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ORIGINAL RESEARCH |
From the Division of Maternal-Fetal Medicine, Spartanburg Regional Healthcare System, Spartanburg, South Carolina; the Department of Obstetrics and Gynecology, University of Mississippi, Jackson, Mississippi; and the Department of Obstetrics and Gynecology, Medical College of Georgia, Augusta, Georgia.
Address reprint requests to: Suneet P. Chauhan, MD Division of Maternal-Fetal Medicine Spartanburg Regional Health Care System 853 North Church Street, Suite 403 Spartanburg, SC 29303 E-mail: mfmchauhan{at}aol.com
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Abstract |
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Methods: Over 2 years, parturients with reliable gestational ages, preeclampsia, and sonographic estimates of birth weights were matched (1:1) for gestational age with women without preeclampsia. Paired and unpaired t tests were used; P < .05 was significant. Odds ratios (ORs) and 95% confidence intervals (CIs) were calculated.
Results: Two hundred eighty-seven preeclamptic women were identified and matched. In each group, mean (± standard deviation [SD]) gestational age was 34.9 ± 4.2 weeks, and 166 (57.8%) infants were born preterm. Fetal growth restriction (FGR) was significantly more common among women with preeclampsia (14.9%) than among controls (5.6%; OR 2.98, 95% CI 1.64, 5.44). The percentage of sonographic estimates within 10% of actual birth weight (57.5% versus 53.6%) was similar in the two groups (OR 1.16; 95% CI 0.84,1.62). Compared with normal growth, the mean (± SD) standardized absolute error was significantly higher among those with FGR regardless of group (preeclampsia 109 ± 100 versus 158 ± 152 g/kg; P = .009; control 117 ± 103 versus 233 ± 206 g/kg; P < .001). Fetal growth restriction was detected more commonly among preeclamptic women than among controls (11.6% versus 0%; OR 4.74 95% CI 0.25, 90.31). The sensitivity and positive predictive value of FGR detection were 10% and 50%, respectively, among women with preeclampsia and 0% each among controls.
Conclusion: Although FGR was detected more frequently in fetuses of women with preeclampsia than in those of controls, the ability to predict it with sonography remained poor.
Among preeclamptic women (those with a sustained elevation of blood pressure [BP] of 140 mmHg systolic or 90 mmHg diastolic after 20 weeks’ gestation, proteinuria, and edema1), detection of growth-restricted fetuses (weight less than 10th percentile for gestational age) is important because, with growth disturbances, mild diseases can become severe, fetal growth restriction (FGR) is associated with increased risk of poor perinatal outcome2 and neurodevelopmental delay,3 and conservative management of severe preeclampsia at 28 to 32 weeks’ gestation depends on clinicians’ ability to diagnose FGR. Fetal growth restriction can be a contraindication for expectant management of severe preeclampsia, especially very preterm disease. 4–6
Despite the importance of antenatal diagnosis of FGR among women with hypertensive disease, there is a paucity of reports on the diagnostic accuracy of identifying a growth-restricted fetus. The purpose of this case control study was to determine the accuracy of sonographic estimates of fetal weight to identify fetuses that will be born with weights below the 10th percentile to women with and without hypertensive disease of pregnancy.
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Materials and Methods |
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Chronic hypertension was defined as a documented BP of 140/90 mmHg or more before 20 weeks’ gestation or prescription of antihypertensive medications before conception. Superimposed preeclampsia was diagnosed if there was an elevation of 30 mmHg of systolic or 15 mmHg of diastolic pressure over the BP recorded in the first trimester with an increase in 24-hour proteinuria of 1 g or more. Women were considered preeclamptic if on two occasions at least 6 hours apart they had systolic BP of at least 140 mmHg or diastolic BP of at least 90 mmHg, and proteinuria of at least 300 mg in 24 hours. Severe preeclampsia was defined according to ACOG criteria.1 A parturient was diagnosed with eclampsia if she had grand mal seizures with other criteria of preeclampsia. Hemolysis, elevated liver enzymes and low platelets (HELLP) syndrome was defined according to the criteria of Martin and coworkers.7 Insulin-dependent diabetes (class A2) was diagnosed if patients prescribed 1800 kcal American Diabetic Association diets had fasting or 2-hour post-prandial glucose values more than 105 or 120 mg/dL, respectively. Diabetes mellitus diagnosed before pregnancy was categorized according to the White classification. Delivery before 37 weeks was considered pre-term. Fetal growth restriction and small for gestational age (SGA) were defined as sonographic predictions and actual birth weights, respectively, below the 10th percentile for gestational age.8
The accuracy of sonographically estimated fetal weights (EFWs) was measured by comparing the mean error (birth weight-the predicted weight in grams), mean standardized absolute error (absolute value of error in grams ÷ birth weight in kilograms x 1000), and percentage of predictions within 10% of birth weights. Assuming 55% of the sonographic estimates are within 10% of actual birth weight in the control group,9 275 patients were required in the two groups to detect a difference of 12% among preeclamptic women (
= .5, and ß = .20). Paired or unpaired t tests or nonparametric tests (if the distribution was not Gaussian) were used and P < .05 was significant. Odds ratios (ORs) and 95% confidence intervals (CIs) were calculated; if there was one value of 0, then to make calculations possible, 0.5 was added to each cell. Sensitivity, specificity, test efficiency (portion of patients correctly assigned normal or abnormal), and positive and negative predictive values were determined.
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Results |
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shows the maternal and neonatal characteristics of the groups. The portion of nulliparas was similar in the study (103 of 287, 35.9%) and control (94 of 287, 32.5%; OR 1.15, 95% CI 0.81, 1.62) groups. In both groups, 19 (6.6%), 147 (51.2%), and 121 (42.2%) delivered before 28, between 28 and 36.9, and at 37 or more weeks, respectively. Although the sonographic estimates of the birth weights were similar between groups, actual birth weights were significantly different. In the two groups, mean error, mean standardized absolute error, and percentage of estimates within 10% of the actual weight were similar.
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). Of the 43 SGA infants in the study group, 63% were from pregnancies with severe preeclampsia (27 of 43), and 37% had mothers with mild disease (16 of 43). Preterm SGA infants were born significantly more often in the study (32 of 43, 74%) than in the control group (three of 13, 23%; OR 9.69 95% CI 2.25, 41.80). In the study group, sensitivity, specificity, test efficiency, and positive and negative predictive values of sonographic estimated fetal weights under 10%, to identify growth-restricted fetuses, were 10%, 98%, 84%, 50%, and 84%, respectively. The corresponding values in the control group were 0%, 99%, 94%, 0%, and 94%.
Table 2 shows the accuracy of sonographic prediction of birth weights among the two groups with and without SGA. Fetal growth restriction was detected more commonly among preeclamptic women than among controls (11.6% versus 0%; OR 4.74, 95% CI 0.25, 90.31). The mean standardized absolute error among the 43 preeclamptic women with SGA infants (158 ± 152 g/kg) was similar to that among the women with SGA infants in the control group (233 ± 206 g/kg; P = .21). Newborns with abnormal growth in the two groups had similar percentages of estimates within 10% of actual weight (51% versus 44%; OR 1.34, 95% CI 0.42, 4.27).
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Discussion |
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Among newborns of preeclamptic women, not only the AC, but BPD, HC, and FL were significantly lower among the SGA newborns than among those of normal weight (Table 2), which was surprising, considering gestational age was similar among hypertensive women with and without FGR, and with hypertensive disease, FGR is asymmetrical. A possible explanation for that finding is that FGR occurs early in gestation and affects all biometric characteristics. That hypothesis is consistent with the abnormal Doppler findings at 12–16 weeks10 among women who subsequently developed hypertensive disease, and the fact that atherosis in the placental vessel is present by 20–22 weeks.11
Overall accuracy of estimating birth weights sonographically was similar between the groups (Table 1) and among newborns with weights above the 10th percentile (Table 2). However, inaccuracy of fetal weight estimation is significantly higher among pregnancies with FGR than among those without (Table 2). Regardless of group, SGA fetuses had similar error, mean standardized absolute error, and percentage of estimates within 10% of birth weights, which indicated that the accuracy of sonographic estimates was related to growth abnormalities and not hypertension or preeclampsia. In the study and control groups, newborns with growth abnormalities had negative mean errors (-205 ± 365 and -286 ± 515 g, respectively) indicating that sonographic predictions were overestimates, whereas neonates with normal growth had positive mean errors (34 ± 413 and 62 ± 394 g, respectively) indicating underestimation. A possible reason for the higher inaccuracy among fetuses with growth below the 10th percentile is that regression equations, used to calculate the sonographic estimates of weight, were derived from fetuses with normal growth.12 Decreased soft tissue with FGR might render predictions of birth weight intrinsically inaccurate.
The most important finding of the study was that antenatal detection of FGR was poor. Overall, the sensitivity of sonographic estimates of fetal weights under 10% for gestational age to detect FGR was only 10%, with a positive predictive value of 50%. Among all preeclamptic women at 28–32 weeks, the corresponding values were 18% and 50%, respectively. For women with severe preeclampsia at 28–32 weeks, the sensitivity of detecting FGR was 20%, and the positive predictive value only 50%. The inability to reliably and consistently detect abnormal growth in a majority of cases was reported by other investigators. David et al13 found that the sensitivity of sonographically estimated fetal weights (derived from 26 models) below 10% for gestational age ranged from 0 to 57%. Using the model proposed by Hadlock et al,12 those authors found a sensitivity of 23%. The following observation also lends credence to the poor ability to identify FGR antenatally: Among 68 women with severe preeclampsia managed expectantly, Chari et al5 reported delivery of 15 (22%) newborns with weight below 10% and none of them were detected before delivery.
If sonographic estimation of birth weight is a poor means of detecting FGR, it is reasonable to ask if other methods have better accuracy. Chang et al14 did a 15-year review of English language journals to find the most appropriate ultrasonic measurement for predicting FGR. After analyzing 117 articles, the authors concluded that placental grading, decreased amniotic fluid volume, reduced total intrauterine volume, and Doppler waveform indices did not have as high a sensitivity or predictive accuracy as AC or estimated fetal weight under 10% for gestational age. Schucker et al6 recently reported that among 136 patients with severe preeclampsia managed expectantly for at least 48 hours, an amniotic fluid index no more than 5.0 cm on admission had a sensitivity of 14% for FGR detection. Thus, accurate antenatal diagnosis of FGR remains problematic.
There are several limitations of this study that should be acknowledged. One shortcoming is that residents, maternal-fetal fellows, and sonographers undertook ultrasonographic examinations. Although it is possible that estimates of sonographers were more accurate than those of physicians in training, prospective studies have not confirmed this.9 It is unlikely that even if sonographers performed all the biometric measurements the detection of FGR would have improved. We acknowledge that the definition of FGR is variable. Some investigators consider FGR to be birth weight below 5%,5 10%,4,6,13,14 or 15%.15 We selected the criterion of neonatal weight of less than 10% for FGR because it is used most commonly. It is unlikely our findings would be different if another threshold were chosen, although future studies are needed to confirm them. Clinical management schemes that presume antenatal diagnosis of FGR is reliable should be used with caution.
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Footnotes |
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Received June 22, 1998. Received in revised form October 8, 1998. Accepted October 29, 1998.
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References |
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2. Piper JM, Langer O, Xenakis EM, McFarland M, Elliott BD, Berkus MD. Perinatal outcome in growth-restricted fetuses: Do hypertensive and normotensive pregnancies differ? Obstet Gynecol 1996;88: 194–9.[Abstract]
3. Spinillo A, Iasci A, Capuzzo E, Stronati M, Ometto A, Guaschino S. Early neonatal prognosis in preeclampsia: A matched case-control study in low birth weight infants. Hypertens Preg 1993;12:507–15.
4. Sibai BM, Mercer BM, Schiff E, Friedman SA. Aggressive versus expectant management of severe preeclampsia at 28 to 32 weeks’ gestation: A randomized controlled trial. Am J Obstet Gynecol 1994;171:818–22.[Medline]
5. Chari RS, Friedman SA, O’Brien JM, Sibai BM. Daily antenatal testing in women with severe preeclampsia. Am J Obstet Gynecol 1995;173:1207–10.[Medline]
6. Schucker JL, Mercer BM, Audibert F, Lewis RL, Friedman SA, Sibai BM. Serial amniotic fluid index in severe preeclampsia: A poor predictor of adverse outcome. Am J Obstet Gynecol 1996;175:1018–23.[Medline]
7. Martin JN Jr, Blake P, Lowry S, Perry KG Jr, Files JC, Morrison JC. Pregnancy complicated by preeclampsia-eclampsia with the syndrome of hemolysis, elevated liver enzymes, and low platelet count: How rapid is postpartum recovery? Obstet Gynecol 1990; 76:737–41.[Medline]
8. Braner WE, Edelman DA, Hendricks CH. A standard of fetal growth for the United States of America. Am J Obstet Gynecol 1976;126:555–64.[Medline]
9. Chauhan SP, Cowan BD, Magann EF, Bradford TH, Roberts WE, Morrison JC. Intrapartum detection of macrosomic fetus: Clinical versus eight sonographic models. Aust N Z J Obstet Gynaecol 1995;35:266–70.[Medline]
10. Harrington K, Carpenter RG, Goldfrad C, Campbell S. Transvaginal Doppler ultrasound of the uteroplacental circulation in the early prediction of preeclampsia and intrauterine growth retardation. Br J Obstet Gynaecol 1997;104:674–81.[Medline]
11. Roberts JM, Redman CW. Preeclampsia: More than pregnancy-induced hypertension. Lancet 1993;341:1447–51.[Medline]
12. Hadlock FP, Harrist RB, Sharman RS, Deter RL, Park SK. Estimation of fetal weight with the use of head, body and femur measurements—A prospective study. Am J Obstet Gynecol 1985; 151:333–7.[Medline]
13. David C, Tagliavini G, Pilu G, Rudenholz A, Bovicelli L. Receiver-operator characteristic curves for the ultrasonographic prediction of small-for-gestational-age fetuses in low risk pregnancies. Am J Obstet Gynecol 1996;174:1037–42.[Medline]
14. Chang TC, Robson SC, Boys RJ, Spencer JAD. Prediction of the small-for-gestational-age infant: Which ultrasonic measurement is best? Obstet Gynecol 1992;80:1030–8.
15. Seeds JW, Peng T. Impaired growth and risk of fetal death: Is the tenth percentile the appropriate standard? Am J Obstet Gynecol 1998;178:658–69.[Medline]
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