The after coming head of the baby in a breech delivery can be arrested by all of the following except:
Extended head
Hydrocephalus
Placenta pre
Incomplete dilatation of the cervix
If a whole baby can make its way from breech onwards till the head, then even if there is a placenta pre, it will not obstruct the fetal head to come though since there was no obstruction to the whole fetus.
Labour - II