A 19 yo primigravid woman at 31 weeeks gestation is admitted to the hospital because of intense uterine contractions every 1 to 2 minutes for 2 hours. The uterus is firm and tender. The fetal HR is 165/min. there is dark blood flowing from the vagina, and the cervix is completely effaced and 7 cm dilated; the vertex is at 0 station. Fundal height is 30 cm. What is the most likely cause of the findings?