A 30 year old G6P3 woman in her second trimester is referred to your obstetrics practice because she has a history of mid-trimester losses as well as a history of short labors. She has three living children, and during those pregnancies, she has had successively earlier deliveries. On examination, you note cervical dilation to 4cm and 80% effacement even though she is only at 22 weeks. She has no painful contractions, vaginal bleeding, ruptured membranes, or signs of infection. There's been a slight change in the color of vaginal discharge from clear to light yellow which also seems thinner, but other than that, she's asymptomatic. Her other gynecologic history is only notable for an abnormal pap smear at age 25 for which she received LEEP.
Challenge: What is your most likely diagnosis at this point?