(479) Differences in time of birth between spontaneous and operative vaginal births: a nationwide population-based analysis

Objective: Previous studies have identified a correlation between obstetric interventions and the time of day. However, they do not account for the granular variation in temporality of such interventions, which may be influenced by healthcare provider and resource availability. Our objective was to assess differences in time of birth among spontaneous versus operative (forceps- and vacuum-assisted) vaginal births using a population-based dataset.

Study Design: We performed a retrospective population-based study using birth certificate data from the National Vital Statistics System from 2016 to 2021, which includes the time of birth and method of delivery for all recorded births in the United States. The number of spontaneous and operative vaginal births at each minute was determined over a 24-hour time frame. To facilitate balanced comparisons between groups, birth numbers were normalized relative to the total births within each group. Trend lines were established based on 60-minute moving averages and the distributions were compared using the Kolmogorov-Smirnov test.

Results: A total of 15,413,957 vaginal births were analyzed, among which 690,912 (4.5%) were operative births. The normalized proportion of spontaneous and operative births over a 24-hour period is presented in Figure 1. There was a statistically significant difference in the time of birth between spontaneous and operative births (p< 0.01). Compared to births at other time intervals, those between 4:30-7:30 p.m. were more likely to be operative births (OR 1.11, 95% CI 1.11-1.12). Conversely, births between 3:00-6:00 a.m. were less likely to be operative births (OR 0.89, 95% CI 0.88-0.89).

Conclusion: In this population-based study, we identified temporal differences between spontaneous and operative vaginal births with increased use of instrumentation during late afternoon and reduced use in the early morning. These results prompt further investigation into the indications for operative vaginal birth and the root causes of these temporal variations, which are likely multifactorial and involve provider and resource availability.