PALM DESERT, CALIF. — There may be a threshold of morbid obesity associated with a sharply increased risk of nonelective cesarean delivery that is not shared by less obese women, according to results of a preliminary study presented at the annual meeting of the Society for Obstetric Anesthesiology and Perinatology.
The issue may have clinical implications for management of women at the lower ranges of morbid obesity who may wish to undergo labor without early and aggressive epidural management in anticipation of a probable cesarean section.
In their study, investigators at the University of Michigan, Ann Arbor, looked for a linear increase in cesarean deliveries as obesity increased, but instead found that nonelective cesarean deliveries did not significantly increase until body mass indexes rose above 46.
At the highest ranges of obesity, a very significant increase in nonelective C-sections was seen in the study of 226 parturients: 58% of those with a BMI of 47–88, compared with 39% for women with BMIs between 30 and 46.
Monica Riesner, M.D., of the department of anesthesiology at the University of Michigan, presented the findings on behalf of a colleague, Jill Mhyre, M.D., who could not attend the meeting.
Dr. Mhyre and associates studied the charts of obese parturients who delivered vaginally or by nonelective C-section at their institution between 1999 and 2002. Women undergoing elective C-sections were not included in the analysis.
Among the patients meeting study criteria, 62 had a BMI between 30 and 39.9 (defined as obese by the Institute of Medicine); 116 had a BMI between 40 and 49.9, and 48 had a BMI between 50 and 88. A BMI greater than 50 has been proposed by some authors to constitute a new category, the “super obese.”
The mean BMI in the cohort was 44.5. The mean age was 28 years.
Fourteen percent of the group had diabetes, 14% had preeclampsia, one-fifth had asthma, and a quarter smoked.
Slightly more than half of the women delivered vaginally.
The nonelective C-section rate was 42% in women with BMIs between 30 and 39, and 45.7% for those with BMIs between 40 and 88, a nonsignificant difference.
In fact, a statistically meaningful difference in C-section rates was not observed in women with BMIs lower than 46, although they were significant at every cut point of BMIs above that level.
The single-institution study was not sufficiently powered to determine an absolute threshold for increased cesarean risk, which investigators hypothesized “may be as high as 50 or even 55,” said Dr. Riesner.
Stepwise logistic regression analyses found that a BMI greater than 46 was independently associated with more than a twofold increase in the risk of C-section.
Parity appeared to be protective in less obese women, but not in those women with a BMI of 47 or higher.
A more comprehensive study is underway using a new electronic records system to capture more cases, with the aim of shedding more light on the findings of this preliminary study.
At this point, it appears to be reasonable to continue to encourage women with very high BMIs to allow early epidural analgesia, Dr. Riesner said.
If the findings are confirmed, less obese women may be safely managed without an epidural if they meet such criteria as a history of vaginal delivery at the same maternal weight; reassuring maternal airway findings upon examination; and good progression of labor and fetal status, she said.
An audience member praised the study and called for more research, since she recently learned that labor nurses at her institution were discouraging super-obese parturients from having epidural anesthesia during labor, since they are difficult to move following a motor block.