Fetal head station and head circumference at second‐stage onset and likelihood of vaginal delivery in nulliparous women

OBJECTIVE: Our objective was to evaluate the combined association of fetal head station (FHS) at second-stage onset and fetal head circumference (HC) with the likelihood of spontaneous vaginal delivery (SVD) in nulliparous women. METHODS: This retrospective cohort study included nulliparous women with singleton, vertex-presenting, term pregnancies (≥37 weeks) who reached the second stage of labor between 2013 and 2023 at a tertiary medical center. FHS was categorized as above, at, or below the ischial spines. HC, measured by ultrasound within 14 days before delivery, was dichotomized as <350 mm or ≥350 mm. Participants were classified into six FHS-HC groups. The primary outcome was SVD. Associations were assessed using multivariable logistic regression. RESULTS: Among 4230 women, SVD rates ranged from 57.7% for FHS above the ischial spines with HC ≥350 mm to 88.7% for FHS below the ischial spines with HC ≥350 mm. Compared with the reference group (FHS below the ischial spines and HC <350 mm), the adjusted odds of SVD decreased with higher station: adjusted odds ratio (aOR) 0.73 (95% confidence interval [CI] 0.60-0.88) for station at the ischial spines and 0.59 (95% CI 0.44-0.80) for station above the ischial spines. Among women with HC ≥350 mm, those with FHS above the ischial spines had the lowest likelihood of SVD (aOR 0.35; 95% CI 0.14-0.82). Higher FHS and larger HC were associated with longer second-stage duration (P < 0.001). CONCLUSION: FHS at second-stage onset was more strongly associated with SVD than was HC. Combined FHS-HC assessment may help identify women at increased likelihood of operative delivery and support closer monitoring of labor progress.