Methods
Data
Data are obtained from European Longitudinal Cohort Study of Pregnancy and Childhood–Czech Republic (ELSPAC-CZ). The study was initiated by World Health Organization to investigate factors in maternal and child health in several European countries. The Czech part was approved for adherence to ethical guidelines by the Scientific Council of Masaryk University. The Czech cohort targeted the entire population of births in two cities between March 1, 1991 and June 30, 199227. Brno, a large metropolis and Znojmo, a smaller nearby town, were chosen to represent urban and rural populations. Medical documentation was available for 7,589 births (96% of all eligible births). To assess prenatal characteristics, the study partnered with local obstetrics/gynecology practices to distribute survey questionnaires to gravidas at 20 weeks (henceforth mid-pregnancy). This effort yielded 4,811 responses for the baseline survey sample. All participating women provided written informed consent. When the focal child reached age six months, eighteen months, three years, five years, and seven years, women were mailed follow-up surveys on child’s health. At child age seven years, 3,312 mother-child pairs remained in the cohort for 67% of the baseline sample. Of these, 1,849 mother-child pairs had complete data and are included in the analytical sample.
Measures
The main outcome, child’s wheeze/wheeze with whistle (henceforth “wheeze”), was collected at age six months, eighteen months, three years, five years, and seven years. At each time point, mothers were asked, “Were there any periods [between observation time points] when there was wheezing or wheezing with whistling on your child’s chest when breathing?” (yes/no). Following Tucson Children’s Respiratory Study (TCRS)28,29, wheeze was coded as four longitudinal phenotypes, including “never” (no wheezing at any time point), “early-onset transient” (EOT) for onset before age three years with resolution by age seven years, “early-onset persistent” (EOP) for onset before age three years persisting at age seven years, and “late-onset” (LO) for onset between age three years and seven years. While TCRS used age six years as a cut-off, we use seven years as the closest available observation point.
Prenatal and postnatal life events, main predictors, were assessed using Social Readjustment Scale30 adapted for Avon Longitudinal Study of Pregnancy and Childhood. The questionnaire, tailored specifically for pregnancy and postpartum and used in prior population-based research31–33 lists 42 types of life events (Figure A1 and A2 in SI). Events range from rare, e.g., death of a spouse, to more common, e.g., problems at work. Respondents report whether each event occurred during a specified period. Prenatal stress was assessed in mid-pregnancy using events that occurred since conception. Postnatal stress was assessed at six months postpartum using events that occurred since the delivery. For each observation period, stress exposure was represented by the total number of reported events categorized as low, medium, and high using terciles.
Mediators. To represent no/short breastfeeding as a risk factor, we used maternal reports of breastfeeding duration collected at eighteen months; for non-responding mothers, breastfeeding duration was extracted from paediatric records. The duration was dichotomized using the cut-off of four months, the approximate mean. Mothers reporting no breastfeeding or breastfeeding <4 months were coded as 1 for no/short breastfeeding; others were coded as 0.
LRTI and allergy diagnoses for age 0-18 months were extracted from paediatric documentation. Both variables were coded 1 for children with the diagnosis and 0 for others.
Current maternal smoking (yes/no) was self-reported in mid-pregnancy.
Child’s SSE was mother-reported at eighteen months, coded as hours per week, and treated as continuous.
Covariates . Demographic background includes maternal education (years), marital status (married vs. non-married), age at delivery (years), child sex (male vs. female), and residential region (Brno vs. Znojmo). Additional covariates, selected using literature on paediatric wheeze/asthma, include child allergy extracted from medical records at age eighteen months, maternal histories of asthma and allergy34 self-reported in mid-pregnancy, and low birth weight (<2,500 grams)35 extracted from obstetric documentation. Parity and singleton birth are used as proxies for siblings in the household11.
Statistical Analysis
Analysis was conducted using Stata 14.0 statistical software. Descriptive statistics were obtained including frequencies, percentages, means, and standard deviations. Bivariate relationships between life events and wheeze phenotypes were assessed using simple multinomial logistic regression models. Next, full multivariate models were fitted including all hypothesized predictors, mediators (prenatal life events, postnatal life events, LRTI, allergy, smoking in pregnancy, SSE, breastfeeding), demographic characteristics (child sex, maternal age and its square, education, region, singleton birth), and all remaining covariates. Final models testing H1 and H2 retained hypothesized predictors, mediators, and demographic characteristics; other covariates were trimmed for parsimony to those with p<0.10 in the full model. These included parity and maternal asthma history. Predicted probabilities for each wheeze phenotype were obtained using marginal effects. Finally, mediation analysis was conducted to test H3 and H4. Wheeze was dichotomized as no wheeze vs. any wheeze in mediation analysis since methodology for mediation with dichotomous outcomes is better developed compared to multi-category outcomes36. Decomposition of total effects into direct and indirect was performed using “ldecomp” command37with 1,000 bootstrap replications to produce 95% bias-corrected confidence intervals.