A primigravida at the term of 20 weeks complains of pain in her lower abdomen, smearing blood-streaked discharge from the genital tracts. Uterine tone is increased, fetus is mobile. On vaginal examination: the uterus is enlarged according to the term, uterine cervix is shortened to 0,5 cm, external cervical orifice is open by 2 cm. What is the most likely diagnosis?