(226) Impaired fetal growth drives high rates of cesarean delivery after failed trial of labor

Some growth restricted fetuses may not tolerate labor. We performed causal-mediation analysis, an innovative statistical methodology, to quantify the impact of small for gestational age (SGA) birth, a surrogate for fetal growth restriction, and the impact of gestational age (GA) at delivery on the risk of cesarean delivery after failed trial of labor (TOL).

Study Design:

We performed a large cross-sectional analysis of all para 0 patients who delivered non-anomalous, singleton live births from 22-42 weeks’ gestation in the US (2015-2019). The primary outcome was cesarean delivery after any labor, termed “failed TOL.” The exposure was SGA birth (sex-specific birthweight < 3^rd [SGA3] and < 5^th [SGA5] percentiles for GA). We performed causal mediation analysis to quantify the impact of GA at delivery on the causal association between SGA (exposure) and cesarean delivery after failed TOL (outcome).

Results:

Of 5,422,214 subjects, n=195,123 (3.6%) were SGA3, n=342,315 (6.3%) were SGA5, and n=4,726,980 (87.2%) were AGA. Overall cesarean risks after failed TOL from 22-42 weeks gestation were higher for SGA3 and SGA5 (16.9% and 15.7%, respectively) compared to AGA (14.3%). The analysis by GA strata revealed similar rates of cesarean delivery after failed TOL for SGA3 versus AGA prior to 32 weeks, but notable increased risk for SGA3 after 32 weeks (TABLE 1). Using 39 weeks as the reference, the mediation analysis revealed that exposure to SGA (the controlled direct effect) was the primary driver of this disparate risk, especially at 37-38 weeks and after 41 weeks. Associations were similar for SGA5 (data not shown).

Conclusion: Compared to AGA births, SGA births are at increased risk for cesarean delivery after failed TOL. This innovative analysis revealed that the risk appears to be the result of exposure to SGA, not GA at delivery. Further study is needed to identify fetuses with pathologic growth that may tolerate labor and avoid morbidity from failed TOL.