Based on the history of exposure to ETS the study population was
divided into two groups. The maternal and foetal variables were compared
in the subjects exposed to ETS with those who were not exposed.
The exposure to ETS was quantified by calculating a cumulative
“exposure index”, which was a product of the number of smokers
in the house to whom the case was exposed, average number of
bidis/cigarettes smoked per day by them in the presence of the case
and the period of gestation in weeks. For the purpose of studying
dose response, the exposed group was divided into four categories
according to the quartiles of the exposure index.