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.