Introduction
Caesarean deliveries are associated with more maternal morbidity than vaginal deliveries, yet more than one-third of pregnancies in the United States (US) are delivered by caesarean (1). Various factors contributed to the caesarean epidemic, including maternal and fetal indications, labour management, and non-medical factors (1). Small for gestational age (SGA), a proxy for fetal growth restriction that includes both constitutionally small and pathologically growth restricted fetuses, is one such fetal indication (2, 3). For pathologically growth restricted fetuses, transient contraction of the placental intervillous space during labour induces hypoxic-ischemic stress that may be poorly tolerated, resulting in category 2 fetal heart rate tracings, reflecting “fetal distress” in labour (4, 5). While some fetuses with severe growth restriction may not tolerate labour, an indication for caesarean delivery (6), others will have normal fetal heart rate patterns. In order to avoid emergent delivery for potential labour complications, obstetricians may encourage some patients to undergo prelabour caesarean deliveries. This counseling, which may be impacted by gestational age, often leads to management decisions that remain unsupported by empirical evidence and evidence-based recommendations (1).
SGA is associated with labour outcomes, including caesarean delivery, regardless of trial of labour (TOL) (7). Gestational age influences this relationship, but the extent to which gestational age impacts labour outcomes remains unknown. Therefore, we undertook this population-based study to determine if gestational age mediates or interacts with exposure to SGA to influence the risk of caesarean delivery. We performed a causal mediation analysis, which is an analytical method that provides a causal framework to disentangle the impact of gestational age as mediator, interaction, or both on exposure to SGA and labour outcomes (8-12). We hypothesized that gestational age would have the largest impact on labour outcomes <28 weeks’ gestation and, particularly, for severely growth restricted fetuses.