Shortened Perineum
Definition and Clinical Terminology
A shortened perineum is clinically defined as a perineal body length of less than 3 cm, measured from the posterior fourchette to the center of the anal orifice. 1, 2
The perineal body represents the anatomical distance between the posterior vaginal fourchette and the anterior margin of the anus. 3 This measurement is typically obtained during labor or prenatal assessment and serves as an important predictor of obstetric outcomes.
Normal Reference Values
- Average perineal body length: 3.9-4.6 cm (±0.7-0.9 cm) 2, 4
- Short perineum threshold: ≤3.0 cm 1, 2
- Very short perineum: ≤2.5 cm (associated with highest risk) 2
Clinical Significance and Associated Risks
Obstetric Anal Sphincter Injuries (OASIS)
Women with perineal body length <3 cm face significantly elevated risks:
- 40% risk of ultrasound-diagnosed anal sphincter tear versus 11.1% in women with normal perineal length (p=0.038) 1
- 40% risk of third- or fourth-degree lacerations when perineal body ≤2.5 cm versus 5.6% in women with normal length (p=0.004) 2
This risk persists even after controlling for operative vaginal delivery and episiotomy use. 2
Operative Delivery Risk
Shortened perineum (≤3.5 cm) is associated with:
- 28.5% incidence of operative vaginal delivery versus 9.2% in women with normal perineal length (p=0.006) 2
Episiotomy Considerations
When episiotomy is performed in women with short perineum:
- The remaining uninjured perineal tissue is significantly shorter than in women with normal perineal length 5
- This increases risk of extension to the anal sphincter 5
- The distance between episiotomy and anus is significantly reduced (p=0.026 compared to normal perineum) 5
Clinical Assessment
Measurement should be performed during the first stage of labor or at 35-37 weeks gestation, measuring from the posterior fourchette to the center of the anal orifice using a sterile ruler or caliper. 1, 2, 4
The anal position index (distance from fourchette to anus divided by distance from fourchette to inferior coccyx margin) provides additional risk stratification, with values <0.42 indicating anterior anal displacement and increased delivery trauma risk. 4
Management Implications
For women identified with shortened perineum:
- Consider mediolateral episiotomy over midline episiotomy to minimize OASIS risk 6
- Heightened vigilance for anal sphincter injury during and after delivery 3, 6
- Mandatory digital rectal examination post-delivery, as up to 35% of sphincter injuries are missed without this examination 3, 7, 6
- Consider endoanal ultrasound at 6 weeks postpartum if any concern for sphincter compromise exists 7, 1