The full Bt20+sample comprises 3273 singleton births. In our analysis
we use data from the antenatal period (n=1595) and delivery reports.
A total of 1228 women had complete data for all exposure
variables (measured prenatally) and had infants who were delivered
within the greater Johannesburg metropolitan area over the specified
7-week period, between April and June of 1990. These 1228 motherinfant
pairs comprised the analytical sample for this study.