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Sonographic Prediction of Fetal Macrosomia
Abstract & Commentary
By John C. Hobbins, MD, Professor and Chief of Obstetrics, University of Colorado Health Sciences Center, Denver, is Associate Editor for OB/GYN Clinical Alert.
Dr. Hobbins reports no financial relationship to this field of study.
Synopsis: The diagnosis of macrosomia by ultrasound tilts decision-making toward a higher rate of cesarean section.
Source: Melamed N, et al. Sonographic prediction of fetal macrosomia. The consequences of false diagnosis. J Ultrasound Med 2010;29:225-230
The management of a patient with suspected fetal macrosomia remains an enigma. Not being able to come up with a consistent plan of management for macrosomia is compounded by our inability to reliably predict it. A group of investigators from Israel have recently attempted to see what the downstream effect would be from false-positive and false-negative diagnoses of macrosomia by ultrasound-derived estimation of fetal weight (EFW).
They reviewed 4 years of data (2002-2006) from one hospital and concentrated on patients whose infants weighed between 3000 g and 4500 g. They then identified those who had EFWs done within 3 days of delivery (using the commonly used Hadlock formula, which incorporates the biparietal diameter, head circumference, abdominal circumference, and femur length). Macrosomia was defined as an EFW of > 4000 g and false-negative (FN) and false-positive (FP) results were determined by whether the actual birth weight (BW) was above or below the 4000 g threshold.
The authors chose a case-control approach to their analysis. Two hundred eighty-two patients whose EFWs were > 4000 g, but whose BWs were < 4000 g (FP) were matched against 1128 true-negative (TN) patients whose EFWs were < 4000 g and had infants who weighed < 4000 g (4:1 ratio). Also, 178 FN patients whose EFW was < 4000 g, but whose infant BWs were > 4000 g were matched against 352 true-positive (TP) patients (2:1 ratio) whose EFW and BW were both > 4000 g. Outcomes included cesarean section rate (CSR), major perineal trauma, 5-minute Apgar scores < 7, and neonatal trauma.
There was a 2.5 times higher CSR in the FP group, but a higher rate of perineal and neonatal trauma in the FN group, mostly related to a higher rate of instrumental deliveries.
The authors then employed a different formula by Woo et al,1 which had been shown to have a better ability to predict macrosomia, to estimate the effect on outcomes using the same data set. As might be expected, they found that it decreased the FP rate but increased the FN rate. The authors estimated that the more accurate diagnosis of macrosomia would result in a 5% decrease in the CSR. However, the trade-off was that it could increase the rate of operative vaginal deliveries and, thereby, adverse perinatal outcomes.
There are scores of formulas in the literature to estimate fetal weight with various biometric parameters some designed for smaller fetuses (SGA) and some for large-for-gestational age (LGA) fetuses. Recently, investigators have added thigh circumference,2 cheek-to-cheek diameter,3 as well as 3-D4 to the mix, with better results. Nevertheless, these new wrinkles are somewhat cumbersome, and it remains to be seen whether they will catch on.
The standard (Hadlock) formula used in this study has been imprinted into the software of virtually every ultrasound machine sold in the United States. Interestingly, of the variables used in most formulas, the abdominal circumference is the one that most appropriately reflects how beefy (or, for that matter, scrawny) a fetus is. Unfortunately, it is the one that is most difficult to measure accurately in very late pregnancy. Crowding results in compression of the abdomen in such a way as to make it difficult to trace the perimeter or, certainly, to fit it into an appropriate circle or ellipse. This is why these formulas are more accurate in assessing EFW in an SGA fetus at 32 weeks than an LGA fetus at 38-39 weeks. Also, a 10% standard error of the method means ± 400 g in a 4000 g baby, compared with a ± 200 g splay in a 2000 g fetus. Lately, if we suspect macrosomia, we have chosen to do our EFW at 35-36 weeks and to project forward based on the fetal weight percentile. Best et al have found that by obtaining an EFW between 34 and 37 weeks and assuming that future fetal weight gain would continue according to the same percentile profile, the factored EFW at birth was more accurate than an EFW obtained closer to delivery especially in diabetics.5
No matter how objective a clinician wishes to be, an ultrasound diagnosis of macrosomia shortens the fuse for doing a cesarean section. In one telling figure in the Israeli paper, the reasons for doing a cesarean section were listed in the study groups. Whether the true birth weight was below or above 4000 g, those with EFW > 4000 g had higher rates of "failure to progress" or "arrested dilatation" than those with similar birth weights but with EFW < 4000 g.
So, what is the solution? The "head in the sand" approach of eschewing any attempt at EFW would allow the clinician to choose the route of delivery based on clinical acumen and intuition. On the other hand, an EFW obtained by the most accurate approach, would add another dimension that could be balanced with other variables in the decision-making process. It would seem than an ultrasound diagnosis of macrosomia could be downplayed as the reason alone to do an early cesarean section, but could be weighted (no pun intended) more heavily in the decision as to whether to use forceps or to do a vacuum extraction. This should target more precisely the right patients for cesarean section, while diminishing perinatal morbidity.