Aw mama, I get your worry kasi first time mo talaga! Honestly, placental grading varies so much from person to person, a few of us mums I know had similar situations and everything turned out fine, pero definitely worth bringing this up sa OB mo sa next check-up kasi sya lang makakasabi for sure based sa ultrasound mo at overall pregnancy. Try not to stress too much in the meantime; kung walang other concerns ang doc, Grade 0 pa rin at 31 weeks ay pwedeng normal pa for your baby.