Background: According to the last reviews Progesterone (P) and (17P) are able to reduce preterm delivery (PTD), either as prophylactic administration in the presence of previous PTD or as a treatment of the actual pregnancy, becoming at risk because of cervical shortening/preterm labour. At present is difficult to distinguish the clinical effects of P from the one of 17P as well as it is impossible to choice among the diverse doses and formulations utilized in the RCTs published so far, as well as in those under recruitment.