The role of fetal viability in perineal and anal sphincter injury: Insights from a large cohort study.

BACKGROUND

Perineal injury is a common complication of vaginal delivery with implications for maternal morbidity. Injuries range from minor lacerations to severe obstetric anal sphincter injuries (OASI). The role of fetal viability in perineal outcomes remains unclear.

METHODS

A retrospective cohort study was conducted at a tertiary center (with approximately 13 000 annual deliveries). Eligible cases were singleton vaginal births ≥24 weeks and ≥ 500 g. Breech, vacuum-assisted deliveries, and episiotomies were excluded. Matching was performed by year of birth, parity, gestational age, birthweight, and epidural use. Analyses included matched and unmatched comparisons, as well as univariate and multivariate analyses.

RESULTS

The unmatched cohort comprised 86 341 viable and 598 non-viable births. Non-viable births occurred at lower gestational age (31.9 vs. 39.5 weeks, P < 0.001) and birthweight (1796 ± 920 vs. 3270 ± 435 g, P < 0.001), with lower spontaneous perineal injury rates (21.7% vs. 53.4%, P < 0.001). In the matched cohort (426 pairs), injury remained lower in non-viable births (29.1% vs. 36.2%, P = 0.028). OASI rates were similar (0.0% vs. 0.9%, P = 0.124). Multivariate analysis showed that non-viable status independently reduced risk of spontaneous perineal injury (odds ratio [OR] 0.46, 95% confidence interval [CI] 0.37-0.57, P < 0.001). Other predictors included nulliparity (OR 4.57, 95% CI 4.40-4.72), birthweight (OR 1.64 per kg, 95% CI 1.57-1.71), and gestational age (OR 1.05 per week, 95% CI 1.04-1.06; all P < 0.001).

CONCLUSION

Non-viable fetal status was associated with reduced spontaneous perineal injury, although the effect lessened after adjusting for birthweight and gestational age. These findings suggest a potential fetal contribution to perineal injury mechanisms.