First of all did the ultrasound which you had was vagina or abdominal. Makes lot of difference. I suggest repeat ultrasound but now vagina probe. As such 2.5 cms is ok although slightly less. Ultrasound person should have given report whether is there funneling or in the sense the inside mouth of uterus is opening? You will need steroid injection to make baby's lungs mature and should take vagina swab to check if there is infection.