When the lowermost poion of the presenting fetal pa is at the level of the spines, fetal station designated is

ACOG adopted the classification of the station that divides the pelvis above and below the spines into fifths. Each fifths represents 1cm above or below the spines. Thus as the presenting fetal pa descends from the inlet toward the ischial spines, the designation is -5, -4, -3, -2, -1, and then 0. Below the spines, as the presenting fetal pa descends, it passes +1, +2, +3, +4, and +5 stations to delivery. Station +5 corresponds to the fetal head being visible at the introitus.(Ref: William's Obstetrics; 25th edition)