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.
The cesarean delivery (CD) rate in the United States is at an all-time high. In 2008, 32.3% of births were by CD, which was a 56% increase from 1996. Utilization of prenatal ultrasonography has risen in parallel. The average number of ultrasound examinations per pregnancy has nearly doubled in the last decade, and the percent of pregnancies with ≥4 ultrasound examinations has almost tripled.
For Editors’ Commentary, see Contents
See related editorial, page 239
Increased use of prenatal ultrasound scanning may be contributing to the rising CD rate. One study suggested that sonographic overestimation of fetal macrosomia is associated with a lower threshold for CD for labor arrest. Reduction in the CD rate is a goal of many national organizations. The Healthy People 2020 Goals, for example, include a 10% reduction in CDs among low-risk women. If obtaining an ultrasound-estimated fetal weight (US-EFW) near term places women at increased risk of CD, it may represent a modifiable risk factor that providers could target. We hypothesized that knowledge of US-EFW may impact provider practice or patient preference even if the fetus is not macrosomic. Our study analyzes whether women with any US-EFW within 1 month of delivery are more likely to be delivered by cesarean section compared with those women without an US-EFW.
Materials and Methods
We created a retrospective cohort of deliveries at a single tertiary care center during 1 fiscal year (Oct. 1, 2009, to Sept. 30, 2010) after obtaining approval from our institutional review board (protocol #2011-P-001261/1). We included women who were delivered by provider groups for whom ultrasound reports were available on the electronic medical record. These groups included 1 private practice, 1 group of certified nurse-midwives, and a faculty group that included patients who were seen by maternal-fetal medicine subspecialists, academic generalists, and obstetric residents.
Our cohort was restricted in several ways. We included only singleton live births and excluded those patients with a scheduled CD (both primary and repeat). Patients with a maternal or fetal contraindication to vaginal delivery (including placenta previa, breech/malpresentation, and previous uterine surgery) and patients with diabetes mellitus (both gestational and pregestational) were also excluded because it is standard practice at our institution to get an US-EFW for all patients with diabetes mellitus. Finally, we excluded women who delivered at <39 weeks’ gestation to create a lower risk cohort by removing anyone with a serious maternal or fetal indication that necessitated delivery at <39 weeks’ gestation. In addition, we assumed that concern for labor dystocia because of fetal size would be greater at ≥39 weeks’ gestation compared with earlier gestational ages; thus, the knowledge of US-EFW would have a greater impact at later gestation.
Ultrasound examinations were performed by maternal-fetal medicine specialists and radiologists. Studies were performed in 1 of 3 centralized ultrasound suites; the ultrasound reports were available immediately on our electronic medical record. EFW was provided in grams, and a weight percentile was given that was based on a curve of hospital-specific gestational age–matched birthweights. We abstracted only ultrasound reports with an EFW assessment that had been performed within 1 month (31 days) of the delivery date. If a patient had >1 US-EFW within the month before delivery, we considered only the most recent. Of note, the provider who ordered the ultrasound scan may not have been the delivering provider. All patients were delivered in group practices.
Demographic and delivery data were abstracted from our computerized medical record system. Missing demographic data were hand-collected by a review of the relevant charts. Gestational ages were rounded to the nearest completed gestational week. Body mass index (BMI) was categorized as obese if prepregnancy BMI was >30 kg/m 2 or, if prepregnancy BMI was not available, if delivery BMI was >35 kg/m 2 , because this was the average BMI increase in our cohort for those women with both sets of data available. The CDs for the midwife group were performed by the faculty practice. Thus, CDs were reviewed, and the provider group was reassigned to the midwives if appropriate. Women with prelabor rupture of membranes and oxytocin administration before the onset of spontaneous contractions were considered to be induced. Ultrasound information was hand-collected, and the date, indication, and EFW were recorded.
We created 2 groups: (1) women with an US-EFW within 1 month of delivery and (2) women with no US-EFW. We reported baseline characteristics for these 2 groups using medians and quartiles (because they were not normally distributed) and compared demographic differences using Wilcoxon 2-sided tests for continuous variables and χ 2 testing for categoric variables. We used logistic regression to analyze whether US-EFW was an independent risk factor for CD. The mode of delivery was the outcome variable; both spontaneous and operative vaginal deliveries were included in the vaginal delivery group. The statistical software package (SPSS version 12.0; SPSS Inc, Chicago, IL) was used for all data analyses.
We performed logistic regressions on several different population subsets to analyze the potential relationship and explanation for our findings in the entire cohort. The different subsets included (1) a low-risk group (we excluded any patient with an ultrasound scan or induction of labor for maternal disease or fetal concerns), (2) a group with smaller EFWs (<3000 g), (3) a group with larger EFWs (>3500 g), and (4) a group with actual birthweights of >4000 g.
Power calculations were performed with the assumption that the average CD rate would be 15% for the entire population and that 50% of the population would have an US-EFW performed within 1 month of delivery. We assumed that a 5% absolute difference in CD rates would be meaningful clinically and used a power of 80%, which required a sample size of 1600 women. We chose to use all the deliveries within 1 fiscal year (approximately 2300 women after exclusions) as a convenience sample, which gave us 92% power to detect a 5% absolute difference in CD rates.
Results
During the study period, 7880 women delivered. Figure 1 displays all our exclusions to arrive at a final cohort of 2329 women. Our cohort was composed of 557 women who were delivered by the private practice, 701 women who were delivered by the faculty practice, and 1071 women who were cared for antepartum by the midwife practice.
In total, 1168 women (50.2%) had an US-EFW within 1 month of delivery. Of those with an US-EFW, 540 women (46%) had the procedure within 1 week of delivery; 276 women (24%) had the procedure 1-2 weeks before delivery; 179 women (15%) had the procedure 2-3 weeks before delivery, and 173 women (15%) had the procedure 3 weeks to 1 month before delivery. The most common indications for an ultrasound scan, as listed on the ultrasound report, were postdates (19%), uterine size less than dates (17%), uterine size greater than dates (14%), and advanced maternal age (16%). The ultrasound findings did not significantly impact delivery timing because there were no new fetal anomalies detected and because only 13 women were induced for oligohydramnios as seen on ultrasound scans within a week of delivery.
Demographic characteristics by ultrasound status are displayed in Table 1 . As shown, women with an US-EFW within 1 month of delivery were significantly older and more likely to be white, to be cared for by an physician as compared with a midwife, to be obese, and to undergo an induction of labor. Women with an US-EFW also delivered at slightly later gestational ages. The median for both groups was 40 weeks’ gestation, but the range was higher in the US-EFW group, which was statistically significant ( P = .03). There were no differences in parity, previous CD, or birthweight.
| Ultrasound-estimated fetal weight within 1 month of delivery | |||
|---|---|---|---|
| Variable | Yes (n = 1168) | No (n = 1161) | P value |
| Maternal age, y a | 30 (24-34) | 27 (22-32) | < .01 |
| Race, n (%) | < .01 | ||
| Black | 254 (22) | 303 (26) | |
| Hispanic | 281 (24) | 433 (37) | |
| White | 497 (43) | 288 (25) | |
| Asian | 89 (8) | 95 (8) | |
| Other/unknown | 47 (4) | 42 (4) | |
| Nulliparity, n (%) | 576 (49) | 575 (50) | .92 |
| Obese, n (%) b | 231 (20) | 182 (17) | < .01 |
| Previous cesarean delivery, n (%) | 49 (4) | 44 (4) | .62 |
| Induction of labor, n (%) | 502 (43) | 263 (23) | < .01 |
| Gestational age, wk a | 40 (40-41) | 40 (40-40) | .03 |
| Provider group, n (%) | < .01 | ||
| Private physician | 356 (30) | 201 (17) | |
| Faculty physician | 400 (34) | 301 (26) | |
| Certified nurse-midwife | 412 (35) | 659 (57) | |
| Birthweight, g a | 3348 (3061-3657) | 3373 (3118-3657) | .30 |
a Results reported as median (25-75% quartiles);
b Prepregnancy body mass index of >30 kg/m 2 or, if not available, delivery body mass index of >35 kg/m 2 .
Table 2 shows the unadjusted CD rates between those women with and without an US-EFW within 1 month of delivery. Women with an US-EFW were 55% more likely to have a CD (15.7% vs 10.2%; P < .01). The increased risk of CD with US-EFW was observed in all provider groups. Women with an US-EFW were more likely to have a CD compared with women without an US-EFW in the private physician, faculty physician, and midwife practices with odds ratios (ORs) of 2.1 (95% confidence interval [CI], 1.1–3.7), 1.3 (95% CI, 0.8–1.9), and 1.6 (95% CI, 1.1–2.3), respectively. For every 18 additional US-EFWs that were obtained, 1 additional CD was performed. The timing of ultrasound ascertainment within the 1-month period before delivery had little impact. The CD rate was 16% with an ultrasound within 1 month of delivery, 16% with an ultrasound within 3 weeks of delivery, 15.5% within 2 weeks of delivery, and 12% within 1 week of delivery.
| Ultrasound-estimated fetal weight within 1 month of delivery | |||
|---|---|---|---|
| Variable | Yes (n = 1168) | No (n = 1161) | P value |
| Cesarean delivery, n (%) | 183 (15.7) | 118 (10.2) | < .01 |
| Indications for cesarean delivery, n (%) a | |||
| Macrosomia | 17 (9) | 0 | < .01 |
| Failure to progress | 108 (59) | 79 (67) | .17 |
| Nonreassuring fetal assessment | 74 (40) | 57 (48) | .18 |
| Other | 7 (4) | 6 (5) | .81 |
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
