Posterior placenta just means that your placentais behind the baby. That's in fact a good position for the placenta. And yours is grade 3 whichmeans it's mature :-) your worry about posteriorbaby, in your results it'll be mentioned as "baby in cephalic posterior position -Google
Malapit ka na manganak mommy. Meaning hinog na placenta mo. Goodluck.
Thank you momsh! ☺
Arlyn Jane Saguid Deguzman