Birth outcome variables

Our primary outcomes were birth weight, LBW (<2500g) and very low birth weight (VLBW)(<1500g),²⁴ gestational age, preterm birth (gestational-age at birth of less than 37 weeks),²⁴ and SGA (birth weight at delivery below the 10th centile for gestation (by sex),²⁵ using centiles derived from all eligible births).

Length of gestation was determined as the completed weeks of gestation according to the World Health Organization definition, which specifies time from the first day of the last menstrual period. If date of the last menstrual period is not available/reliable, an estimate is provided in the HES maternity database. We defined trimester 1 as weeks 0-13 and trimester 3 from week 27 to birth, calculated by counting back from gestational age at birth.