Maternal BMI, smoking status and infant birth weight: An analysis of an Australian Cohort

Maternal nutritional status and body composition have important implications for foetal growth and development. Women who smoke while pregnant are at increased risk of a range of adverse outcomes including miscarriage and premature labour. The aim of this study was to investigate the dual impact of smoking status and BMI on birthing outcomes. This was a retrospective cohort design using data from the Birthing Outcome System at an Australian hospital. A total of 14 487 births were included. Within this cohort 715 (4.9%), 7268 (50.2%), 3658 (25.3%), 1558 (10.8%), 711 (4.9%) and 576 (3.9%) were underweight, of normal BMI, overweight, obese class I, II or III respectively. In bivariate adjusted models, and regardless of BMI, women that smoked during pregnancy were at an increased risk of delivering low birth weight (LBW) infants. Women who had a BMI of ≥40 kg/m2 and smoked were 4.5 times more likely to deliver small for gestational age (SGA) babies compared to non-smoking women within the same BMI category (AOR 4.508 95% CI: 2.068–9.828). This increased risk was not observed in women that ceased smoking before or during pregnancy. This study suggests that the effect of smoking during pregnancy on LBW occurs in women of all BMIs. However, delivering an SGA infant is markedly increased among the morbidly obese. Health professionals need to encourage women not to start smoking and support cessation during pregnancy, especially for women with a higher BMI.