The term "anterior placenta" refers to the location of the placenta within your uterus
. Sometimes, though, the egg implants on the opposite side of the uterus, closest to your abdomen. When the placenta develops, it grows on the front (or anterior) side of your uterus, with the baby behind it.
Close to os means it is close to the cervical opening.
Lying close to" is good. As your womb grows bigger, the placenta stays in the same place but the top bit grows and the bottom bit doesn't grow much, so the placenta ends up a lot further away from the os.
Usually towards the end of the pregnancy
it will move higher. all this means is that she may have to have a c section delivery because her placenta may and up blocking the exit. It may also mean an early delivery.