Small for Gestational Age Births, Gestational Age, and Labour Outcomes:
A Population-Based Study in the US
Objective: To estimate the causal impact of small for gestational age
(SGA) births on caesarean delivery, with and without trial of labour
(TOL); and to quantify how much of the association is mediated through
gestational age at delivery. Design: Cross-sectional analysis. Setting:
Para 2 women who delivered non-anomalous, singleton live births from
22-44 weeks’ gestation in the US (2015-2018). Main outcomes and
measures: Caesarean delivery with and without TOL. The exposure was SGA
births (sex-specific birthweight <5th and <3rd
percentiles for gestational age), and AGA births (10-89th percentile).
We performed causal mediation analysis to determine the impact of
gestational age at delivery (22-33, 34-36, 37-38, 39-40 and ≥41 weeks)
as intermediate. Results: Of the 3,755,798 subjects, compared to AGA
(29.6%), caesarean risks were higher for SGA <5th (34.3%)
and SGA <3rd (36.4%) percentiles. For SGA <5th
percentile, the adjusted excess risk of caesarean delivery without TOL
had a “U” shaped association, with increased risk at preterm
gestations, nadir at 39-40 weeks, and increased thereafter. The
decomposition analysis revealed the driver of this excess risk was SGA
births. The risk of caesarean delivery with TOL was highest
<34 weeks’ gestation and was primarily an interaction effect.
As gestation advanced, SGA births contributed proportionately greater to
the risk. Associations were stronger for SGA <3rd percentile.
Conclusions: Exposure to SGA drives high rates of prelabour caesareans
and contributes to high risks of caesarean deliveries after TOL at
>41 weeks gestation; a different mechanism drives high
rates of caesareans after TOL at preterm gestations.