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

Fluid dynamics during menstrual cycle

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12. 0ian P, MaltauJM, Noddeland H, Fadnes HO. Oedemapreventing mechanisms in subcutis in normal pregnant women. Br J Obstet Gynaecol 1985;92: 1113. 13. Landis EM, Pappenheimer JR. In: Hamilton WF, Dow P, eds. Handbook of physiology. Washington DC: American Physiolo~ical Society, 1963:961-1034.

14. Noddeland H, Riisnes SM, Fadnes HO. Interstitial fluid colloid osmotic and hydrostatic pressures in subcutaneous tissue of patients with nephrotic syndrome.· Scand J Clin Lab Invest 1982;42:139.

Estimating fetal age using multiple parameters: A prospective evaluation in a racially mixed population Frank P. Hadlock, M.D., Ronald B. Harrist, Ph.D., Yogesh P. Shah, M.D., David E. King, M.D., Seung K. Park, M.D., and Ralph S. Sharman, M.D. Houston, Texas 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. (AM J 0BSTET GYNECOL 1987;156:955-7.)

Key words: Ultrasonography, fetal age The use of multiple fetal measurements to determine menstrual age is now an accepted concept, although very few investigators have evaluated its validity. 1· 6 In a previous report, we demonstrated the efficacy of this technique using complex regression equations that were developed on a middle-class private white population.6 In the present study these equations were evaluated prospectively on a mixed racial population from a county hospital.

Material and methods The study group consisted of 300 indigent black (38%) and Hispanic (62%) patients seen at a county hospital in the Houston area. All of these patients knew the beginning day of the last menstrual period and denied oral contraceptive use in the 3-month period before the examination. Patients with maternal disease that might adversely effect fetal growth (e.g., hypertension and diabetes mellitus) and patients with mul-

From the Department of Radiology, Baylor College of Medicine, and the Department of Biometry, University of Texas School of Public Health. Received for publication July 18, 1986; accepted October 30, 1986. Reprint requests: Frank P. Hadlock, M.D.,jefferson Davis Hospital, Department of Radiology, 1801 Allen Pkwy., Houston, TX 77019.

Table I. Comparison of variability in fetal age prediction (21 to 42 weeks) with single versus multiple parameters Feta[ parameters

BPD HC AC FL BPD,FL HC, FL BPD,AC,FL HC, AC, FL BPD, HC, AC, FL

Variability (mean ± 2 SD) in weeks

-0.05 -0.60 -0.30 -0.20 -0.30 -0.40 -0.30 -0.30 -0.30

± ± ± ±

± ±

± ±

±

2.84 2.76 2.80 2.82 2.32 2.30 2.22 2.20 2.18

BPD = Biparietal diameter; HC = head circumference; AC = abdominal circumference; FL = femur length.

tiple gestations (e.g., twins) were excluded from the study. No fetuses included in the study had apparent fetal anomalies, and measurements of head size, abdominal size, femur length, and weight were within the normal range ( + 2 SD) for the stated menstrual age. 6 Each fetus was measured only once in gestation. All examinations were performed by physicians using a commercially available linear-array realtime ultrasonographic system with a 3.5 MHz focused transducer 955

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April 1987 Am J Obstet Gynecol

Hadlock et al.

MIDDLE OF INTERVAL -3.5 -3.0

-2.5 -2.0 -1.5 -1.0

-.5

o.o .5 1. 0 1. 5

2.0 2.5 3.0

NUMBER OF OBSERVATIONS 1

0 9 22 27 42 51 58

41 26 11 9 1

2

* ***** *********** ************** ********************* ************************** ***************************** ********************* ************* ****** *****

* *

Fig. l. This histogram of the residuals (differences between actual menstrual age and predicted menstrual age) is from the regression model based on all four fetal measurements used together. Note that the residuals are almost perfectly centered on zero, indicating very little systematic bias. Also note that 96% of the observations fall within ± 2 weeks, which is a relatively narrow band of variability for patients in the second half of pregnancy. (Residuals are rounded to the nearest half week; asterisk indicates two observations.)

Table II. Subgroup variability ( ± 2 SD) in predicting fetal age with single versus multiple parameters Fetal parameters

20-24 Wk (N = 32)

24-30 Wk (N = 105)

30-36 Wk (N = 118)

36-42 Wk (N = 45)

BPD HC AC FL BPD,FL HC, FL BPD,AC,FL HC, AC, FL BPD, HC, AC, FL

± 1.70 ± 1.80 ± 1.65 ±2.40 ± 1.80 ± 1.76 ± 1.57 ± 1.61 ± 1.54

±2.70 ±2.88 ±2.64 ±2.82 ±2.35 ±2.42 ±2.24 ±2.30 ±2.28

±2.76 ±2.78 ±2.80 ±2.40 ±2.14 ±2.06 ±2.12 ±2.04 ±2.04

±3.02 ±2.62 ±3.46 ±2.98 ±2.22 ±2.26 ±2.36 ±2.34 ±2.24

BPD

=

Biparietal diameter; HC

=

head circumference; AC

(ADR, Tempe, Arizona). The techniques for imaging and measuring the biparietal diameter, femur length, head circumference, and abdominal circumference are described in detail elsewhere. 6 Measurements of the biparietal diameter and femur length were made with electronic calipers, and circumferences of the head and abdomen were calculated from two diameters according to the formula for the circumference of an ellipse. Regression models developed from our previous study of 361 white fetuses seen between 14 and 42 menstrual weeks were used to predict menstrual age from the sonographic measurements obtained in this study. Regression equations using the four individual parameters alone and the optimal combinations of these parameters, based on our previous study, 6 were evaluated. The errors in predicting menstrual age were calculated by subtracting the actual menstrual age from the predicted menstrual age. The t test was used to determine if the mean errors were significantly different from zero, and the F test was used to determine if

= abdominal circumference;

FL

=

femur length.

the SDs of the mean errors were significantly different from each other.

Results The results from this study are similar to those reported in our previous study of white fetuses 6 (Tables I and II). The mean differences were significantly different from zero (p ~ 0.05) except for that of the bi parietal diameter, but those of the optimal models do not appear to be clinically significant (2 to 3 days). Of major interest is the comparison of the SDs, which are an index of the random errors that may be observed with these parameters to predict menstrual age. No differences (p ~ 0.05) could be demonstrated between the four models in which individual parameters were used alone, and no significant differences could be demonstrated between the models that used more than one parameter. However, all the regression equations that used more than one parameter demonstrated significant increases (p ~ 0.05) in the accuracy in pre-

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dieting menstrual age in comparison with the models in which individual parameters were used alone. Using the model based on all four parameters in combination, 96% of the predicted ages were within 2 weeks of the true menstrual age; this finding is identical to that observed in the original study 6 (Fig. 1). Comment

The concept of using more than one fetal measurement to predict menstrual age is now well accepted, but the number of studies demonstrating the efficacy of this technique is quite small. 1-5 Our previous work in evaluation of middle-class white patients demonstrated unequivocally that the use of more than one fetal measurement can significantly increase the accuracy of predicting menstrual age. 6 The major importance of the current study is that it validates prospectively the concept that the use of multiple parameters in estimating fetal age offers a significant advantage over any single parameter used alone, and 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. A surprising finding, as yet unexplained, is the slightly higher variability observed for all models in the 24- to 30-week subinterval as compared with the 30- to 36-week subinterval; further studies are planned to evaluate this finding (Table II). As we have indicated in our previous report, 6 the choice of parameters used to predict age in a given case should depend on the number of technically satisfactory measurements obtained. Images that indicate abnormal fetal anatomy (e.g., hydrocephalus, ascites, and femur fractures) should not be measured for purposes of fetal dating. The biparietal diameter should not be included when the cephalic index is outside the normal range. 7 Similarly, the femur length/head circumference ratio should be used to eliminate possible microcephaly or dwarfism, 8 and the femur length/abdominal circumference ratio should be used to make sure abdominal growth is not affected by macrosomia or growth retardation. 9 Thus if the cephalic index and the two body ratios are normal, we choose to use all four measure-

Estimating fetal age with multiple parameters

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ments to predict age. If the cephalic index is abnormal but the body ratios are normal, we would use the model based on head circumference, abdominal circumference, and femur length. If cephalic index and femur length/abdominal circumference are abnormal but femur length/head circumference is normal, we would use the model based on head circumference and femur length. In summary, one must critically examine the images and various ratios to ensure that the optimal model for predicting age is chosen in a given case. 6 - 1° For those who adopt our equations for use in clinical practice, it would be prudent to test these models in several hundred patients with known dates so that any potentially excessive systematic or random errors can be detected. If such errors are found, it will be necessary for the individual laboratory to develop its own equations using multiple parameters.

REFERENCES 1. Guidelines for obstetrical sonography. American College of Radiology (adopted 1985). 2. Guidelines for obstetrical sonography. American Institute of Ultrasound in Medicine (adopted 1985). 3. Queenan JT. OB ultrasound in the office. Contemp OB/GYN 1982;20: 134-66. 4. Oman SD, Wax Y. Estimating fetal age by ultrasound measurements: an example of multivariate calibration. Biometrics 1984;40:94 7-60. 5. Hadlock FP, Deter RL, Harrist RB, Park SK. Computer assisted analysis of fetal age in the third trimester using multiple fetal growth parameters. J Clio Ultrasound 1983;11:313-6. 6. Hadlock FP, Deter RL, Harrist RB, Park SK. Estimating fetal age: computer-assisted analysis of multiple fetal growth parameters. Radiology 1984;152:497-501. 7. Hadlock FP, Deter RL, Carpenter RJ, Park SK. Estimating fetal age: effect of head shape on BPD. AJR 1981;137: 83-5. 8. Hadlock FP, Harrist RB, Shah Y, Park SK. The femur length/head circumference relation in obstetric sonography. J Ultrasound Med l 984;3:439-42. 9. Hadlock FP, Deter RL, Harrist RB, et al. A date independent predictor of intrauterine growth retardation: femur length/abdominal circumference ratio. AJR 1983; 141 :979-84. 10. Hohler CW. Cross-checking pregnancy landmarks by ultrasound. Contemp OB/GYN 1982;20:169-75.