Statistical Analysis
Quantitative variables such as mean maternal age, birth weight and gestational age were evaluated by Student’s t test. Chi square test was used for the evaluation of birth order and employment status. For categorical variables like pregnancy complications adjusted odds ratios (OR) with 95% confidence intervals (CI) were estimated. A multivariable conditional logistic regression model was used to compare maternal risk factors of cases with controls. Among confounding factors maternal age (continuous variable), birth order (parity) (2 vs. 1, 3+ vs. 1) and employment status (skilled/semiskilled worker vs. professional/managerial, unskilled worker/other vs. professional/managerial) as an indicator of socio-economic status were considered.11