Strengths and limitations
This study had several strengths and limitations. Strengths were the novel statistical methods, the large availability of variables and the large and representative sample. Inclusion in Generation R was aimed at the first trimester, but was still possible until birth ensuring that those with a late antenatal care initiation were still eligible.20 Some limitations should also be taken into account. We were limited by self-report of pregnancy recognition. This may be subject to recall bias or socially acceptable answers, which may have diluted the associations. In addition, as mentioned in the methods section, one of the assumptions of this causal study design is consistency, which means that the hypothetical intervention (early pregnancy recognition) must be well-defined. In practice, an intervention that ensures everyone will recognize the pregnancy within 6 weeks does not exist. In the third paragraph of the discussion we explained that varying reasons may complicate timely recognizing a pregnancy, therefore potentially violating the consistency assumption. To reduce the possibility of bias, we performed sensitivity analyses in those with a regular cycle, because they would be more likely to know when they miss their period. The results indicated that the reductions in inequalities in antenatal care initiation were only significant for pregnancy intention, migration background and neighborhood deprivation, showing that our initial results may have been slightly overestimated.