Evaluation of Fetal Weight Estimation Formulas in Assessing Small-for-Gestational-Age Fetuses

Abstract

Purpose: The aim of this study was to compare the accuracy of multiple sonographic fetal weight estimation models in assessing small-for-gestational-age (SGA) fetuses.

Materials and Methods: The cohort included all singleton pregnancies recorded at a single medical center from January 2004 to September 2011, with a minimum of 24 weeks of gestation. SGA was defined as a fetal weight of less than the 10th percentile. We used birth weight (BW) distribution curves for our population, matched according to fetal gender and gestational age. Predicted birth weights were calculated using 26 sonographic fetal weight estimation models, including targeted formulas for SGA fetuses.

Results: 1218 cases of SGA fetuses that underwent sonographic fetal weight estimation within one week prior to delivery were found. Prediction of fetal weight was significantly less accurate in SGA fetuses than in the general population. The random error for SGA fetuses ranged from 7.2 % to 13.9 % in different models, while the systematic error ranged from –12.8 % to 26 %. Most non-targeted formulas showed a specificity of over 90 % but a sensitivity of only 20 – 35 % in the detection of SGA fetuses, while most targeted formulas had a low specificity but a high sensitivity. The model by Scott et al. was found to be the most accurate in assessing SGA fetuses in our population.

Conclusion: Estimation of fetal weight in SGA fetuses is less accurate than in the general population. Some formulas which are designed for SGA are more accurate than others and their use might increase the sensitivity in identifying SGA fetuses, with only a small decline in specificity.

Zusammenfassung

Ziel: Das Ziel dieser Studie war es, die Genauigkeit der verschiedenen sonografischen Modelle zur Gewichtsschätzung von „Small-for-Gestational-Age“(SGA)-Feten zu vergleichen.

Material und Methoden: Die Kohorte schloss alle Einlingsschwangerschaften ein, die in einem einzelnen medizinischen Zentrum zwischen Januar 2004 und September 2011 registriert wurden und die ein Gestationsalter von mindestens 24 Wochen aufwiesen. Als SGA wurde ein Fetalgewicht unterhalb der 10. Perzentile definiert. Für unsere Population verwendeten wir Geburtsgewicht(BW)-Verteilungstabellen, die auf das Geschlecht und das Gestationsalter des Feten abgestimmt waren. Die Geburtsgewichte wurden durch 26 Modelle zur sonografischen Schätzung des fetalen Gewichts vorausberechnet, darunter auch spezielle Formeln für SGA-Feten.

Ergebnisse: Es wurden 1218 SGA-Feten ermittelt, bei denen eine Woche vor Entbindung eine sonografische Gewichtsschätzung durchgeführt wurde. Im Vergleich zur Normalpopulation war die Vorhersage des fetalen Gewichts von SGA-Feten wesentlich weniger genau. Der statistische Fehler lag bei den verschiedenen Modellen zwischen 7,2 % und 13,9 %, während die systematische Abweichung in einem Bereich zwischen –12,8 % und 26 % lag. Die meisten der nicht-speziellen Formeln zeigten in Bezug auf die Feststellung von SGA-Feten eine Spezifität von über 90 %, aber eine Sensitivität von 20 – 35 %, während die meisten speziellen Formeln eine niedrige Spezifität, dafür jedoch eine hohe Sensitivität aufwiesen. In unserer Population war das Modell von Scott et al. das zuverlässigste für die Feststellung von SGA-Feten.

Schlussfolgerung: Die fetale Gewichtsschätzung bei SGA-Feten ist weniger genau als in der Normalpopulation. Einige speziell für SGA entwickelte Formeln sind genauer als andere und können die Sensitivität für die Erkennung von SGA-Feten erhöhen, mit nur geringen Einbußen bei der Spezifität.

Key words

weight - estimation - ultrasound - SGA - small for gestational age

¹ Department of Obstetrics and Gynecology, Part of this publication was performed in partial fulfillment of the MD thesis requirements of the Sackler School of Medicine, Tel-Aviv University, Tel-Aviv, Israel

• References

• 1 Barker DJP. Long-term outcome of retarded fetal growth. In: Divon MY, (ed.) Clinical Obstetrics and Gynecology. Philadelphia: PA: Lippincott-Raven; 1997: 853-863
• 2 Dudley NJ. A systematic review of the ultrasound estimation of fetal weight. Ultrasound Obstet Gynecol 2005; 25: 80-89
• 3 ACOG Practice Bulletin number 12 – Intrauterine Growth Restriction. Clinical Management Guidelines for Obstetricians-Gynecologists. January 2000.
• 4 Chauhan SP, Gupta LM, Hendrix NW et al. Intrauterine growth restriction: comparison of American College of Obstetricians and Gynecologists practice bulletin with other national guidelines. Am J Obstet Gynecol  2009; 200: 409
• 5 Mandruzzato G, Antsaklis A, Botet F et al. Intrauterine restriction (IUGR). J Perinat Med  2008; 36: 277-281
• 6 Lee PA, Chernausek SD, Hokken-Koelega ACS et al. for the International SGA Advisory Board International Small for Gestational Age Advisory Board Consensus Development Conference Statement: Management of Short Children Born Small for Gestational Age. Pediatrics 2003; 111: 1253-1261
• 7 Oros D, Figueras F, Cruz-Martinez R et al. Longitudinal changes in uterine, umbilical and fetal cerebral Doppler indices in late-onset small-for-gestational age fetuses. Ultrasound Obstet Gynecol 2011; 37: 191-195
• 8 Kurmanavicius J, Burkhardt T, Wisser J et al. Ultrasonographic fetal weight estimation: accuracy of formulas and accuracy of examiners by birth weight from 500 to 5000 g. J Perinat Med 2004; 32: 155-161
• 9 Barel O, Vaknin Z, Tovbin J et al. Assessment of accuracy for multiple sonographic fetal weight estimation formulas, a ten year experience from a single center. J Ultrasound Med 2013; 32: 815-823
• 10 Sabbagha RE, Minogue J, Tamura RK et al. Estimation of birth weight by use of ultrasonographic formulas targeted to large-, appropriate-, and small-for-gestational-age fetuses. Am J Obstet Gynecol 1989; 160: 854-860
• 11 Schild RL, Fell K, Fimmers R et al. A new formula for calculating weight in the fetus of < or =1600 g. Ultrasound Obstet Gynecol 2004; 24: 775-780
• 12 Scott F, Beeby P, Abbott J et al. New formula for estimating fetal weight below 1000g: comparison with existing formulas. J Ultrasound Med 1996; 15: 669-672
• 13 Siemer J, Hilbert A, Hart N et al. A new sonographic weight formula for fetuses <or= 2500 g. Ultraschall in Med  2009; 30: 47-51
• 14 Thurnau GR, Tamura RK, Sabbagha R et al. A simple estimated fetal weight equation based on real-time ultrasound measurements of fetuses less than thirty-four weeks’ gestation. Am J Obstet Gynecol 1983; 145: 557-561
• 15 Robson SC, Gallivan S, Walkinshaw SA et al. Ultrasonic estimation of fetal weight: use of targeted formulas in small for gestational age fetuses. Obstet Gynecol 1993; 82: 359-364
• 16 Chitty LS, Altman DG, Henderson A et al. Charts of fetal size, 2: head measurements. Br J Obstet Gynaecol 1994; 101: 35-43
• 17 Chitty LS, Altman DG, Henderson A et al. Charts of fetal size, 3: abdominal measurements. Br J Obstet Gynaecol 1994; 101: 125-131
• 18 Chitty LS, Altman DG, Henderson A et al. Charts of fetal size, 4: femur length. Br J Obstet Gynaecol 1994; 101: 132-135
• 19 Campbell S, Wilkin D. Ultrasonic measurement of fetal abdomen circumference in the estimation of fetal weight. Br J Obstet Gynaecol 1975; 82: 689-697
• 20 Hadlock FP, Harrist RB, Carpenter RJ et al. Sonographic estimation of fetal weight. The value of femur length in addition to head and abdomen measurements. Radiology 1984; 150: 535-540
• 21 Hadlock FP, Harrist RB, Sharman RS et al. Estimation of fetal weight with the use of head, body, and femur measurements – a prospective study. Am J Obstet Gynecol 1985; 151: 333-337
• 22 Warsof SL, Gohari P, Berkowitz RL et al. The estimation of fetal weight by computer-assisted analysis. Am J Obstet Gynecol  1977; 128: 881-892
• 23 Warsof SL, Wolf P, Coulehan J et al. Comparison of fetal weight estimation formulas with and without head measurements. Obstet Gynecol 1986; 67: 569-573
• 24 Higginbottom J, Slater J, Porter G et al. Estimation of fetal weight from ultrasonic measurement of trunk circumference. Br J Obstet Gynaecol 1975; 82: 698-701
• 25 Jordaan HVF. Estimation of fetal weight by ultrasound. J Clin Ultrasound 1983; 11: 59-66
• 26 Woo JSK, Wan CW, Cho KM. Computer-assisted evaluation of ultrasonic fetal weight prediction using multiple regression equations with and without the fetal femur length. J Ultrasound Med 1985; 4: 65-67
• 27 Vintzileos AM, Campbell WA, Rodis JF et al. Fetal weight estimation formulas with head, abdominal, femur, and thigh circumference measurements. Am J Obstet Gynecol 1987; 157: 410-414
• 28 Shinozuka N, Okai T, Kohzuma S et al. Formulas for fetal weight estimation by ultrasound measurements based on neonatal specific gravities and volumes. Am J Obstet Gynecol 1987; 157: 1140-1145
• 29 Combs CA, Jaekle RK, Rosenn B et al. Sonographic estimation of fetal weight based on a model of fetal volume. Obstet Gynecol 1993; 82: 365-370
• 30 Mielke G, Pietsch-Breitfeld B, Salinas R et al. A new formula for prenatal ultrasonographic weight estimation in extremely preterm fetuses. Gynecol Obstet Invest 1995; 40: 84-88
• 31 Dollberg S, Haklai Z, Mimouni FB et al Birth weight standards in the live-born population in Israel. Isr Med Assoc J 2005; 7: 311-314
• 32 Bland JM, Altman DG. Applying the right statistics: analyses of measurement studies. Ultrasound Obstet Gynecol 2003; 22: 85-93
• 33 Jouannic JM, Grange G, Goffinet F et al. Validity of sonographic formulas for estimating fetal weight below 1,250g: a series of 119 cases. Fetal Diagn Ther 2001; 16: 254-258
• 34 Abele H, Hoopmann M, Wagner N et al. Accuracy of sonographic fetal weight estimation of fetuses with a birth weight of 1500g or less. Eur J Obstet Gynecol Reprod Biol 2010; 153: 131-137
• 35 Proctor LK, Rushworth V, Shah PS et al. Incorporation of femur length leads to underestimation of fetal weight in asymmetric preterm growth restriction. Ultrasound Obstet Gynecol 2010; 35: 442-448