INTRODUCTION: The objective of this study was to evaluate the prevalence of large for gestational age (LGA) and polyhydramnios found on ultrasound performed between 34 and 38 weeks of gestation and investigate the prevalence of gestational diabetes and delivery outcomes in this population. METHODS: A retrospective cross-sectional study (IRB #4869) was performed including chart review of late third-trimester ultrasounds performed on singleton pregnancies between 34 and 38 weeks of gestation from April 2020 to April 2022 at a hospital-owned private practice. Patients were grouped as no LGA (estimated fetal weight >90%) or polyhydramnios (amniotic fluid index ≥25 or maximum vertical pocket ≥8) or LGA and/or polyhydramnios. Differences in cesarean delivery (CD), gestational diabetes diagnosis, and macrosomia between groups were assessed via χ2 and Fisher's exact tests. RESULTS: Of 1,510 pregnancies, 15.0% (227/1,510) were diagnosed with LGA and/or polyhydramnios on late third-trimester ultrasound. The overall prevalence of CD in the group without LGA and/or polyhydramnios was 25.5%, whereas the rate of CD in patients having LGA and/or polyhydramnios was 44.1% (P<.001). The overall prevalence of macrosomia in the group without LGA and/or polyhydramnios was 8.8%, whereas the rate of macrosomia in patients having LGA and/or polyhydramnios was 17.2% (P<.001). The prevalence of diabetes in pregnancy was not significantly different between groups (without LGA and/or polyhydramnios 12.2% versus having LGA and/or polyhydramnios 16.7%; P=.081). CONCLUSION: Patients with LGA and/or polyhydramnios on late third-trimester ultrasound had significantly higher rates of CD and macrosomia. They were not more likely to have a diagnosis of diabetes during pregnancy. Late third-trimester ultrasound findings of LGA and/or polyhydramnios could be useful in detecting undiagnosed and/or missed diabetes. Future work will evaluate the timing of gestational diabetes screening and subsequent testing for diabetes if findings suggest LGA and/or polyhydramnios.