Perinatal Advantages and Disadvantages of Being Underweight before Pregnancy: A Population-Based Study

To evaluate the advantages and disadvantages of being underweight before pregnancy.

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).

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.

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.