Aims: To evaluate the advantages and disadvantages of being underweight before pregnancy. Methods: Cohort study of a large population-based dataset of singleton births was used to compare maternal and neonatal outcomes of pre-gravid underweight body mass index (BMI <18.5 kg/m2) women with pre-gravid normal weight controls (BMI 18.5-24.9 kg/m2). Results: A total of 10,995 pre-gravid underweight and 146,155 pre-gravid normal weight mothers were compared. The mean maternal age and gestational age were not different but lean mothers were significantly and more frequently primiparous, had a higher incidence of births at <36 and <32 weeks' gestation, and had a significantly higher incidence of low and very low birth weight infants. Lean mothers had a significantly lower incidence of birth weights >4,000 g, less cesarean births and a lower incidence of gestational diabetes and hypertensive disorders. Conclusions: A tradeoff exists between the advantages of being lean before pregnancy in terms of less maternal morbidity in return for gaining a more advanced gestational age and higher birth weight.

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