Perineal tear classification is a system used to quantify the severity of trauma to the perineum

Perineum

In human anatomy, the perineum is a region of the body including the perineal body and surrounding structures...

 during vaginal childbirth

Childbirth

Childbirth is the culmination of a human pregnancy or gestation period with the birth of one or more newborn infants from a woman's uterus...

. Delivery may lead to overstretching of the vagina

Vagina

The vagina is a fibromuscular tubular tract leading from the uterus to the exterior of the body in female placental mammals and marsupials, or to the cloaca in female birds, monotremes, and some reptiles. Female insects and other invertebrates also have a vagina, which is the terminal part of the...

, causing tears in the perineal tissue between the vagina and rectum. Lacerations are classified into four categories:

• First degree tear: laceration is limited to the fourchette
  Frenulum labiorum pudendi
  In female human anatomy, the frenulum labiorum pudendi is a frenulum where the labia minora meet posteriorly.-Pathology:...
  
   and superficial perineal skin or vaginal mucosa
• Second degree tear: laceration extends beyond fourchette, perineal skin and vaginal mucosa to perineal muscles and fascia, but not the anal sphincter
• Third degree tear: fourchette, perineal skin, vaginal mucosa, muscles, and anal sphincter
  Anus
  The anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest,...
  
  are torn
• Fourth degree tear: fourchette, perineal skin, vaginal mucosa, muscles, anal sphincter, and rectal mucosa are torn

Third degree tears may be further subdivided into three subcategories:

• 3a: partial tear of the external anal sphincter involving less than 50% thickness
• 3b: greater than 50% tear of the external anal sphincter
• 3c: internal sphincter is torn