Short cervix

I went in for an ultrasound and ended up in triage.I was told my cervix was short at 1.96.I'm hoping somebody can shed some light on this.I'm 31 weeks ,I was 30 weeks when the doc told me.But she wasn't very explanatory.my kid is only 2lbs 8ozs kinda small do u think.They percribed me progesterone 200 mg but what if I forget/miss doses will it effect my baby