March 28, 2014 — Summary of "The Association Between Obstetrical Interventions and Late Preterm Birth," Bassil et al., American Journal of Obstetrics and Gynecology, Feb. 28. 2014.
Preterm birth is the No. 1 cause of infant morbidity and mortality, according to Kate Bassil of the Maternal-Infant Care Research Institute at Mount Sinai Hospital and colleagues. They noted that late preterm (LP) births -- those between 34 and 36 weeks of gestation -- account for the majority of preterm births and put infants at greater risk for adverse outcomes relative to term births.
"There is concern that obstetrical interventions may be driving the increase in LP birth in recent years and are responsible for a substantial proportion of LP births," the researchers wrote. They developed a study to assess associations between obstetrical interventions and LP births.
The researchers conducted a population-based cohort study of women who gave birth in hospitals in Ontario, Canada, from April 2005 through March 2012. They used an Internet-based surveillance system in the province to extract maternal, fetal and obstetrical data on women who gave birth from 34 weeks to 40 weeks and six days of pregnancy during the study period.
The researchers defined late preterm (LP) births as those occurring between 34 weeks and 36 weeks and six days of gestation, while term births were those occurring between 37 weeks and 40 weeks and six days.
The primary exposure variable was "any obstetrical intervention," including pre-labor cesarean sections or induced deliveries. The secondary exposure variables were pre-labor c-section and induced delivery, considered separately. The researchers defined pre-labor c-section as any c-section delivery performed before labor had been induced or started naturally. Induced delivery was defined as any medical or surgical intervention to initiate contractions before labor began on its own, including induced deliveries that ended in c-sections.
The researchers also assessed various factors known or thought to be associated with preterm birth, including maternal characteristics, such as age, socioeconomic status and smoking; maternal health problems, such as diabetes; and obstetrical complications, such as preeclampsia and breech positioning of the fetus.
To analyze the data, the researchers randomly assigned births into one of two roughly equally sized cohorts: a derivation cohort and a validation cohort, which "was used to test the associations identified through the derivation cohort." For both cohorts, they conducted a primary analysis that tested the association between obstetrical interventions and LP birth, adjusted for the factors associated with preterm birth. They then conducted a secondary analysis that repeated the first analysis but assessed pre-labor c-section and induced delivery separately.
After excluding births that were less than 500 grams and those with incomplete data capture, 917,013 live births were included in the study, including 49,157 that were LP. The researchers split the births into the derivation and validation cohorts, finding that in both groups, 38% of births involved "any obstetrical intervention," 21% were induced and 17% were pre-labor c-sections.
In the primary analysis, after adjusting for factors associated with preterm birth, the study found that "any obstetrical intervention" was negatively associated with LP birth relative to term birth in the derivation cohort. In the secondary analysis, which assessed pre-labor c-section and induction separately, both interventions also were negatively associated with LP birth relative to term birth.
The study also found that risk factors known or thought to be linked with preterm birth were independently associated with LP birth, including "several potentially modifiable risk factors," such as smoking during pregnancy, previous c-section, and material and social deprivation.
Discussion and Conclusions
"[I]nduction and pre-labour cesarean section were associated with a lower likelihood of LP birth relative to term birth for pregnancies with similar maternal and fetal risk," the researchers wrote, noting that "this trend persisted through each LP gestational week up to 37 weeks."
"There has been concern that the escalation in LP birth over the past 20 years may be related to obstetrical interventions and iatrogenic preterm birth," the researchers noted, adding that a recent multicountry analysis found that c-sections and induction of labor "together accounted for approximately 20% of the change in LP birth between 1989 and 2004."
They continued, "However, maternal and fetal health problems are also increasing and expedited delivery through obstetrical intervention in the setting of maternal or fetal compromise is generally accepted practice to avoid potentially disastrous maternal or neonatal outcomes."
The findings in the study suggest "that obstetrical care providers may be preferentially avoiding interventions to bring about LP birth in the setting of equivalent maternal and obstetrical risk" and "that increased awareness among obstetrical care providers about the harms of unnecessary LP birth may be a contributing factor," according to the researchers. They speculated that elective c-sections and inductions, "which obstetrical care providers may be more liberal with in the term than in the late period," might also be a factor.
The researchers also noted that "potentially modifiable factors" that "were independently associated with LP birth" -- such as smoking during pregnancy, unnecessary c-sections, and "material and social deprivation" -- could "be useful targets for interventions to reduce LP birth."