I am 20weeks 5days pregnant; my ultrasound report states that placenta is in posterior wall, low lying, lower margin reaching almost upto the level of internal Os with Grade I maturity. Cervix measures 3.5cms and internal cervix is closed. Foetal Biometry *bpd-50mms; HC-185mms, ac-168mms fl-32mms..Firstly what does Os mean, is this risky? what are the precautions to be taken? Secondly,what are the chances of normal delivery?