Vitamin D deficiency affects more than half of the general population. During pregnancy vitamin D insufficiency is associated with a higher risk of complications such as an increased incidence of miscarriages. Preterm delivery, preeclampsia, gestational diabetes, growth retardation and low birth weight as well as postpartum hemorrhage are all pathologies whose incidence seems to be increased with hypovitaminosis D. This could be linked to the pregnancy changes of the immune system, on which vitamin D plays a well-known modulating role. Substitution, even if its benefit is not clearly established, should be considered as there are no side effects. Although lack of evidence regarding the timing of the introduction of treatment, substitution may be proposed before conception.