"spoljašnje ušće" - refers to cervix, and the term " ulozivo" - refers to the openness of the birth canal during the labour, which some OBs measure in cm, and some in the number of fingers that can be inserted into vagina during the regular, multiple/hourly(if not even frequent) check-ups during the different stages of labour.
Usually, it is said that the "women is ready to start pushing when she is FULLY DILATED,meaning - the cervix is fully opened(for the lack of better term). It means "opened for 10cm", enough for the baby's head to pass through, "the baby is crowning" (in the ref.below, v.intr./noun in Obstetrics)
I don't know the context, nor can I guess the preposition following the source phrase "...ulozivo za...cm/prsta", so I wrote all that I know on this subject.
I sincerely hope this was helpful - most of which I learned through my own experience of the same kind :)