Tissues of the birth canal and the area around it will stretch during the baby's passage, and sometimes they can tear. These tears can occur not only during a normal vaginal delivery, but also with the use of forceps or a vacuum.
Often, tears are not serious, but if they are deep in the vagina or rectum they will need to be closed with stitches that will later dissolve on their own. You may need to take stool softeners and pain medications after delivery if this happens.
In rare cases, extremely serious tears can cause muscle control problems and sexual dysfunction later on. You might want to discuss the possibilities and prevention methods with your doctor before labor.