Impact of an Umbilical Artery S/D Ratio ≤ 5th Percentile on Pregnancy Outcomes: A Retrospective Cohort Study

Objective To evaluate the impact of persistent versus non-persistent umbilical artery S/D ratios at or below the 5th percentile on pregnancy outcomes, and to explore its predictive value for adverse pregnancy outcomes through gestational age-stratified analysis. Methods This was a single-center retrospective cohort study that included 1,790 singleton pregnant women who underwent umbilical artery Doppler examinations at Shandong Provincial Maternal and Child Health Care Hospital Affiliated to Qingdao University between January 2023 and December 2025. Inclusion criteria were: gestational age ≥ 36 weeks and complete S/D ratio records. Exclusion criteria included persistently elevated S/D ratios, absent or reversed end-diastolic flow; multiple pregnancies; and fetal structural or chromosomal abnormalities. Low S/D ratio was defined as an initial measurement ≤5th percentile. Persistent low S/D was defined as all measurements ≤5th percentile, while non-persistent low S/D was defined as only some measurements meeting this criterion. After random 1:1 matching by gestational age, 444 cases were included in each group. Multivariable logistic regression analysis was performed to examine the role of umbilical cord abnormalities, fetal distress, neonatal birth weight, and maternal comorbidities. Primary outcomes included Apgar score < 7 at 5 minutes, admission to neonatal intensive care unit (NICU), and neonatal respiratory distress. Secondary outcomes comprised mode of delivery, fetal distress rate, and the interval from first detection of low S/D to delivery. Outcomes were compared after stratifying by the gestational age at first detection of low S/D into two-week intervals (36–37, 38–39, and 40–41 weeks). Results Among the 1,790 included cases, 1,056 had documented low S/D ratios (based on 2,125 total measurements), comprising 612 cases in the persistent group and 444 in the non-persistent group. After matching, 444 cases were included in each group. No stillbirths occurred in either group. Compared with the non-persistent group, the persistent group had a shorter interval from detection to delivery (20.4 ± 8.7 days vs. 26.8 ± 6.5 days, P < 0.001) and lower neonatal birth weight (3312 ± 518 g vs. 3428 ± 462 g, P = 0.012). There were no statistically significant differences between groups in the rates of Apgar score < 7 at 5 minutes, NICU admission, or neonatal respiratory distress. The rate of umbilical cord abnormalities was higher in the persistent group (15.3% vs. 10.1%, P = 0.05). Stratified analysis revealed that among the 40–41 weeks group, the persistent low S/D subgroup had a higher rate of fetal distress (6.1% vs. 2.4%). Gestational age at first detection of low S/D was negatively correlated with the interval to delivery (r=-0.32, P < 0.001). Multivariable logistic regression analysis demonstrated that the pattern of low S/D (persistent vs. non-persistent) was not an independent predictor (OR = 1.08, 95% CI: 0.75–1.56, P = 0.68); however, umbilical cord abnormalities (OR = 3.12, 95% CI: 1.89–5.15, P < 0.001) and fetal distress (OR = 4.32, 95% CI: 2.18–8.56, P < 0.001) were identified as independent risk factors. Conclusions This large-sample retrospective cohort study demonstrates that persistent low S/D ratio, compared with non-persistent low S/D, is associated with a higher rate of umbilical cord abnormalities. Gestational age-stratified analysis suggests that persistent low S/D at advanced gestational ages may increase the risk of fetal distress. Gestational age at first detection of low S/D was negatively correlated with the interval to delivery. Multivariable analysis indicates that low S/D ratio accompanied by umbilical cord abnormalities and fetal distress are independent predictors of adverse pregnancy outcomes. Trial registration: not applicable.