On Exposure Definition and Temporal Interpretation in the Reported Association Between Number of Pregnancies and Rheumatoid Arthritis Letter

We read with interest the cross-sectional NHANES analysis by Sheng et al, who reported that a higher number of pregnancies were associated with higher odds of self-reported rheumatoid arthritis (RA) among 2383 women, including an adjusted odds ratio of 1139 per additional pregnancy and a sharper increase beyond three pregnancies. 1Their effort to address an important question at the interface of reproductive history and autoimmune disease is commendable.We nevertheless believe that several aspects of exposure definition and clinical interpretation would benefit from further clarification before the findings are used as a basis for a threshold-like or screening interpretation.This is particularly relevant because clinicians may intuitively read "number of pregnancies" as a proxy for parity or repeated postpartum exposure, which is not necessarily what the current variable captures.Our first concern is that the study evaluates lifetime number of pregnancies, but much of the discussion interprets the results through mechanisms that are more closely linked to live birth or postpartum physiology.The authors appropriately acknowledge that pregnancy outcomes such as miscarriage and induced abortion were unavailable. 1This matters because a broad pregnancy count is a heterogeneous construct, and the broader literature has not shown a consistent association between gravidity or parity and incident RA overall.In a systematic review and meta-analysis, Chen et al found no consistent evidence that either gravidity or parity was associated with RA development 2 More recently, a large prospective cohort reported no association for ever pregnancy, although women with four or more children had a modestly higher RA risk after extensive adjustment, suggesting that different reproductive phenotypes should probably not be assumed to be interchangeable. 3For this reason, the reported "more than three pregnancies" threshold may be better framed as a hypothesisgenerating pattern within a broad pregnancy-history measure rather than a definitive clinical or biological cut-point.Second, the outcome was self-reported RA subtype rather than validated incident RA.This is not a trivial distinction.Validation studies in population cohorts have shown that self-reported RA alone has limited positive predictive value, and that accuracy improves substantially when medication data or medical-record confirmation are incorporated. 4,5In this context, some misclassification between RA and other arthritides is plausible.Because the analysis included 143 RA cases, even modest misclassification may have influenced the magnitude of the observed odds ratios and the apparent dose-response pattern.We therefore wonder whether the discussion should more explicitly acknowledge that the study concerns self-reported prevalent RA rather than clinically adjudicated RA, and that this uncertainty may affect both the estimated effect size and the proposed threshold-like interpretation.Third, the cross-sectional use of lifetime pregnancy count and lifetime RA history makes temporal interpretation more difficult.The timing of pregnancies relative to RA diagnosis was not reported in a way that resolves sequence. 1Yet the literature suggests that reproductive timing matters: in the Swedish EIRA study, parity-related associations differed by age and ACPA status, with increased risk concentrated in younger women with ACPA-negative RA. 6 Likewise, incident RA has been reported to peak during the first 24 months postpartum. 7These observations do not refute the authors'