These Women Face Higher Risk for Anal Sphincter Tears During Birth

Certain clinical and demographic factors were associated with higher rates of obstetric anal sphincter injury (OASI) among first-time mothers during vaginal birth, a large cohort study found.

In a study of more than half a million previously nulliparous women giving birth in California, OASI occurred in 4.4% with operative delivery being the strongest risk factor, reported Christina Chambers, PhD, of the University of California San Diego, and colleagues.

OASI occurred in 20% of forceps-assisted deliveries (adjusted relative risk [aRR] 4.38, 95% CI 4.11-4.68) and 12.3% of vacuum-assisted deliveries (aRR 2.95, 95% CI 2.86-3.04), compared to just 3.5% of nonoperative vaginal deliveries, they wrote in JAMA Network Open.

Demographics also affected the OASI risk estimates. Compared to non-Hispanic white women, those of Asian ancestry showed nearly doubled risk (aRR 1.92, 95% CI 1.86-1.99) while Black and Hispanic women both had lower risk of OASI.

Also, women with maternal diabetes, preeclampsia, and delivery of a large-for-gestational-age infant were at heightened risk, while those with obesity prior to pregnancy had lower risk.

Co-author Lindsey Burnett, PhD, MD, also of the University of California San Diego, told MedPage Today that, as a urogynecologist, she sees a lot of women with OASI and its sequelae.

“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.

Approximately 216,000 U.S. women experience OASI annually. Severe perineal injuries like OASI often result in long-term pelvic floor disorders — such as pelvic organ prolapse, stress urinary incontinence, and overactive bladder — and about three in 10 women report impaired quality of life from anal incontinence afterwards, the paper noted. Cesarean delivery on maternal request is the only known intervention that completely eliminates OASI risk.

“The inspiration for this research approach was to think about what we can do to better guess who would benefit from preventive intervention, and to personalize the discussions about the risks of having these types of tears that put people at risk of having really severe pelvic floor disorders later,” Burnett explained.

“When we’re trying to design better ways to engage patients in personalized care, we really need to be tailoring counseling towards their specific risk rather than overall risk,” she continued.

It was already known that operative delivery was a risk factor for OASI, which makes sense considering forceps and vacuums can “come into the sphincter complex and really compromise the ability for it to function later,” Burnett said.

To better describe the risk factors associated with OASI, the research team conducted a population-based analysis. They looked at all nulliparous women with singleton, full-term pregnancies (37 to 44 weeks’ gestation) who delivered live-born infants in California between 2016 and 2021, linking birth certificates to hospital discharge records.

From the birth certificate data, authors collected maternal information, including age at delivery, self-reported race and ethnicity, education, expected payer for delivery, and type of birth attendant, as well as neonatal factors including gestational age at delivery and birth weight. Maternal diabetes and hypertensive disorders were determined from birth certificates and maternal discharge records.

A total of 577,510 nulliparous women met the eligibility criteria, of whom 85% were age 18-34. In this cohort, 42% of patients were Hispanic, 30% non-Hispanic white, 18% Asian, and 4% were Black, with the rest classified as “other” or unknown.

OASI was identified via ICD-10 codes for third- and fourth-degree perineal lacerations. The reference group were those with no or first-degree lacerations, who were compared against those with second-degree lacerations and those with OASI.

“This large population-based cohort study confirmed established risk factors for OASI and identified several nonmodifiable risk factors,” the investigators wrote.

Chambers and colleagues noted some limitations, including the retrospective study design, the probable underreporting of OASI in administrative data, and potential confounding by indication for birth attendant type. The available data also lacked information on episiotomy use, fetal head position, second-stage labor duration, clinician experience, or perineal protective maneuvers that could influence OASI risk.