There are several strengths in our study. First, being a case–control study nested within an ongoing prospective cohort, we could ascertain the temporality between the exposure and the outcome and thereby apply models to allow estimation of potential causal effects of exogenous female sex hormones on asthma risk. Second, we built causal DAGs based on published literature and our
a priori subject-matter knowledge to identify potential confounding variables for each hormonal exposure [
26,
30], which provides an explicit framework to minimize confounding. Third, in order to reduce the potential bias introduced by item non-response, we implemented MI to impute the missing data [
39]. Fourth, we applied Bayesian statistical model which incorporated our prior background knowledge into the analysis; in this way, the current analysis provides a solid basis for future analyses. Finally, we adopted open and reproducible research practices [
107]—we developed
a priori statistical analysis protocol, documented in detail the research process, and made R scripts publicly available.
Certain limitations need to be taken into account in the interpretation of our findings. First, in our study, women with ever asthma at baseline were excluded, and for many women, hormonal exposures occurred before the study had started. This could have introduced selection bias, especially among older women,
only if hormonal exposures had a causal effect on new-onset asthma (Additional file
2: Fig. S3) [
27]. For example,
if hormonal contraceptives increased the risk of new-onset asthma, the more susceptible individuals would have developed asthma before baseline in the exposed group than in the unexposed group; consequently, restricting to individuals who had not developed asthma by baseline would likely result in less susceptible individuals after baseline in the exposed group than in the unexposed group, thereby attenuating the magnitude of the effect estimate for the true harmful effect or even biasing the effect estimate towards the opposite direction [
27]. Contrary to the hypothetical example, in our study, we found that when the age at baseline increased, the magnitude of point estimate for use of hormonal contraceptives consistently increased. We suspect that selection bias due to selection of women based on baseline asthma status may likely be the main explanation for the increase of the point estimate with increasing baseline age. This suggests that use of hormonal contraceptives may in fact have a protective effect on new-onset asthma, as opposed to a harmful effect indicated by our results. Notably, this type of selection bias may arise in any study that attempts to estimate the causal effect of an (lifetime) exposure that occurs before the study has started [
27,
94,
108,
109]. Second, because some identified confounding variables were not available in our dataset, we had to rely on proxy variables (e.g., for socioeconomic status), or could not adjust for them at all (e.g., diet, alcohol) for use of MHT and new-onset asthma. Third, the information on hormonal exposures and asthma status was obtained retrospectively by questionnaire survey. Thus, an individual’s ability to recall their medical history may affect the measurement of both hormonal exposures and asthma (Additional file
2: Fig. S4) [
92]. In addition, because hormonal exposures were ascertained by recall after asthma had occurred, asthma status might affect the recall of hormonal exposures. However, we expect that the influence was likely to be minimal (if present), because the causal relationship between asthma and hormonal exposures had not been well established. Fourth, more than half of the cases did not report age at asthma onset, which could potentially affect estimation of the temporal relationship between hormonal exposures and new-onset asthma. However, for the cases who reported age at asthma onset, asthma occurred after use of hormonal contraceptives or MHT. Fifth, although WSAS is a population-based study of a representative sample in western Sweden, given the baseline participation rate of 66% and the follow-up rate of 72%, potential selection bias (i.e., bias due to unit non-response) may have arisen, which could potentially affect the estimation of causal effects of exogenous female sex hormones on asthma risk as well as the generalization of our results to the source population. Unfortunately, we were unable to account for this type of bias in our study. Sixth, we could not investigate the potential varying causal effects of use of hormonal contraceptives and MHT by subtypes, doses, durations of use, and routes of administration, because the information on these factors could not be reliably determined from the questionnaire survey. Seventh, the study population for MHT use included women aged
\(\ge\) 45 years at baseline, which was used as a proxy measure to identify menopausal women. Since it was not based on the actual menopausal status, this may affect the generalization of our results to the menopausal group of women. Finally, due to data unavailability, we were unable to address the potential modifying effect of BMI for the effects of exogenous female sex hormones on asthma risk.