This was categorized according to International Standard Classification of Occupations-88 groups (1 -9), [ 20] and used as a 4 category variable (with 1 representing higher socio-economic position – used as the reference category - and 4 the lowest).
The main exposure variable was volunteering, reported as a four category variable.
We construct household income as a four category variable: < $20 K (reference), $20-$20-$50$50 K+, and income missing.
Age was described as a three category variable 20-24 yearss, 25-29 yeand, and 30-35 years), then grouped into 2 categories 20-29 yearss and 30-35 years) for further analyses.
To maximise the use of available data, birth weight was included in all models as a three category variable (normal (BW > = 2500 gm), low birth weight (BW <2500 gm) and birth weight not recorded).
We estimated the odds ratios (ORs) for PIH and PE through polytomous logistic regression models, treating the four exposure categories as a class variable.
Mode of delivery was recorded in the Atlantic DIP database as a four-category variable: a) normal vaginal delivery, b) assisted vaginal delivery (including forceps and/or ventouse), c) elective caesarean section, and d) emergency caesarean section.
Each variable was incorporated in the statistical models as a three-category variable, the breakpoints being chosen a priori on the basis of the quintile distribution of the expected numbers, to isolate the highest quintile.
Age was included as a three-category variable, where patients were classified as being aged younger than 30 years, between 30 and 40 years, or being older than 40 years.
