Obstetric ultrasound for estimated fetal weight: is the information more harm than benefit?




The option to perform a safe cesarean delivery has dramatically reduced both neonatal and maternal morbidity and mortality over the past 100 years. There have been reductions in maternal mortality related to obstetric anesthesia even over the past 3 decades. Certainly, in the developing world, the capability to perform timely, safe cesareans would impact injury during childbirth to both mothers and babies. However, in the developed world it seems that our cesarean rate is too high–perhaps we have become cavalier about the indications for cesarean. Further, our medical-legal system puts enormous pressures on clinicians to guarantee safe passage for neonates that are neither feasible nor optimal for the bulk of our patients.




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With these pressures, clinicians have an increasingly lower threshold to offer, recommend, and perform cesarean deliveries. The prevention of intrapartum hypoxic injury is a common indication for cesarean, despite the fact that our most commonly used tool for identifying such injury, intrapartum fetal monitoring, is limited and understudied. Another common indication is the cesarean performed for failure to progress in labor or for cephalopelvic disproportion in general. Such cesareans are even offered prior to the onset of labor, most commonly if the estimated fetal weight (EFW) is >5000 g in a woman without gestational diabetes to prevent shoulder dystocia. Unfortunately, our ability to accurately predict birthweight at this extreme value is poor, with a less than 50% positive predictive value for ≥5000 g, and our ability to predict shoulder dystocia is even worse. In a landmark study by Rouse et al, it was estimated that, utilizing a policy that offered elective cesarean to all women with an EFW >4500 g, the number of cesareans needed to prevent a brachial plexus injury would be 3695 with a cost of $8.7 million per injury prevented.


Regardless, we continue to utilize obstetric ultrasound as a tool for estimating fetal growth. With respect to how clinicians actually use these EFWs, there are studies that have found that having an increased EFW on ultrasound increases the risk of cesarean delivery, regardless of actual birthweight. For example, despite the poor predictive value of such ultrasounds, one study demonstrated that the cesarean rates may be doubled in women with an EFW of ≥4000 g. These studies are interesting and tell us that clinicians act differently based on the results of the obstetric ultrasound’s EFW, but do not speak to the impact of getting an obstetric ultrasound at all. In the current edition of The Grey Journal, Little et al examine how having an obstetric ultrasound within a month of delivery affects mode of delivery. The cohort consisted of nondiabetic women who either underwent obstetric ultrasound within a month of delivery or did not undergo such a study. Interestingly the 2 groups had a similar mean birthweight. However, they differed in terms of racial/ethnic breakdown with more white women in the ultrasound group. There were also more obese women, more inductions of labor, and more women cared for by private physicians in the ultrasound group.


The authors found that those with a recent ultrasound were about 50% more likely to undergo a cesarean delivery (15.7% vs 10.2%, P < .01). Given the potential confounders listed above, this is not particularly surprising. However, even when controlling for all of these factors as well as actual birthweight, this risk difference persisted (adjusted odds ratio, 1.44; 95% confidence interval, 1.1−1.9). This risk increased to an adjusted odds ratio of 1.85 if the ultrasound EFW was >3500 g. While 9% of the cesareans in the ultrasound group were performed for suspected macrosomia, this did not account for the entire difference. The patients with an ultrasound also had higher rates of cesarean for both failure to progress in labor and nonreassuring fetal monitoring. This suggests that the knowledge about the EFW may have contributed to how the clinicians thought about the patient and perhaps pushed them to look for an indication for cesarean or gave them a lower threshold to consider a cesarean. If actual patient birthweight had differed, then this would be reasonable. However, given that the mean birthweights did not differ, it raises concern that the ultrasound findings biased the clinicians’ judgment.


There may be other factors involved as well. Perhaps clinicians who are more likely to order an obstetric ultrasound for fetal weight are also more likely to perform cesarean deliveries. However, as these were primarily patients cared for in large groups of providers, this seems unlikely. It may be that the 2 groups differed in ways that were not or could not be measured leading to confounding that could not be controlled for even in the multivariable models. For example, it was interesting that the mean birthweights were similar even though nearly a third of the ultrasounds were performed for size greater than or less than dates. While it is true that fundal height measurements are quite poor at predicting abnormal fetal weight, it may be that the extremes of birthweight were different in the 2 groups. One would expect a population getting ultrasound near delivery for fetal weight to have more of both macrosomia and growth restriction which could lead, respectively, to higher rates of failure to progress and nonreassuring intrapartum fetal monitoring.


Interestingly, there were fewer midwifery patients in the ultrasound group and having a midwifery provider was associated with a 50% reduction in the risk of cesarean. However, it does not appear that this factor accounted for the difference in the 2 groups as it was controlled for in the multivariable model. One interesting factor about the multivariable model was the lack of an association between induction of labor and cesarean. This has been reported recently by other authors, and it is intriguing given the national concern over induction and the purported risk of cesarean.


In the end, this study, like much of the most interesting research, brings up more questions than it answers. Is the effect of obstetric ultrasound purely a confounding issue, serving as a marker for differences in the populations? Or is there an actual effect from obtaining an ultrasound EFW near delivery? This latter effect could actually be addressed in a prospective, randomized, controlled trial of ultrasound versus no ultrasound at term. While we wait for such studies to be conducted, it is imperative for us to realize that while we may assume that more information can only be better, we must consider that inaccurate, poorly predictive information can lead to potentially unindicated interventions.

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May 15, 2017 | Posted by in GYNECOLOGY | Comments Off on Obstetric ultrasound for estimated fetal weight: is the information more harm than benefit?

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