In women whose labor is started with oxytocin, a long early (latent) phase before reaching active labor raises the chance of a C‑section and both mother‑ and baby‑related complications, especially for first‑time mothers, though most still deliver vaginally.

We sought to assess whether a prolonged latent phase is associated with increased risk of cesarean delivery (CD) and adverse outcomes among patients undergoing induction of labor (IOL) reaching the active phase. This was a secondary analysis of a prospective cohort study from 2018-2022 of term, singleton gestations with intact membranes and unfavorable cervix undergoing IOL at two large academic hospitals. Patients who reached active labor (&#x2265;6cm dilation) were included. Prolonged latent phase was defined as &lt;6&#x2009;cm dilated after &#x2265;12&#x2009;h with ruptured membranes and oxytocin. The primary outcome was CD after reaching active phase. Secondary outcomes included CD for labor arrest and maternal/neonatal morbidity. Analyses were stratified by parity and mode of delivery, and multivariable logistic regression was performed to adjust for confounders. Of 8,509 patients, 7,451 (87.6%) reached active phase. Among them, 472 (6.3%) had a prolonged latent phase, with a median duration of 15.4&#x2009;h [13.3-26.8]. 74.8% of patients who reached active labor after a prolonged latent phase delivered vaginally, though they had a higher rate of CD than those without a prolonged latent phase (25.2% vs 9.9%, <i>p</i>&#x2009;&lt;&#x2009;0.001). This remained significant after adjusting for confounders (aRR 1.69 95%CI [1.41-2.01]). Stratifying by parity, only nulliparous patients with a prolonged latent phase remained at increased risk for CD (aRR 1.67 95%CI [1.39-2.00]). Patients with a prolonged latent phase were more likely to have clinical chorioamnionitis (20.3% vs. 9.4%, <i>p</i>&#x2009;&lt;&#x2009;0.001) and to experience the composite maternal and neonatal morbidities (26.9% vs. 15.1%, <i>p</i>&#x2009;&lt;&#x2009;0.001; 4.2% vs. 2.3%, <i>p</i>&#x2009;=&#x2009;0.01). Among patients with a vaginal delivery, a prolonged latent phase was associated with higher maternal morbidity. However, patients who underwent CD had the highest risk of morbidity, regardless of latent phase duration. Among term patients undergoing induction with an unfavorable cervix, patients with a prolonged latent phase who ultimately enter active labor have a high rate of vaginal delivery. However, a prolonged latent phase is associated with increased morbidity.