Anal Sphincter Tear Risk Factors in Birth
A large study identifies key risk factors for severe perineal tears during vaginal birth in first-time mothers, including operative delivery and Asian
A major study of over half a million first-time mothers in California has pinpointed specific factors that increase the risk of a severe perineal tear during vaginal birth. Operative delivery was the strongest risk factor, with Asian women also showing nearly double the risk compared to non-Hispanic white women.
The research, published in JAMA Network Open, analyzed data from 577,510 nulliparous women with full-term pregnancies who delivered in California between 2016 and 2019. Obstetric anal sphincter injury (OASI), which involves severe tears to the anal sphincter, occurred in 4.4% of these vaginal births. These injuries are a leading cause of long-term pelvic floor disorders.
Key Risk Factors Identified
The study confirmed that the method of delivery dramatically alters risk. Compared to nonoperative vaginal births, the use of instruments significantly increased the likelihood of injury.
| Delivery Method | OASI Occurrence Rate | Adjusted Relative Risk (95% CI) |
|---|---|---|
| Forceps-assisted | 20% | 4.38 (4.11-4.68) |
| Vacuum-assisted | 12.3% | 2.95 (2.86-3.04) |
| Nonoperative vaginal | 3.5% | Reference group |
Demographic factors also played a clear role. Dr. Christina Chambers of UC San Diego and her colleagues reported that Asian women had a 92% higher risk of OASI than non-Hispanic white women. Black and Hispanic women both had a lower risk. Certain maternal health conditions, like diabetes and preeclampsia, as well as delivering a large-for-gestational-age infant, were linked to higher risk. Interestingly, women with obesity prior to pregnancy had a lower risk.
The Impact and Patient Questions
As a urogynecologist, co-author Dr. Lindsey Burnett frequently sees the lasting consequences of these tears. She told MedPage Today that patients often ask why they weren't warned. "The number one question that I get from patients is, 'Why did no one tell me this could happen to me, and what was my risk of getting it?'" Burnett said.
These injuries are not rare. Approximately 216,000 U.S. women experience OASI each year. The sequelae often include pelvic organ prolapse, stress urinary incontinence, and anal incontinence, with about 30% of affected women reporting a reduced quality of life. Cesarean delivery on maternal request remains the only known intervention that completely eliminates the risk of OASI.
Toward Personalized Risk Counseling
The study's goal was to move beyond general warnings. "The inspiration for this research approach was to think about what we can do to better guess who would benefit from preventive intervention," Burnett explained. She emphasized the need for tailored discussions based on a patient's specific risk profile rather than overall population risk.
The research team conducted a population-based analysis by linking California birth certificates to hospital discharge records. They identified OASI cases using diagnostic codes for third- and fourth-degree lacerations. The cohort was diverse: 42% Hispanic, 30% non-Hispanic white, 18% Asian, and 4% Black.
The authors acknowledged limitations, including the retrospective design and probable underreporting of OASI in administrative data. The data also lacked details on episiotomy use, labor duration, and specific protective maneuvers used during delivery, which could influence risk. The study confirms established risks and highlights nonmodifiable factors to better inform patient care.
