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Bigger babies are more at risk for getting stuck, which is called "shoulder dystocia." Although the actual numerical risk for shoulder dystocia with big babies is not very great (and small babies get stuck too), the risk for shoulder dystocia is higher with a bigger baby. Most of the time it is resolved without damage, but in some cases babies or moms are hurt, sometimes seriously. In rare cases, babies even die. Damage from shoulder dystocia is one of the main reasons care providers get sued.
As a result, a big baby make many care providers nervous. So they routinely employ estimation of fetal weight near the end of pregnancy, either by hands-on estimation or by ultrasound estimation. But the critical question is whether estimating fetal weight actually improves outcomes.
Here is yet another study that shows that estimating fetal weight does NOT improve outcomes and that it increases the risk for cesareans.
In this very large multi-center MFMU study, a baby estimated to be above 4000g (8 lbs., 13 oz.) was more than twice as likely to be born by cesarean. A diabetic mother with a baby estimated to be above 3500g (about 7 lbs. 12 oz.) was more than nine times as likely to have a cesarean.
This relationship persisted even when adjusted for actual birth weight and other variables, showing it was not just about the baby's size. The mere prediction of a large baby (whether baby was actually large or not) increased the risk for cesarean, suggesting that it is the way big babies are managed that makes the difference.
This is a particularly pertinent issue for women of size. Because "obese" women tend to have larger babies on average, care providers can get very anxious about fetal size in their pregnancies. Fear of big baby (and the resulting interventions) is one of the top drivers around the outrageously high cesarean rate in high-BMI women.
Sadly, this study does not directly address the effect of fetal weight estimation in the sub-population of obese women. It would be great to have data actually examining a practice that is so common in the management of obese women.
However, this is a very large and very powerful study showing that the practice of estimating fetal weight is harmful in pregnant women in general.
And it is just the latest in a long series of studies that show that the common practice of estimating fetal weight increases the cesarean rate.
Given that it is such a common practice in obstetrics these days, the practice of estimating fetal size before birth needs to be re-evaluated.
There are times when estimating fetal size can be appropriate, but most of the time it is more harmful than helpful. Care providers need to stop using it routinely.
And its use especially needs to be evaluated in the management of women of size.
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.