Hydrogest is a synthetic progestin, specifically an ester derivative of 17α-hydroxyprogesterone formed from caproic acid (hexanoic acid). It is formulated as a sterile, long-acting preparation in an oil solution for intramuscular use. The injection is viscous and oily, necessitating slow administration over one minute or longer. This medication is primarily used to reduce the risk of preterm birth in women with a singleton pregnancy who have a history of spontaneous preterm delivery.