Study design and assumptions
In this study, we are not interested in a potential causal effect of the
predictors on timing of antenatal care initiation via a mechanism of
timing of pregnancy recognition, which is what would be obtained when
using mediation methods. From a public health point of view, it would be
too complicated or even impossible to intervene on age, relationship
status, migration background or socioeconomic position for people to
initiate antenatal care earlier. Early pregnancy recognition might be a
factor that may have a causal relationship with timing of antenatal care
initiation, is amenable to intervention, and may reduce inequalities in
antenatal care initiation. To answer this research question, we assessed
the effect of early pregnancy recognition on timing of antenatal care
initiation, and we used this information to estimate the reduction in
inequalities in antenatal care initiation if everyone in our study
population had recognized the pregnancy early. This design is also
called a hypothetical intervention,32 because we used
data from an observational study to estimate the counterfactuals, i.e.
what would have happened if we had intervened in such a way that all
participants would recognize the pregnancy within 6 weeks of gestation.
Three causal assumptions apply in respect to the intervention and the
outcome.33 First, exchangeability refers to no
residual confounding or bias in the association between pregnancy
recognition and timing of antenatal care initiation. We therefore
adjusted for all predictors as potential confounders. Second, positivity
means that the probability of receiving the intervention (early
pregnancy recognition) conditional on the covariates must be greater
than 0. This was tested by estimating propensity scores in a model of
the non-imputed dataset with early pregnancy recognition as dichotomous
outcome and all the covariates as predictors. As the participant with
the smallest propensity score had a score of 0.217 to receive the
intervention, the positivity assumption was satisfied. Third,
consistency requires the intervention to be clearly defined. In
practice, our intervention would require sexually active women of
reproductive age to take a pregnancy test in case of pregnancy suspicion
(e.g. their menstruation did not start in the week that they expected).
This would be complicated for those with an irregular cycle, thereby
potentially violating the consistency assumption. Therefore, in order to
relax this assumption, we repeated the analyses in only those with a
regular cycle (28 days plus minus 4 days) in sensitivity
analyses.34