The Dangers of Estimating Fetal Weight Near Term

Here's a new, interesting study that shows that the routine practice of ordering a fetal weight estimate near term can have negative consequences.

Background: Many care providers order an ultrasound to estimate the baby's weight when the mother is near term, even though research shows this is not a very accurate way to predict the baby's weight.  

Women with babies predicted to be big often are pressured to schedule a planned cesarean to avoid the possibility of shoulder dystocia (shoulders getting stuck, causing injury to the baby), despite little research to show that a cesarean improves outcome.

Or they are encouraged to induce labor early "while the baby is smaller," even though research shows that this actually may increase the risk for cesarean.

Here is a new study that shows that even just doing the fetal-weight ultrasound in the first place may increase the risk for cesarean.

Why?  Probably because knowledge of the results (and nervousness over fetal size) changes the behavior of the care provider so that they have a lower surgical threshold during labor.  

This is particularly an issue for women of size. Even though the majority of "obese" women do not have big babies at all, we do tend to have bigger babies on average than "normal" weight women.  This makes care providers nervous. And since many believe that fat women eat mostly junk food or are practically diabetic anyhow, this increases their fear of big babies even more. So many providers routinely order ultrasounds to estimate fetal weight near term in the pregnancies of obese women.

In this most recent study, 50% of all pregnant women had an ultrasound that calculated Estimated Fetal Weight (EFW) within a month of delivery. Those that did had a higher cesarean rate than those who didn't, especially those whose babies were predicted to be big by EFW.

The rate of EFW ulstraounds is probably even higher in obese women.  How does that then affect our cesarean rates? It is likely another example of management variables that result in increased cesarean rates in obese women.  Yet this is a management variable that can be modified.

An ultrasound to estimate fetal weight near term does not improve outcomes and is a risk factor for an increased cesarean rate.  

References

Am J Obstet Gynecol. 2012 Jul 9. [Epub ahead of print] Estimated fetal weight by ultrasound: a modifiable risk factor for cesarean delivery? Little SE, Edlow AG, Thomas AM, Smith NA.  PMID: 22902073

OBJECTIVE: The purpose of this study was to investigate whether knowledge of ultrasound-obtained estimated fetal weight (US-EFW) is a risk factor for cesarean delivery (CD).
STUDY DESIGN: Retrospective cohort from a single center in 2009-2010 of singleton, term live births. CD rates were compared for women with and without US-EFW within 1 month of delivery and adjusted for potential confounders.
RESULTS: Of the 2329 women in our cohort, 50.2% had US-EFW within 1 month of delivery. CD was significantly more common for women with US-EFW (15.7% vs 10.2%; P < .001); after we controlled for confounders, US-EFW remained an independent risk factor for CD (odds ratio, 1.44; 95% confidence interval, 1.1-1.9). The risk increased when US-EFW was >3500 g (odds ratio, 1.8; 95% confidence interval, 1.3-2.7).
CONCLUSION: Knowledge of US-EFW, above and beyond the impact of fetal size itself, increases the risk of CD. Acquisition of US-EFW near term appears to be an independent and potentially modifiable risk factor for CD.

Am J Obstet Gynecol. 2009 Mar;200(3):340.e1-3. Overestimation of fetal weight by ultrasound: does it influence the likelihood of cesarean delivery for labor arrest? Blackwell SC, Refuerzo J, Chadha R, Carreno CA.  PMID: 19254597

OBJECTIVE: We sought to determine whether the overestimation of ultrasound-derived estimated fetal weight (EFW) is associated with increased diagnosis of labor arrest. STUDY DESIGN: This is a historical cohort study of nulliparous women with term pregnancies who underwent bedside ultrasound examination for EFW before labor induction. Labor outcomes of women with EFW overestimation > 15% the actual birthweight were compared with those with EFW not overestimated. RESULTS: Overestimation of EFW occurred in 9.5% of cases (23/241). The rate of cesarean delivery (CD) for labor arrest was higher for those with EFW overestimation (34.8% vs 13.3%; P = .01) even though there were no differences in length of the induction duration. After adjusting for confounding factors, EFW overestimation remained associated with CD for labor arrest (odds ratio, 4.8; 95% confidence interval, 1.5-15.2). CONCLUSION: Our finding suggests that an overestimation of EFW may be associated with a lower threshold for CD for labor arrest.

Am J Obstet Gynecol. 1995 Oct;173(4):1215-9. Fetal macrosomia: does antenatal prediction affect delivery route and birth outcome? Weeks JW, Pitman T, Spinnato JA 2nd.  PMID: 7485323

OBJECTIVE: Our purpose was to determine whether clinical or ultrasonographic prediction of fetal macrosomia influences subsequent delivery route and birth outcome in a clinical setting where macrosomia is not considered an indication for cesarean delivery. STUDY DESIGN: The hospital records of 504 patients delivered of infants weighing > or = 4200 gm between October 1989 and March 1994 were reviewed. Statistical comparisons were made between patients in whom fetal macrosomia was predicted before delivery (n = 102) and those in whom it was not (n = 402). Cesarean delivery, shoulder dystocia, and birth trauma rates were the variables of interest. RESULTS: Cesarean sections were performed in 52% of the "predicted" group deliveries and in 30% of the "not predicted" group (p < 0.01). The increased cesarean delivery rate in the predicted group appeared to be related to an increased incidence of labor inductions (42.5% vs 26.6%, p = 0.005) and a greater proportion of failed inductions. The proportion of patients delivered by cesarean section without a trial of labor was similar in the predicted and not predicted groups (14.7% vs 10.2%, p = 0.21). There was no significant differences in the incidence of shoulder dystocia or the occurrence of birth trauma. CONCLUSIONS: The antenatal prediction of fetal macrosomia is associated with a marked increase in cesarean deliveries without a significant reduction in the incidence of shoulder dystocia or fetal injury. Ultrasonography and labor induction for patients at risk for fetal macrosomia should be discouraged.

Eur J Obstet Gynecol Reprod Biol. 2002 Oct 10;105(1):20-4. Clinical and ultrasonographic weight estimation in large for gestational age fetus. Weiner Z, et al. PMID: 12270559

OBJECTIVE: To examine prospectively the effect on pregnancy outcome of a management protocol, that adds ultrasonographic weight estimation in fetuses suspected clinically as large. STUDY DESIGN: Prospective follow up study of all singleton deliveries during a 1 year period. All patients underwent routine clinical estimation of fetal weight. When clinical estimation of fetal weight was > or = 3700 g, patients were referred for ultrasonographic estimation of fetal weight. When the latter was > or = 4000 g the patient was informed about the risks of birth trauma. Cesarean section was recommended only when > or = 4500 g. Ultrasonography was repeated every 4 days when possible. Predictive values of clinical and ultrasonographic estimations of fetal weight for diagnosing macrosomia, defined for the purpose of this study as 4000 g or more, and their effect on the rate of cesarean sections. RESULTS: Five hundred fifty-five (14.4%) out of 3844 singletons were estimated as 3700 g or more. Only 315 fetuses had ultrasonographic estimation of weight within 3 days of delivery. The sensitivity of clinical and ultrasonographic prediction of macrosomia was 68 and 58%, respectively. Cesarean section rate in newborns weighing 4000 g or more was 22% when macrosomia was clinically suspected compared to 11% when it was not (P<0.05). In fetuses estimated ultrasonographically as 4000 g or larger the cesarean section rate was doubled (50.7% versus 24.9%, P<0.05) compared to those estimated as smaller than 4000 g, although actual weight of 4500 g or more was recorded in 10.6 and 8.5% of these groups, respectively. There were no cases of shoulder dystocia in macrosomic babies when macrosomia was not detected by ultrasound compared to two cases of shoulder dystocia (2.7%) when macrosomia was detected by ultrasound. CONCLUSION: Antenatal suspicion of macrosomia increased the cesarean section rate while the associated improvement in pregnancy outcome remains questionable. The contribution of ultrasound, added to routine clinical estimation of fetal weight, was clinically insignificant apart from a further increase in cesarean section rate.

Arch Gynecol Obstet. 2008 Sep;278(3):225-30. Epub 2008 Feb 26. Suspected macrosomia? Better not tell. Sadeh-Mestechkin D, et al.  PMID: 18299867

OBJECTIVE: To evaluate the management policy of delivery in a suspected macrosomic fetus and to describe the outcome of this policy. STUDY DESIGN: For this prospective observational study we followed the management by reviewing the medical records of 145 women and their infants. The study population included women at term admitted to the obstetrics department with suspected macrosomic infants, as was diagnosed by an obstetrician and/or by fetal sonographic weight estimation of > or =4,000 g. The comparison group (n = 5,943) consisted of all women who gave birth during the data collection period. RESULTS: Induction of labor and cesarean delivery rates in the macrosomic pregnancies (actual birth weight >4,000 g) of the study group were significantly higher when compared with the macrosomic pregnancies of the comparison group. When comparing the non-macrosomic to the macrosomic pregnancies (actual birth weight </>4,000 g) of the study group no significant difference was demonstrated regarding maternal or infant complications. The sensitivity, specificity and positive predictive value of the methods used for detecting macrosomia were 21.6, 98.6 and 43.5%, respectively. CONCLUSION: Our ability to predict macrosomia is poor. Our management policy of suspected macrosomic pregnancies raises induction of labor and cesarean delivery rates without improving maternal or fetal outcome.

Am J Obstet Gynecol. 2006 Sep;195(3):657-72. Epub 2006 Apr 21. Shoulder dystocia: the unpreventable obstetric emergency with empiric management guidelines. Gherman RB, et al.   PMID: 16949396

OBJECTIVE: Much of our understanding and knowledge of shoulder dystocia has been blurred by inconsistent and scientific studies that are of limited scientific quality. In an evidence-based format, we sought to answer the following questions: (1) Is shoulder dystocia predictable? (2) Can shoulder dystocia be prevented? (3) When shoulder dystocia does occur, what maneuvers should be performed? and (4) What are the sequelae of shoulder dystocia? STUDY DESIGN: Electronic databases, including PUBMED and the Cochrane Database, were searched using the key word "shoulder dystocia." We also performed a manual review of articles included in the bibliographies of these selected articles to further define articles for review. Only those articles published in the English language were eligible for inclusion. RESULTS: There is a significantly increased risk of shoulder dystocia as birth weight linearly increases. From a prospective point of view, however, prepregnancy and antepartum risk factors have exceedingly poor predictive value for the prediction of shoulder dystocia. Late pregnancy ultrasound likewise displays low sensitivity, decreasing accuracy with increasing birth weight, and an overall tendency to overestimate the birth weight. Induction of labor for suspected fetal macrosomia has not been shown to alter the incidence of shoulder dystocia among nondiabetic patients. The concept of prophylactic cesarean delivery as a means to prevent shoulder dystocia and therefore avoid brachial plexus injury has not been supported by either clinical or theoretic data....