Outcome measures and definitions
Our primary outcome measure was a composite adverse maternal outcome that included the occurrence of one or more of the following: shoulder dystocia, obstetrical anal sphincter injury (OASIS), retained placenta, post-partum hemorrhage (PPH), blood products transfusion, prolonged duration of hospitalization and puerperal fever.
Even though, the association between lower obstetrician annual volume and the increased risk for CD has been shown7, we decided not to include CD or VAVD as an outcome measure as the decision to perform CD or VAVD in our department is determined by the attending obstetrician.
CNM practice duration was defined by the time interval between her first attended delivery and index delivery. Shoulder dystocia was defined as a protraction of head-to-body delivery time of >60 seconds, and/or the use obstetrical maneuvers to facilitate safe and rapid delivery8. OASIS was classified as third-degree lacerations which involve external ±internal sphincter and fourth-degree rupture which involves the rectal mucosa. OASIS is diagnosed by an obstetrician and repaired in the operating room 9. “Retained placenta” was defined as any third stage of delivery that included a diagnosis of complete or partial (fragments) remnants of the placenta10. PPH was defined by one of two means: a subjective assessment of blood loss (over 500 ml) or hemoglobin drop >3gr/dL11. Prolonged hospitalization – length of postpartum hospital stays > 5 days (routine hospitalization length is 2-3 days). Puerperal febrile morbidity - temperature of ≥100.4°F [≥38°C] from the second day after delivery to discharge12.
Secondary outcomes were various maternal and neonatal outcomes. The maternal outcomes assessed were those that composed the composite adverse maternal outcome, as described above. The neonatal outcomes included the following composite adverse neonatal outcomes defined by at least one of the following: : rates of 1-min Apgar score<  7, 5-min Apgar score <  7, neonatal asphyxia, meconium aspiration, jaundice, transient tachypnea of the newborn (TTN), mechanical ventilation, convulsions, neonatal intensive care unit (NICU) admission, sepsis, hypoxic ischemic encephalopathy (HIE).