How Fetal Head Circumference Affects the Need for Cesarean Section
By John C. Hobbins, MD
Professor, Department of Obstetrics and Gynecology, University of Colorado School of Medicine, Aurora
Dr. Hobbins reports no financial relationships relevant to this field of study.
SYNOPSIS: A recent study shows that infant head circumference has a greater effect on rates of cesarean section and instrumental delivery than birth weight.
SOURCE: Lipschuetz M, et al. A large head circumference is more strongly associated with unplanned cesarean or instrumental delivery and neonatal complications than high birth weight. Am J Obstet Gynecol 2015;213:833 e1-12.
A recent article in the American Journal of Obstetrics and Gynecology may provide some clues regarding how to avoid an unnecessary cesarean section in patients suspected of having a large fetus. A team from Israel reviewed medical records of patients delivering at two Hebrew University campuses from 2010 to 2012.1 They divided the 24,780 patients into groups depending on infant birth weights (BW) and head circumferences (HC). The dependent variables involved delivery events — the rate of cesarean section (CSR), unplanned cesarean sections (UPC), and instrumental deliveries (ID).
When the HC was > 95th percentile, the CSR was 38%, with a 16% UPC rate and an 11.2% ID rate. For pregnancies where the HC was < 95th percentile, the same rates were 21.6%, 7.8%, and 6.7% (roughly half in each category), respectively. Based on the size of the baby alone, if the BW > 95th percentile, the CSR, UPC, and ID rates were essentially the same at 19.7%, 10.2%, and 3.4% vs 22.4%, 8.2%, and 7.1% when BW < 95th percentile.
Only 24% of large babies had HC > 95th percentile. Although an HC in this category doubled the risk of unplanned cesarean section, a BW of > 95th percentile had a negligible effect.
If the authors used babies with normal HC and BW as a control group, other babies having large HC and normal BW were more likely to have an UPC (odds ratio [OR], 3.08; 95% confidence interval [CI], 2.52-3.75) or ID (OR, 3.03; 95% CI, 2.46-3.75). However, if HC was normal and BW > 95th percentile, there was no difference in UPC (OR, 1.18; 95% CI, 0.91-1.54). The most common reason for UPC with large heads was failure to progress (27.7% vs 14.1%; P = 0.001) and for normal size heads was fetal distress (23.4% vs 16.9; P < 0.001).
COMMENTARY
The cesarean section rate is still way too high (32.7%),2 and although the American College of Obstetrics and Gynecology and the World Health Organization have been pushing to decrease the rates of cesarean section, consumers are now becoming involved in this issue. In January, two articles appeared in The New York Times3,4 that should make obstetricians uncomfortable. The authors suggested that physicians may be more primed to perform cesarean sections since the financial rewards are greater, less in-hospital provider time is required, and the medical-legal liability is less than with a vaginal delivery. They contended that the clinical suspicion of macrosomia provides a ready excuse to intervene, and the author of the second article even cited examples of patients being “bullied” into having cesarean sections because of misperceived macrosomia. Although these messages had the feel of a journalistic shiv, many of the authors’ points were well taken.
I chose to review this article because it adds a piece of information that can be very helpful in avoiding cesarean section in patients with suspected fetal macrosomia. In this situation, the clinician is confronted with a classic “damned if you do, damned if you don’t” decision, which could end in two unwanted outcomes: 1) performing a cesarean section that is unnecessary, and 2) encountering shoulder dystocia if a cesarean section is not done.
Knowing the HC should give the clinician a leg up regarding which patients may ultimately need cesarean section for failure to progress. However, the HC needs to be assessed in conjunction with the abdominal circumference (a surrogate for the size of the shoulders), since a major body-to-head disproportion would predispose the patient to shoulder dystocia.
In suspected fetal macrosomia, below is a proposal that should allow greater opportunity for vaginal delivery along with a lower risk of shoulder dystocia:
- In patients at risk for macrosomia (large fundal height, diabetes, a previous macrosomic infant, or an ultrasound exam earlier in pregnancy suggesting macrosomia), do an estimated fetal weight (EFW) at about 35 weeks and extrapolate forward using a method by Best et al.5 This has been shown in macrosomia to be more accurate than doing an EFW shortly before or during labor.
- If the EFW appears to exceed 4500 g and the HC is > 95th percentile, then there is a greater chance of cephalopelvic disproportion and the threshold may be a bit lower to do cesarean section if progress is sluggish in the first stage of labor. However, if the HC is < 95th percentile, then the chance of failure to progress is diminished appreciably.1
- If the average abdominal diameter minus the biparietal diameter of a macrosomic fetus exceeds 2.5 cm (the Cohen index),6 then a prolonged second stage of labor should be avoided and instrumental delivery would be contraindicated, since the chances of shoulder dystocia in this setting is 25% in unselected patients and 38% in diabetics.7
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Despite frequently cited inaccuracies of EFW in macrosomia, if the EFW is > 5000 g, especially if there is body-to-head disproportion and/or diabetes, an elective cesarean section should be offered.8
- If both HC and the Cohen index are reassuring, then it should be reasonably safe to apply a liberal approach to the lengths of the first and second stages of labor.
Most importantly, a trial of labor with a plan tailor-made to each patient based on all the above variables, along with the patient’s past obstetrical history and stature, should allow vaginal delivery in the overwhelming majority of patients with suspected fetal macrosomia.
REFERENCES
- Lipschuetz M, et al. A large head circumference is more strongly associated with unplanned cesarean or instrumental delivery and neonatal complications than high birth weight. Am J Obstet Gynecol 2015;213:833.e1-12.
- Osterman MJ, Martin JA. Trends in low-risk cesarean delivery in the United States. Natl Vital Stat Rep 2014;63:1-16.
- Rosenberg T. Reducing Unnecessary C-section Births. The New York Times Jan. 19, 2016.
- Rabin RC. Let Me Tell You About My Big Baby. The New York Times Jan. 21, 2016.
- Best G, Pressman ER. Ultrasonographic prediction of birth weight in diabetic pregnancies. Obstet Gynecol 2002;99;740-744.
- Cohen B, et al. The incidence and severity of shoulder dystocia correlates with sonographic measurements of asymmetry in patients with diabetes. Am J Perinatol 1999;16:197-201.
- Miller RS, et al. Sonographic fetal asymmetry predicts shoulder dystocia. J Ultrasound Med 2007;26:1523-1528.
- ACOG Practice Bulletin No.22: Fetal Macrosomia. American College of Obstetricians and Gynecologists Washington DC; 2000.
A recent study shows that infant head circumference has a greater effect on rates of cesarean section and instrumental delivery than birth weight.
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