Harvard Medical School, Boston, Massachusetts USA. Revision ... EFW, estimated fetal weight; FL, femur length; GA, ges- ... risk for adverse neonatal outcomes, sonography has been used to identify ...... The role of ultrasound in the diagnosis.
Article
Adverse Birth Outcomes in Relation to Prenatal Sonographic Measurements of Fetal Size Rebecca Smith-Bindman, MD, Philip W. Chu, MS, Jeff Ecker, MD, Vickie A. Feldstein, MD, Roy A. Filly, MD, Peter Bacchetti, PhD Objective. To evaluate and quantify the prediction of multiple neonatal outcomes by sonographically estimated fetal weight across a broad range of gestational ages. Methods. A retrospective cohort analysis was conducted among women with certain gestational age (n = 1376) seen at the University of California San Francisco from 1994 through 1997. The relative risks for small size at birth, small (low birth weight) for gestational age, and adverse neonatal outcomes were compared between small and average-sized fetuses. Results. Fetuses with an estimated fetal weight in the 5th percentile or less for gestational age were at increased risk of a birth weight less than 2000 g (relative risk, 6.5), a birth weight in less than the 3rd percentile for gestational age (relative risk, 10.1), preterm birth (relative risk, 2.2), extreme preterm birth (relative risk, 5.7), prolonged neonatal hospital stay (relative risk, 2.7), neonatal intensive care unit admission (relative risk, 3.2), and stillbirth or neonatal death (relative risk, 7.7) compared with averagesized fetuses (all P < .0001). With intrauterine growth restriction defined as an estimated fetal weight in the 5th percentile or less for gestational age, up to 29% of fetuses with adverse neonatal outcomes were detected, for false-positive rates of only 4% to 5%. After adjusting for confounding variables, low estimated fetal weight remained a significant predictor of neonatal morbidity and mortality. Conclusions. Morbidity and mortality are significantly increased among fetuses with an estimated fetal weight in the 5th percentile or less for gestational age. Key words: birth outcomes; fetal biometry; fetal weight; intrauterine growth restriction; neonatal morbidity; neonatal mortality; neonatal outcomes; prenatal sonography. Abbreviations AC, abdominal circumference; BPD, biparietal diameter; EFW, estimated fetal weight; FL, femur length; GA, gestational age; HC, head circumference; IUGR, intrauterine growth restriction; LBW, low birth weight; LMP, last menstrual period; NICU, neonatal intensive care unit; SGA, small for gestational age; UCSF, University of California San Francisco Received October 1, 2002, from the Departments of Radiology (R.S.-B., P.W.C., V.A.F., R.A.F.) and Epidemiology and Biostatistics (R.S.-B., P.B.), University of California, San Francisco, California; and Department of Obstetrics and Gynecology (J.E.), Massachusetts General Hospital, Harvard Medical School, Boston, Massachusetts USA. Revision requested November 11, 2002. Revised manuscript accepted for publication December 12, 2002. We thank David Rock and Jeannie Rabold (Acuson; Siemens Medical Solutions, Mountain View, CA) for generous assistance in obtaining the data from the Aegis data system to complete these analyses and Travis Seawards for assistance in preparing the manuscript. Address correspondence and reprint requests to Rebecca Smith-Bindman, MD, Department of Radiology, University of California San Francisco, UCSF/Mount Zion Campus, 1600 Divisadero St, San Francisco, CA 94115 USA.
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nfants who are small for gestational age (SGA) at birth (low birth weight [LBW]) have higher neonatal morbidity and mortality and worse long-term prognoses than infants who are appropriate size for gestational age (GA).1,2 In an attempt to identify fetuses at greatest risk for adverse neonatal outcomes, sonography has been used to identify small size in utero at varying points in gestation. Because it is often difficult to distinguish constitutionally small fetuses from fetuses whose sizes are altered by a pathologic process (such as placental insufficiency), all small fetuses are usually classified together and considered to have intrauterine growth restriction (IUGR).3,4 Although many definitions of IUGR have been used, the most common definition is an estimated fetal weight (EFW) in less than the 10th percentile for GA at any point in gestation.4 Sonographic measurements of the fetal head, abdomen, and extremities are used to estimate weight,5 which is compared with a population-derived
© 2003 by the American Institute of Ultrasound in Medicine • J Ultrasound Med 22:347–356, 2003 • 0278-4297/03/$3.50
Sonographic Estimated Fetal Weight and Birth Outcomes
distribution of fetal weight at the same GA. Fetuses with weight below a certain percentile are defined as having IUGR. Hence, size, rather than growth, is most often used to define IUGR,6 although other definitions of IUGR have been suggested.7–9 As a routine part of obstetric care, sonography is widely used to assess IUGR in the second and third trimesters of pregnancy. However, few studies have actually quantified the association between sonographically defined IUGR and neonatal morbidity and mortality.10 A number of studies have described the association of EFW immediately before delivery with birth weight, suggesting that sonography is a fairly accurate method of estimating fetal weight late in pregnancy.11–15 Other studies have evaluated the association of IUGR diagnosed on the basis of sonography and LBW but not other adverse neonatal outcomes.16–18 However, most of these reports were case-control studies, making it difficult to evaluate the predictive value of sonography. One study identified an association between sonographic EFW in the first trimester of pregnancy and birth weight, birth weight for GA, and prematurity.19 However, in clinical practice, most women do not have ultrasound examinations until the second or third trimester of pregnancy. We sought to determine and quantify which aspects of neonatal morbidity and mortality could be predicted using second- and thirdtrimester sonographic measurements of fetal size. We also sought to ascertain which sonographic measurements of size best identify those fetuses at greatest risk of neonatal morbidity and mortality.
Materials and Methods Study Population Our population included 1836 women with singleton pregnancies who underwent at least 1 second- or third-trimester obstetric ultrasound examination in the Department of Radiology at the University of California San Francisco (UCSF) Medical Center at 13 to 37 weeks’ gestation from July 1994 through March 1997 and who subsequently gave birth at the UCSF Medical Center. We excluded examinations performed only to assess amniotic fluid volume, cervical length, or biophysical profile. We also excluded women with 5 or more ultrasound examinations during a single pregnancy, multiple pregnancies reduced 348
to singleton pregnancies, women who had undergone fetal surgery, and women transferred to UCSF for delivery, as well as fetuses with major congenital or chromosomal anomalies (noted in the UCSF ultrasound database), because such fetuses are more likely to be small and to have neonatal morbidity unrelated to their size. Among these 1836 women, a subset of 236 had 2 or more ultrasound examinations, with which we evaluated growth (change in weight over time), as described elsewhere.9 The UCSF Institutional Review Board approved the study, and a waiver for informed consent was obtained. Sources of Data The results of all ultrasound examinations performed at UCSF are available on an Acuson Aegis computerized database (Siemens Medical Solutions, Mountain View, CA) including ultrasound examination date, last menstrual period (LMP) date, fetal biometric measurements (crown-rump length, head circumference [HC], biparietal diameter [BPD], abdominal circumference [AC], and femur length [FL]), the presence of fetal structural abnormalities, and indication for examination. A clinician records GA at the time of the initial ultrasound visit. All examinations were performed with commercially available real-time, high-resolution sonographic equipment (Acuson 128XP and Sequoia). The UCSF ultrasound database was linked with the UCSF obstetric database, which includes demographic information and details of obstetric histories and neonatal outcomes for all births at UCSF. Variables abstracted from the obstetric database included maternal race or ethnicity, prepregnancy height and weight, history of substance abuse, neonatal birth weight, sex, GA at delivery, length of neonatal hospital stay, neonatal intensive care unit (NICU) admission, length of NICU stay, assisted ventilation at birth, stillbirth or neonatal death, and the presence of fetal anomalies. Neonatal outcomes are obtained in this database through linkage with the UCSF neonatal database. A senior obstetrician assigned GA on the basis of the patient’s records considering the following variables in descending order of importance: (1) in vitro fertilization date (assigned as the date of fertilization); (2) certain and regular LMP date consistent within 7 days of first-trimester sonography or within 14 days of second-trimester sonography; (3) first-trimester sonography; (4) secondJ Ultrasound Med 22:347–356, 2003
Smith-Bindman et al
trimester sonography20; and (5) third-trimester sonography. We compared estimated GA in the obstetric database with GA in the ultrasound database to ensure consistency. One of the authors reviewed inconsistent records, and 2 authors reached a consensus on how to date these pregnancies ( .05). The risks of adverse birth outcomes were higher than national averages. For example, 15.3% were born at a birth weights of less than 2500 g (compared with 6.4%–13.1% nationally depending on racial or ethnic group)23; 3.8% were
Figure 1. Incidence of pregnancy outcomes in relation to prenatal sonographic EFW for low absolute birth weight (A), LBW for GA (B), and adverse neonatal outcomes (C).
Table 1. Characteristics of the Study Population (N = 1836) Characteristic
Age, y, mean (SD) Height, cm, mean (SD) Weight, kg, mean (SD) Race/ethnicity, % (n) White Black Hispanic Other Primipara, % (n) Maternal substance abuse, % (n) Known GA, % (n) Neonatal outcomes, % (n) Small size at birth Birth weight
