Tvs measured cervical length 56mm@20wks to 38mm@27wks, ffn+, cramping/backache, spotting. normal shortening of cl? what's the risk of preterm labor?

Very real risk: The positive fetal fibronectin suggests that the risk is very real and treatment to prevent/stop preterm labor +/- steroids to hasten lung maturity should be considered based on your particular case. If one is available I would guess your OB wants to consult a maternal-fetal medicine specialist ( a doctor who specializes in high risk pregnancies). Best wishes to you and the baby. Hope this helps.