estimated fetal weight (EFW). METHODS: Between May 2005 and February 2006, MPA and branch artery diameters were prospectively estimated with ultrasonography in 220 healthy fetuses from 19 to 40 weeks' gestation and correlated with GA and EFW. RESULTS: Mean MPA, RPA, and LPA diameters ranged from 2.93, 1.71, and 1.66 mm, respectively, at 19 weeks to 9.23, 5.49, and 5.65 mm at 40 weeks. Linear regression yielded the following formulas for the expected diameters according to GA: MPA=-2.77 + 0.30 x GA; RPA=-1.71 + 0.18 x GA; and LPA=-1.95 + 0.19 x GA. The following formulas were obtained by logarithm distribution for the expected diameters according to EFW: MPA=-1.165 + 0.247 x log of estimated fetal weight (lnFW); RPA=-0.651 + 0.141 x lnFW; and LPA=-0.718 + 0.150 x lnFW. CONCLUSIONS: Nomograms of MPA and branch pulmonary artery diameters according to GA and EFW have been described, generating reference values.     

Staging of intrauterine growth-restricted fetuses.

OBJECTIVE: The purpose of this study was to evaluate the value of cardiovascular, ultrasonographic, and clinical parameters for developing a staging classification of intrauterine growth-restricted (IUGR) fetuses delivered at 32 weeks or earlier. METHODS: Intrauterine growth restriction was defined as the presence of an estimated fetal weight below the 10th percentile. Intrauterine growth-restricted fetuses were staged according to the following parameters, with the presence of any 1 parameter in a stage placing the fetus in that stage: stage I, an abnormal umbilical artery or middle cerebral artery pulsatility index; stage II, an abnormal middle cerebral artery peak systolic velocity, umbilical artery absent/reversed diastolic flow, umbilical vein pulsation and an abnormal ductus venosus pulsatility index; and stage III, reversed flow at the ductus venosus or reversed flow at the umbilical vein, an abnormal tricuspid E wave (early ventricular filling)/A wave (late ventricular filling) ratio, and tricuspid regurgitation. Each stage was divided into A (amniotic fluid index [AFI] <5 cm) and B (AFI >5 cm). The presence of maternal abnormalities was also reported. RESULTS: Seventy-four IUGR fetuses delivered at 32 weeks or earlier were included. Gestational age at delivery was greater in stage I fetuses compared with the other stages. Birth weight decreased with advancing stages. Stage III fetuses had the lowest AFI. There was a direct correlation between the severity of staging and both perinatal mortality and mortality occurring between 20 weeks' gestation and before the neonates were discharged from the hospital (P < .05). CONCLUSIONS: The staging system proposed here may allow comparison of outcome data for IUGR fetuses and may be valuable in determining more timely delivery for these high-risk fetuses.     

Three-dimensional sonographic measurement of liver volume in the small-for-gestational-age fetus.

OBJECTIVE: To assess the growth of the fetal liver in normal pregnancies and to evaluate the ability of fetal liver volume measurement for prediction of small-for-gestational-age fetuses. METHODS: Three-dimensional sonographic examinations were performed on 14 appropriate-for-gestational-age and 10 small-for-gestational-age fetuses. Liver volume and liver length were measured every 2 weeks after 20 weeks' menstrual age until delivery. RESULTS: A curvilinear relationship was found between the menstrual age and liver volume (R2 = 88.4%; P < .0001), and a normal range of liver volume measurement for estimating the growth of the fetal liver during normal pregnancy was generated. Liver length was normal in 7 of 10 small-for-gestational-age fetuses, whereas liver volume values in all small-for-gestational-age fetuses were below normal ranges in the mid to late third trimester. CONCLUSIONS: Our findings suggest that liver volume may be a useful measurement for diagnosing small-for-gestational-age fetuses in the mid to late third trimester but that liver length may not be predictive. Further studies involving a larger sample size would be needed to confirm this suggestion.     

Use of the amniotic fluid index combined with estimated fetal weight within 10 days of delivery for prediction of macrosomia at birth.

OBJECTIVE: The purpose of this study was to assess the value of combining the sonographically estimated fetal weight (EFW) and amniotic fluid index (AFI) measured within 10 days of term delivery for prediction of macrosomia at birth. METHODS: Prospective sonographic fetal biometric measurements and delivery ward data of a single center, uploaded separately over a 4-year period, were retrospectively linked to yield an unselected sample of nondiabetic pregnancies with live-born term neonates. RESULTS: Of the 1925 pregnancies evaluated, 140 (7.2%) were macrosomic (birth weight > or =4000 g). The AFI was significantly higher in the macrosomic group (P < .001). On receiver operating characteristic curve analysis, the area under the curve was larger for predictions based on the EFW alone than on the AFI. An EFW of 4000 g or higher had a positive predictive value of 46.6% for macrosomia at birth. Use of the previously suggested combined EFW and AFI cutoffs of 3689 g and 119 mm, respectively, yielded a positive predictive value of 30.3%. CONCLUSIONS: Combined use of the EFW and AFI rather than the EFW alone does not improve prediction of macrosomia at birth.     

Is there a relationship between fetal weight and amniotic fluid index?

OBJECTIVE: To establish whether there is a relationship between the amniotic fluid index and estimated fetal weight in the third trimester. The presence of a relationship would require adjustment of amniotic fluid index to take account of estimated fetal weight with potential improvement in its prediction of adverse perinatal outcomes. METHODS: Paired measurements of amniotic fluid index and estimated fetal weight from 274 low-risk pregnancies enrolled in a longitudinal study of fetal growth. Measurements were made at fortnightly intervals from 30 weeks' gestation until delivery. A relationship between amniotic fluid index and estimated fetal weight was sought at gestational age week intervals of 30-32, 33-35, 36-38 and 39-41. RESULTS: One thousand and three pairs of measurements of amniotic fluid index and estimated fetal weight were available for analysis. Mean amniotic fluid index decreased towards term as expected. There was no correlation between amniotic fluid index and estimated fetal weight. Furthermore, there was no correlation between amniotic fluid index and estimated fetal weight at any of the gestational age intervals. CONCLUSIONS: There is no clinically relevant correlation between amniotic fluid index and estimated fetal weight. It should remain clinical practice to take account of gestational age when interpreting amniotic fluid index but it is not necessary to make adjustments for estimated fetal weight.     

Sonographic measurement of the fetal cerebellum,cisterna magna,and cavum septum pellucidum in normal fetuses in the second and third trimesters of pregnancy

PURPOSE: Absence of the cavum septum pellucidum (CSP), the cisterna magna (CM), or both, and enlargement of either or both structures are associated with various central nervous system malformations. In an effort to determine normal sizes and relationships between these cranial structures, we measured the CSP and CM in normal fetuses in the second and third trimesters of pregnancy using transabdominal sonography. METHODS: Women with uncomplicated pregnancies and normal singleton fetuses between 16 and 38 weeks' menstrual age were included in this prospective study. The width and anteroposterior (AP) diameters of the CSP were measured on the transverse transventricular plane, and the AP diameter of the CM was measured on the transcerebellar plane from the posterior aspect of the cerebellar vermis to the inner edge of the cranium. The transverse and AP diameters of the cerebellum were also measured. In addition, we measured the biparietal diameters (BPDs). RESULTS: In total, 130 women participated; 64 were examined in their second trimester and 66 in their third trimester. The mean age of the women was 27.4 +/- 4.8 years (range, 18-38 years), and the mean menstrual age of the fetuses was 26.9 +/- 6.7 weeks (range, 16-38 weeks). The mean BPD was 66.8 +/- 18.7 mm (range, 30-96 mm). The mean width and AP diameter of the CSP and the mean AP diameter of the CM differed significantly between the second and third trimesters (p < 0.001). All measured parameters correlated significantly with menstrual age and BPD. CONCLUSIONS: In normal fetuses, the CSP and CM should be visible on transabdominal sonography between 16 and 38 weeks' menstrual age. Because abnormalities in these cranial structures may be indicative of central nervous system malformations, the availability of mean sonographic measurements from normal fetuses should be helpful in determining the need for additional testing in fetuses with abnormal measurements.     

Sonographic detection of undescended testes in the third trimester.

OBJECTIVE: To assess the sonographic detection rate of fetal undescended testes among a low-risk population during the third trimester. METHODS: A sonographic evaluation, which included biometric studies and a detailed examination of the fetal genitalia, was performed prospectively on 332 male fetuses of singleton pregnancies between 34 and 40 weeks' gestation. A qualified neonatologist examined the presence of the testes within the scrotum within 3 days after birth. RESULTS: The scrotum was visible in 294 (89%) of the 332 fetuses who were examined. It was visible in all fetuses evaluated between 34 and 36 weeks' gestation. Nine cases of undescended testes were detected (3%). Of these, the diagnosis of 1 case, examined at 34 weeks' gestation, was revealed after birth to be false-positive. There were no false-negative results. CONCLUSIONS: Sonographic examination during the late third trimester of pregnancy appears to allow accurate diagnosis of undescended testes prenatally. This early identification will alert the neonatologist of the possibility of cryptorchidism and will permit early postnatal identification and treatment.     

The development of the fetal sternum: a cross-sectional sonographic study.

OBJECTIVE: To assess the relationship between gestational age and sonographic appearance of the various sternal components and establish growth during human gestation. DESIGN: A prospective cross-sectional study. METHODS: The study was performed on 252 consecutive normal singleton pregnancies from 19 weeks of gestation until term, using transabdominal high-resolution ultrasound techniques. The sternal length, as well as the number of ossification centers at each gestational age, were recorded. RESULTS: The first occasion at which a fetal human sternum could be visualized with two to three ossification centers was at 19 weeks' gestational age. The fifth ossification center was first visualized at 29 weeks' gestation. The mean +/- SE of sternal length varied from 15 +/- 0.98 mm (95% confidence interval (CI) 12.79-17.21) at 19-20 weeks, to 36.50 +/- 0.29 mm (95% CI 35.58-37.42) at 37-38 weeks' gestation. Sternal length as a function of gestational age was expressed by the regression equation: sternal length (mm) = -11.06 + 1.39 x gestational age (weeks). The correlation coefficient, r = 0.924 for sternal length, was found to be highly statistically significant (p < 0.0001). CONCLUSIONS: The presented data offer normative measurements of the fetal sternum which may be helpful in the prenatal diagnosis of congenital syndromes that include, among other manifestations, abnormalities of sternal development.     

High perinatal survival in monoamniotic twins managed by prophylactic sulindac, intensive ultrasound surveillance, and Cesarean delivery at 32 weeks' gestation.

OBJECTIVES: Increased perinatal mortality in monoamniotic twin pregnancies is attributed to cord accidents in utero and at delivery. We evaluated the following parameters in monoamniotic pregnancies: (1) the incidence of cord entanglement; (2) the effect of sulindac on amniotic fluid volume and stability of fetal lie; and (3) the perinatal outcome with our current management paradigm. METHODS: This is a retrospective review of monoamniotic pregnancies of >or=20 weeks' gestation managed with serial ultrasound surveillance, medical amnioreduction and elective Cesarean delivery at 32 weeks' gestation. Mean amniotic fluid index (AFI) and change in AFI in monoamniotic pregnancies managed with oral sulindac was compared with 40 gestation-matched monochorionic-diamniotic controls. RESULTS: Among 44 monoamniotic pregnancies, 20 with two live structurally normal twins at 20 weeks' gestation satisfied the inclusion criteria. All fetuses survived to 28 days postnatally despite early prenatal cord entanglement in all but one case. Whereas AFI remained stable throughout gestation in the controls, the AFI fell in those patients on sulindac from a mean value of 21.0 cm (95% CI, 18.5-23.6 cm) at 20 weeks to a mean of 12.4 cm (95% CI, 10.1-14.6 cm) at 32 weeks (ANOVA P across gestation = 0.001) but mainly remained within normal limits. Fetal lie was stabilized in 11/20 cases in the monoamniotic group compared with 13/40 in the control group (P < 0.0001). CONCLUSIONS: Cord entanglement appears unpreventable, as it typically occurs in early pregnancy. Sulindac therapy reduces AFI, leads to more stable fetal lie, and may prevent intrauterine death by diminishing the risk of constricting cords that are already entangled. Perinatal survival in monoamniotic pregnancies managed by a regime of sulindac from 20 weeks' gestation, close ultrasound surveillance and elective abdominal delivery at 32 weeks' gestation seems empirically higher than that in the literature.     

Fetal scapular length in the ultrasonographic assessment of gestational age.

A prospective cross-sectional study of 515 singleton fetuses of ages between 15 and 42 weeks' gestation was performed. At gestational ages greater than 26 weeks, only fetuses with a sonographic estimated fetal weight between the 10th and 90th percentiles for growth were included. Scapular length (cm) as a function of gestational age (weeks) was expressed by the regression equation: SL = 0.3289 + 0.9553 (GA) with a Pearson correlation coefficient of R2 = 0.942. This study defines the normal limits of scapular length, demonstrates a high correlation between scapular length, gestational age, and other standard measurements of fetal growth, and indicates that scapular length can predict gestational age in fetuses with normal growth.     

Sonography of fetal choroid plexus cysts: detection depends on cyst size and gestational age.

OBJECTIVE: To establish diagnostic criteria for the diagnosis of fetal choroid plexus cysts across gestation. METHODS: Prenatal sonographic images of 166 fetuses without choroid plexus cysts were prospectively collected from 13 weeks to term. Texture characteristics of the choroid plexus regions were analyzed to quantify changes as a function of gestational age. A set of 20 fetal sonographic images with choroid plexus cysts were used to create cyst prototypes (1.5-2.5 mm), which were randomly embedded into normal choroid plexus images from varying gestational ages. A test set of 544 images was created, which included 408 images with choroid plexus cysts and 136 images without choroid plexus cysts. Four observers following a blinded study design evaluated the presence of choroid plexus cysts in the images. The influence of cyst size and gestational age on the detection of cysts was measured with receiver operating characteristic analysis and analysis of variance. Observer agreement was characterized by agreement kappa statistics. RESULTS: Texture analysis indicated greater echo texture heterogeneity at earlier gestational ages. The receiver operating characteristic analysis showed a corresponding decrease in diagnostic accuracy for cyst detection at earlier gestational ages. The cyst detection threshold (area under the receiver operating characteristic curve, 0.8) was 2.4 mm (SE, 0.2 mm) for 13 to 21 weeks' gestation and decreased to 1.9 mm (SE, 0.17 mm) for 22 to 38 weeks' gestation. Average interobserver and intraobserver kappa statistics were 0.37 and 0.53, respectively. CONCLUSIONS: Because of the changing echo texture of the choroid plexus through gestation, choroid plexus cysts must be at least 2.5 mm in diameter for confident diagnosis before 22 weeks' gestation and at least 2 mm after 22 weeks.     

Relationship between cerebroplacental Doppler ratio and birth weight in postdates pregnancies.

OBJECTIVES: To explore the relationship between cerebroplacental Doppler impedance index and birth weight in postdates pregnancies, and to evaluate the use of a combination of Doppler parameters and ultrasound biometry in the prediction of large-for-gestational age (LGA) fetuses at 41 weeks of gestation. METHODS: The pulsatility indices of the umbilical (UA-PI) and middle cerebral (MCA-PI) arteries, the cerebroplacental pulsatility index ratio (CPR) and the estimated fetal weight (EFW) were obtained in a cohort of 181 ultrasound-dated pregnancies at 41 weeks' gestation, 2 days before induction of delivery. A regression equation was established and the correlation between umbilical artery impedance and different birth-weight centile groups was determined. A receiver-operating characteristics (ROC) curve was used to compare prediction of LGA fetuses using biometry alone with that using biometry and UA-PI. RESULTS: UA-PI was inversely related to EFW (Spearson's correlation coefficient rho = -0.28, P < 0.001). Logistic regression showed an independent contribution of UA-PI to the birth-weight estimation (birth weight = 1356.8 - 232.0 x UA-PI + 0.65 x EFW). On ROC curve analysis, the prediction of LGA with the regression equation was comparable to that using ultrasound biometry alone. CONCLUSION: UA-PI was inversely correlated to EFW, but the combination of ultrasound biometry and UA-PI compared with biometry alone showed similar prediction of LGA fetuses in postdates pregnancies. Further prospective trials on larger populations or groups with a higher prevalence of LGA fetuses would be needed to validate the use of the new formula.