Other covariates including maternal age, height, year of delivery and smoking habit were grouped as detailed in Table 1.
Potential confounders included maternal variables (see Additional file 1), infant's sex, paternal height, and season of delivery.
In the first prenatal visits, the date of the last menstrual period, uterine fundal height, assumed date of delivery and length of gestation (best estimate) were noted.
Gestational age was determined from a combination of maternal history (expected date of delivery, height of fundus, first trimester ultrasound) and the Ballard score, which was done by attending clinical staff.
Personal and physical data on pregnancy provided by the mother included maternal age at birth, planned and actual date of delivery, height, body weight, occupation, birth order and gestational age of the infant, and multiple births.
The maternal demographic characteristics analysed were the ages of women in complete years at the time of delivery, their height (in cm) recorded at booking for antenatal care, and their weight prior to delivery in Kgs.
Then it was practical to use the concept of delivery head (delivery height) as a criterion of the pumps performance ability.
*Adjusted for maternal characteristics (age, parity, smoking, height, education, country of origin, year of delivery).
Odds ratios were adjusted for maternal age, parity, smoking, education, height, country of birth, and year of delivery.
Adjusted models were restricted to observations with complete information on covariates, including maternal BMI, age, parity, smoking, education level, height, country of birth, and year of delivery.
Despite that we could adjust the BMI related risks for several maternal characteristics (age, parity, smoking, education level, height, country of birth, and year of delivery), we cannot exclude the possibility of residual social confounding.
