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Estimated Fetal Weight by Ultrasound: A Modifiable Risk Factor for Cesarean Delivery?

Little, Sarah E.; Edlow, Andrea G.; Thomas, Ann M.; Smith, Nicole A.

Obstetrical & Gynecological Survey: February 2013 - Volume 68 - Issue 2 - p 75–77
doi: 10.1097/OGX.0b013e3182812880

Use of prenatal ultrasonography (US) has risen concomitantly with the increase in cesarean delivery (CD) rates. Sonographic overestimation of fetal macrosomia may be associated with a lower threshold for CD for labor arrest. If obtaining the US estimated fetal weight (US-EFW) near term increases the risk of CD, this could be a modifiable risk factor. This retrospective cohort study was undertaken to determine whether women with a US-EFW within 1 month of delivery are more likely to undergo CD compared with women without a US-EFW.

The women had singleton live births; their demographic and delivery data were obtained from medical records, and the date, indication, and EFW were recorded. Patients were divided into those with a US-EFW within 1 month of delivery and women with no US-EFW. Logistic regression was used to analyze whether US-EFW was an independent risk factor for CD. The mode of delivery was the outcome variable. The statistical software package SPSS version 12.0 was used for all data analyses. Subsets created were groups at low risk, those with EFWs of less than 3000 g, those with EFWs greater than 3500 g, and those with actual birth weights of greater than 4000 g.

This cohort included 2329 women with term pregnancies who met the inclusion and exclusion criteria, of whom 1168 (50.2%) had a US-EFW within 1 month of delivery. Of those with a US-EFW, 540 (46%), 276 (24%), 179 (15%), and 173 women (15%) had the US within 1 week of delivery and 1 to 2 weeks, 2 to 3 weeks, and 3 weeks to 1 month before delivery, respectively. The most common indications were “postdates” (19%), uterine size less than dates (17%), uterine size greater than dates (14%), and older maternal age (16%). Women with a US-EFW within 1 month of delivery were significantly older and more likely to be white, be cared for by a physician rather than a midwife, be obese, undergo labor induction, and deliver at slightly later gestational ages. No differences in parity, previous CD, or actual birth weight were found between the 2 groups. The unadjusted CD rates showed that women with a US-EFW were 55% more likely to have a CD (15.7% vs 10.2%; P < 0.01). The timing of US within the 1-month period before delivery had little impact. The CD rates were 16% with a US within 1 month of delivery and 16%, 15.5%, and 12% for a US within 3, 2, and 1 week(s) of delivery, respectively. The main indications for CD were failure to progress and nonreassuring fetal heart tracings in both groups. Of 17 CDs performed for presumed macrosomia, all in the US-EFW group, only 3 deliveries had an EFW of greater than 5000 g. When controlling for birth weight, maternal age, race, body mass index, previous CD, parity, gestational age, induction versus spontaneous labor, and provider group, US-EFW was a statistically significant independent risk factor for CD. Women with a US-EFW within 1 month of delivery were 44% more likely to have a CD. Other independent predictors of CD were birth weight, advanced maternal age, black or Hispanic race, nulliparity, obesity, previous CD, and faculty provider group. In the entire cohort, US-EFW was associated with increased odds of CD (odds ratio [OR], 1.44; 95% confidence interval [CI], 1.1–1.9). For those with fetuses of less than 3000 g, US-EFW was not associated with an increased risk of CD (OR, 0.97; 95% CI, 0.45–2.1). With EFW greater than 3500 g, the OR was greater and statistically significant (OR, 1.8; 95% CI, 1.3–2.7) and was stronger still for the population with an actual birth weight greater than 4000 g (OR, 3.1; 95% CI, 1.2–8.3).

For women with a US-EFW within 1 month of delivery, the association between the US and CD seems to be due to provider concern over larger US-EFWs because the risk of CD was not increased if the US-EFW was less than 3000 g. However, for women with a US-EFW of greater than 3500 g, an 85% increased risk of CD was apparent with US-EFW determination. The rising CD rate is a great concern, but many of the known reasons contributing to the rise are not readily modifiable. These results indicate that provider knowledge of larger US-EFW may be associated with a higher rate of CD. Limiting the use of US to determine fetal weight near term may help to reduce the CD rate in the United States.

Division of Maternal-Fetal Medicine, Department of Obstetrics and Gynecology (S.E.L., A.M.T., N.A.S.), Brigham and Women’s Hospital; and Division of Maternal-Fetal Medicine, Department of Obstetrics and Gynecology (A.G.E.), Tufts Medical Center, Boston, MA.

© 2013 Lippincott Williams & Wilkins, Inc.