Underweight, defined as BMI ≤ 18.5 kg/m2, is found in 4.2% of pregnancies in Denmark. Pre-pregnancy underweight is more often seen in relation to psychiatric disorders, e.g. anorexia nervosa, and diet restrictions and associates with adverse pregnancy outcomes, such as antepartum haemorrhage requiring blood transfusion, preterm birth, small for gestational age infants, and impaired ability to breastfeed. In this review, we recommend identification of possible underlying medical or psychiatric disorders, focus on sufficient gestational weight gain, and relevant vitamin and mineral substitution.