Third-Trimester Ultrasound Diagnosis of Large for Gestational Age and Risk of Cesarean Delivery.

Objective: Determine if knowledge of a third-trimester ultrasound diagnosis of large for gestational age (LGA) independently increases the risk of cesarean delivery (CD).

Study Design: Historical cohort comparing CD rate among patients diagnosed with an LGA fetus on a clinically indicated ultrasound from January 2017 to July 2021 with those without an LGA diagnosis at 34 weeks or later. LGA was defined as an ultrasound-estimated fetal weight greater than or equal to the 90th percentile for the gestational age. Univariate analysis was performed to identify significant confounding variables and was utilized as covariates for binary regression with CD rate as the primary outcome, and adjusted odds ratios (AOR) with 95% confidence intervals (CI) were calculated. Nulliparous term singleton vertex (NTSV) and multiparous CD rates were also compared.

Results: There were 447 patients diagnosed with an LGA fetus and 1971 patients without an LGA diagnosis on third-trimester ultrasound. The positive predictive value of LGA diagnosis was 50.1% and the false positive rate was 10.6%. Patients with a diagnosis of LGA had higher AOR of CD (OR 2.11, 95% CI 1.56-2.83), and higher AOR of NTSV CD (OR 1.88, 95% CI 1.14-3.13) compared with those without an LGA diagnosis. There was no difference in the rates of non-medically indicated CD, multiparous primary CD, and attempted and successful TOLAC.

Conclusion: Our results suggest third-trimester ultrasound diagnosis of LGA independently increases odds of CD, specifically among nulliparous patients, and the potential bias may be one factor contributing to excessive CDs and NTSV CDs.