Hi, the nidagen contains the hormone progesterone and progesterone is the chief hormone secreted by corpus luteum and causes induction of secretory activity in endometrium of oestrogen primed uterus. It causes glandular development of breast, increases the basal body temperature, and inhibits uterine contractions. Adverse Effects:- Gastrointestinal disturbances, changes in appetite or weight, fluid retention, oedema, acne chloasma (melasma), allergic skin rashes, urticaria, mental depression, breast changes including discomfort or occasionally gynaecomastia, hirsutism, fatigue, drowsiness, fever, headache, premenstrual syndrome-like symptoms, and altered menstrual cycles or irregular menstrual bleeding. Anaphylaxis or anaphylactoid reactions may occur rarely. Nidagen should not be given to patients with undiagnosed vaginal bleeding, nor to those with a history or current high risk of arterial disease, in hepatic impairment, hypersensitivity to the active substances or to any of the excipients. In your case it is likely that it has been prescribed to maintain the pregnancy.