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