Circadian rhythm characteristics of spontaneous labor onset in full-term primiparas and its impact on maternal and neonatal outcomes

To characterize the circadian rhythm of spontaneous labor onset in full-term primiparas, assess its consistency across demographic and obstetric subgroups, and evaluate the association between labor onset timing (nighttime vs. daytime) and maternal/neonatal outcomes, thereby informing evidence-based optimization of obstetric clinical resource allocation. This retrospective study included 1,079 full-term primiparas with spontaneous vaginal delivery at Beijing Tongren Hospital between January 2022 and December 2024. Labor onset time was operationally defined as the first documented time of regular uterine contractions accompanied by cervical effacement ≥ 50% and dilation ≥ 1 cm. Participants were divided into nighttime (22:00–06:59, n = 630) and daytime (07:00–21:59, n = 449) onset groups. General characteristics, labor duration, postpartum blood loss, perineal status, neonatal birth weight, Apgar scores, and NICU admission were compared. Cosinor analysis was applied to detect and quantify circadian rhythmicity in the overall cohort and across prespecified subgroups (maternal age, gestational age, neonatal sex, birth weight, season of birth). Cosinor analysis confirmed a statistically significant circadian rhythm in spontaneous labor onset (F-test for amplitude, P < 0.05), with acrophase at approximately 02:14. A significantly higher proportion of labors commenced during nighttime (58.4% vs. 41.6%, P < 0.05). This nocturnal predominance was consistently observed across all subgroups examined (all P for rhythm < 0.05). No significant differences were found between nighttime and daytime groups in maternal age, labor stage durations, postpartum hemorrhage, neonatal weight, 5-minute Apgar score, perineal laceration, or NICU admission rate (all P > 0.05). However, the daytime group demonstrated a marginally higher mean 1-minute Apgar score (9.85 ± 0.025 vs. 9.76 ± 0.026, P = 0.019; mean difference 0.09, 95% CI 0.02–0.16), and the proportion of neonates with 1-minute Apgar ≤ 7 was significantly greater in the nighttime group (2.5% vs. 0.9%, OR 2.92, 95% CI 1.01–8.44, P = 0.048). Spontaneous labor onset in full-term primiparas exhibits a robust and universal circadian rhythm with a distinct nocturnal peak. While this rhythm does not substantially compromise most key maternal or neonatal outcomes, the modest increase in early neonatal transitional difficulty (lower 1-minute Apgar) during nighttime deliveries signals a potential system-level vulnerability. These findings support the need for dynamic, rhythm‑synchronized adjustments in obstetric staffing, timely availability of labor analgesia, and enhanced neonatal resuscitation preparedness during night hours to optimize perinatal safety and care quality.