Pregnancy in underweight women: implications, management and outcomes

Despite attention on obesity in pregnancy, within the UK and globally, many women enter pregnancy underweight. There remains a paucity of evidence‐based guidance on the optimal care of these women. Maternal underweight is associated with low birthweight and preterm birth, both spontaneous and iatrogenic, but appropriate gestational weight gain may mitigate low body mass index (BMI). Although being underweight may be protective against several antenatal and intrapartum complications, low BMI can be related to underlying organic disease and/or eating disorders, or there may be modifiable lifestyle factors that should be addressed in pregnancy. BMI at booking should assist as a screening tool. Body image, genetic, socioeconomic and cultural factors may affect BMI, making underweight women a heterogeneous group requiring individualised assessment.