Statistical analysis
In absence of previous pilot study to calculate the sample size for the prevalence of preeclampsia in women exposed to H2S pollution, we have considered that incidence of preeclampsia of women in non-exposed areas was 6%. Assuming alpha=0.05 and power of 85% with a non-exposed/exposed ratio of 4, a sample of 3020 women was planned to be able demonstrate a 3-point difference between non-exposed/exposed women. For all descriptive analyzes, assumption of normal distribution of the data was analyzed. Mean and standard deviation, or median and interquartile range were reported when appropriate. Categorical variables were presented as absolute values and percentages. The following tests were used for group comparisons depending of the conditions of application: Student t-test, Mann-Whitney test, Chi-square test or Fisher’s exact test. Kaplan-Meier analysis was conducted to describe preeclampsia free survival time and compared using the log-rank test. The level of statistical significance was set at p<0.05. All statistical analyses were conducted using SAS software 9.4 for Windows (SAS Institute, Cary North Carolina, USA).