Complex sentence: Because there is virtually no resistance of the pelvis to the size of a newborn, only the soft parts resist the birth--and with an almost instant vaginal birth, there can be tears of the perineal soft tissues.
Split into simpler sentences: Because there is virtually no resistance of the pelvis to the size of a newborn, only the soft parts resist the birth. With such an extremely rapid vaginal birth, there can be tears of the perineal soft tissues.