The overall rate of OASIs amongst our study population was 4.24% which is similar to the approximate rate found in the full CSL cohort at 5.8% [1]. We discovered in this cohort, physicians were serving a slightly older, Whiter, and more affluent population than the midwives. Physician patients were also more likely to have medical co-morbidities documented and undergo labor induction and/or augmentation compared to certified nurse-midwife patients. Even when controlling for these population differences and known risk factors for OASIs, we found OASIs at the time of vaginal delivery was more likely with an obstetrician physician as the delivering provider when compared to certified nurse-midwives.
This is one of the first studies to examine the delivering provider as a risk factor for OASIs in the U.S. Only a few studies have examined delivery provider as a risk factor for OASIs which were based in England and Ireland, respectively. One prospective, observational study in compared rates of OASIs in a hospital obstetric unit to those in several freestanding midwifery-led units [10]. Women who delivered in the midwifery-led units more frequently had intact perineum at the time of vaginal delivery. In comparison, a recent study in a university-affiliated district hospital found the risk of OASIs was twice as likely in the midwifery-led units compared to the hospital’s obstetrician-led units [9]. Not only do these data conflict, but likely are not generalizable to a diverse patient population and an American treatment paradigm.
We suspect there are a few differences between physicians and midwives that have led to the differences in OASIs. First, a mentioned above, the patient populations, at least in this cohort, are different. Physician patients in our study had more pre-existing and antenatal medical comorbidities, which may have led to more intrapartum complications or higher risk of maternal/fetal distress requiring an expedited delivery. There was a higher frequency of fetal distress and intrapartum magnesium use within the physician cohort comparatively, which may support this theory. Although, interestingly midwife patients had more meconium-stained fluid, which can traditionally be a sign of fetal distress.
Secondly, there are also major differences in practice style. Our findings also reaffirm the pattern of decreased use of labor induction and augmentation agents as well as delivery interventions amongst midwives when compared to physicians [13]. A study exploring the effect of midwives on perinatal outcomes in the U.S. demonstrated that midwife practice favors waiting for spontaneous onset of labor, hospital admission once active labor is reached, and more conservative with use of pharmacologic and/or surgical interventions during the progression of labor which is consistent with our findings [14]. Our analysis demonstrates that with the use of less intrapartum interventions, we see a lower frequency of OASIs specifically amongst the midwife patients. It is possible that our findings reflect and support limited labor or birth interventions, a core aspect of midwifery philosophy, as a tool to reduce OASIs.
A third major consideration for the decreased OASIs amongst midwife providers is that they may be underdiagnosed. A 2012 survey found only 34% of midwives reported they were confident in OASI assessment at the time of delivery and just 22% indicated they felt prepared to repair the laceration [15]. An additional survey conducted in 2020 revealed the overall accuracy of perineal laceration identification amongst the midwife respondents ranged from 49–99% and a high frequency of OASIs misidentification was accompanied by subsequent inappropriate use of the OASIs severity grading scale [16]. Although these surveys took place in the U.K. where the midwifery model differs from the U.S., there is a possibility that OASIs may be underrepresented amongst our midwife patient cohort.
A final consideration is the presence and participation of physician trainees (fellows, residents, and medical students) in patient care at the time of delivery. This is a delivery factor impacting the physician cohort in our study which we were unable to measure. As the institutions associated with this database are teaching hospitals, we must consider the possibility that by allowing physician trainees particularly those early in their careers to participate in vaginal deliveries this may increase the risk of more advanced obstetric lacerations. While there is not considerable research comparatively examining the role of physician trainees and the incidence of OASIs, a 2016 retrospective cohort study examined the role of midwife experience. Highly experienced midwives, those with greater than 10 years of experience, had the lowest incidence of OASIs when compared to midwives deemed moderately experienced or inexperienced, between 2 and 10years and less than 2 years of experience respectively [3].
A major strength of this study is our large, racially, and socioeconomically diverse patient population from multiple centers. This is more reflective of the general population of the U.S. than previous studies examining the frequency of OASIs amongst varying provider types. In addition, we identified a risk factor that has not been fully assessed for OASIs. It is common knowledge that operative deliveries and episiotomies increase the risk. However, our study findings ask an important question about the way physicians manage labor and how that increases the risk of OASIs.
A major limitation is the exclusion of data from CSL sites with an absence of certified nurse-midwife providers. Although there has been a steady increase in the rate of midwife attended hospital births in the United States since 1975 reaching a peak of 8.6% in 2016, a majority of sites included in the CSL did not include midwives as lead birth-attendants at the time of delivery [14, 17]. Secondly, absent or missing data from the database and inconsistent documentation of factors of interest such as length of induction and length of labor was an additional limitation which prevented full examination of these factor. In addition, in our analysis we were unable to determine if patients underwent intrapartum transfers from midwifery to physician care. This commonly occurs on Labor and Delivery due to evolving complications and may impact the risk of OASIs. Lastly, the only midwife deliveries considered in this database were hospital deliveries which may not be reflective of midwifery care in birth-centers and home births where the incidence of OASIs may differ. Although these limitations may affect the generalizability of our results, there is an association that needs to be further explored to prevent negative impact of OASIs on postpartum women.
We have identified physician providers as a potential modifiable risk factor for OASIs. While women may not be able to choose their provider, differences in risk of OASIs are likely not innate to physicians and thus can be modified. Ultimately, further prospective research is needed to confirm our findings particularly differences in OASIs amongst physician obstetricians and certified nurse-midwives in the U.S. It would also allow for more comprehensive examination of the role of labor induction and augmentation agents, presence of trainees, transfer of care from midwives to physicians, provider experience, as well as standardized OASIs assessment at the time of delivery. The growth of the field of midwifery in the U.S. is supported by published research promoting safe and effective care and is thought to be a possible solution to the reported national shortage of reproductive and obstetric providers [18]. Our findings may support the idea that collaborative practice amongst physician obstetricians and certified nurse-midwives can improve maternal outcomes, specifically that of anal sphincter injury, at the time of vaginal delivery.