Think twice about doing ultrasounds to estimate fetal weight before birth.
In this very large, multi-center study, just the act of estimating fetal weight raised the cesarean rate if the baby was predicted to be big, even when controlling for actual fetal size. It doubled the risk for cesarean in non-diabetic mothers who were thought to be carrying large babies.
Most women predicted to have a large baby will not actually have a large baby, yet fear of a large baby lowers the surgical threshold for many providers, resulting in unnecessary cesareans. Despite limited evidence of improved outcomes, estimating fetal weight is a very common intervention in most obstetric practices, particularly for women of size who tend to have larger babies on average. It is likely a major driver of the high cesarean rate in "obese" women.
Providers need to stop doing so many fetal weight estimates and over-managing the labors of suspected big babies. This is especially important in women of size.
Reference
Obstet Gynecol. 2016 Sep;128(3):487-94. doi: 10.1097/AOG.0000000000001571. Association of Recorded Estimated Fetal Weight and Cesarean Delivery in Attempted Vaginal Delivery at Term. Froehlich RJ1, Sandoval G, Bailit JL, Grobman WA, Reddy UM, Wapner RJ, Varner MW, Thorp JM Jr, Prasad M, Tita AT, Saade G, Sorokin Y, Blackwell SC, Tolosa JE; MSCE, for the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD) Maternal-Fetal Medicine Units (MFMU) Network. PMID: 27500344
OBJECTIVE: To evaluate the association between documentation of estimated fetal weight, and its value, with cesarean delivery. METHODS: This was a secondary analysis of a multicenter observational cohort of 115,502 deliveries from 2008 to 2011. Data were abstracted by trained and certified study personnel. We included women at 37 weeks of gestation or greater attempting vaginal delivery with live, nonanomalous, singleton, vertex fetuses and no history of cesarean delivery. Rates and odds ratios (ORs) were calculated for women with ultrasonography or clinical estimated fetal weight compared with women without documentation of estimated fetal weight. Further subgroup analyses were performed for estimated fetal weight categories (less than 3,500, 3,500-3,999, and 4,000 g or greater) stratified by diabetic status. Multivariable analyses were performed to adjust for important potential confounding variables. RESULTS: We included 64,030 women. Cesarean delivery rates were 18.5% in the ultrasound estimated fetal weight group, 13.4% in the clinical estimated fetal weight group, and 11.7% in the no documented estimated fetal weight group (P<.001). After adjustment (including for birth weight), the adjusted OR of cesarean delivery was 1.44 (95% confidence interval [CI] 1.31-1.58, P<.001) for women with ultrasound estimated fetal weight and 1.08 for clinical estimated fetal weight (95% CI 1.01-1.15, P=.017) compared with women with no documented estimated fetal weight (referent). The highest estimates of fetal weight conveyed the greatest odds of cesarean delivery. When ultrasound estimated fetal weight was 4,000 g or greater, the adjusted OR was 2.15 (95% CI 1.55-2.98, P<.001) in women without diabetes and 9.00 (95% CI 3.65-22.17, P<.001) in women with diabetes compared to those with estimated fetal weight less than 3,500 g. CONCLUSION: In this contemporary cohort of women attempting vaginal delivery at term, documentation of estimated fetal weight (obtained clinically or, particularly, by ultrasonography) was associated with increased odds of cesarean delivery. This relationship was strongest at higher fetal weight estimates, even after controlling for the effects of birth weight and other factors associated with increased cesarean delivery risk.