Among several possible confounders (including sociodemographic variables, number of previous uterine incisions, diabetes, hematocrit, neonatal gender, and birth weight), four—African American race, birth weight, parity, and smoking status—were significantly associated with both BMI and fetal acid-base parameters (umbilical arterial pH and base deficit). When analyzed continuously and adjusted for these four factors, umbilical arterial pH decreased (P<.001) and base deficit increased (P=.005) per unit increase in BMI. For every 10-unit increase in BMI, the pH decreased by 0.01 and the base deficit increased by 0.26 mmol/L. Umbilical arterial pH and base deficit results, analyzed by BMI category, are shown in Table 2. Of note, none of the neonates in this cohort were diagnosed with hypoxic–ischemic encephalopathy.
The type of abdominal incision was missing for 3,098 of the 5,742 women. Therefore, this variable was not included in the analyses reported. However, as a post hoc analysis, we repeated the logistic regression analyses for only those 2,644 women with known abdominal incision types. Ethnicity, birth weight, parity, and smoking status again were the confounders associated with both BMI and acid-base parameters (defined either by pH or base deficit). In the collective group, abdominal incision type was not a significant variable in these models (P=.795 for pH model; P=.295 for base deficit model). For women with a BMI of less than 35, Pfannenstiel incisions, compared with vertical incisions, were not associated with umbilical artery base deficit of 12 mmol/L or more (relative risk [RR] 0.64, 95% confidence interval [CI] 0.35–1.18) or pH less than 7.1 (RR 0.94, 95% CI 0.60–1.49). However, for women with a BMI of 35 or more, Pfannenstiel incisions, compared with vertical incisions, were associated with a higher proportion with umbilical artery base deficit of 12 mmol/L or more (RR 3.40, 95% CI 1.56–7.42). In the women with BMI of 35 or more, those with Pfannenstiel incisions, compared with vertical incisions, had a higher proportion with pH less than 7.1, but this difference did not quite reach statistical significance (RR 1.74, 95% CI 0.99–3.07) (Table 3).
The data from this study demonstrate that, for women undergoing nonemergent prelabor cesarean delivery under spinal anesthesia, obesity is an independent risk factor for lower fetal pH and higher fetal base deficit and that pH declines and base deficit rises as BMI increases. We chose to restrict our analysis to nonlaboring patients to avoid the difficulty in estimating the effect of labor of varying durations and intensity. Likewise, we excluded women who were delivered by cesarean as a result of a nonreassuring fetal heart rate tracing. Furthermore, we excluded women who had medical or obstetric complications that might be associated with uteroplacental insufficiency. Therefore, we demonstrated an association among obesity, spinal anesthesia, and fetal acid-base parameters in a selected population at low risk for acidemia; the effect could be more clinically relevant in higher risk groups. For the reasons described here, our enrollment criteria were quite restrictive. Consequently, most women in the database undergoing cesarean delivery were excluded from our analysis. Although excluding such a large proportion of the cohort could introduce unmeasured bias, this potential weakness was necessary to limit unmeasured bias introduced by labor, fetal heart rate tracing abnormalities, and medical or obstetric conditions that could be associated with uteroplacental insufficiency.
As with all such analyses, our study is limited by its retrospective nature and the fact that these data were collected with the purpose of conducting other analyses. Biases such as misclassification potentially affect our results. As an example, we cannot be certain that all cord gases classified as arterial are correctly classified. However, we think that it is very unlikely that such misclassification could have introduced a systematic bias related to BMI. The large proportion of women that were excluded from the analysis is a potential concern, but we anticipated that a large number of women would be excluded as a result of our restrictive enrollment criteria. The inability to evaluate type of abdominal incision as an exposure in the majority of women is a limitation. Likewise, cord gas data were not available for many women who otherwise would have met our enrollment criteria, because some of the centers in the network did not routinely assess cord gases. It would be anticipated that there would be center-related differences between those women who had cord gas data available for analysis and those for whom these data are missing (eg, demographic differences between centers that collect cord gases universally and those that do not). However, there is no variable in the data set to account for site of delivery, thus precluding assessment of that factor in the analysis. Ideally, time from skin incision to delivery of the neonate would be included as a separate variable. However, having already included the number of prior uterine incisions as a variable, time from skin incision to delivery of the neonate is in the causal pathway of obesity being associated with lower pH and higher base deficit in the umbilical artery. Therefore, it was not evaluated in the models. Finally, we do not have information related to the details of anesthesia management (eg, management of spinal anesthesia-related hypotension) at participating centers.
Among the strengths of our study, these data were carefully collected prospectively by trained research staff at centers experienced in clinical research. In addition, despite very strict enrollment criteria for the current analysis, we had a fairly robust sample size.
Others have reported on the association between obesity and fetal acid-base status. However, the findings have not been consistent. Galan and colleagues,19 in a retrospective cohort study, reported that women with class II or III obesity (BMI 35 or higher) were more likely to have an umbilical artery pH less than 7.10 than were normal-weight women (adjusted odds ratio 2.15, 95% CI 1.05–4.38). In contrast, Maisonneuve and colleagues,20 in a case–control study, reported that cases (umbilical artery pH less than 7.00) were more likely to be obese (BMI 30 or higher) than controls, but the difference did not remain statistically significant in multivariate analysis. Both of these studies included women from the general obstetric population, so factors such as labor varying fetal status might have had an effect on the results.
It is possible that the effect of BMI on fetal acid-base status is the result of impaired pulmonary function caused by obesity. However, we speculate that at least part of the cause of the association between obesity and fetal acid-base status that we found in our analysis is the weight of the abdominal wall causing impaired venous return and decreased placental perfusion. Furthermore, we postulate that retraction of the panniculus to create a Pfannenstiel abdominal incision in selected extremely obese pregnant women might exacerbate the effect. This idea seems to be supported by our finding that Pfannenstiel incisions were associated with lower pH and higher base deficit in the neonates of women with a BMI of 35 or higher but not those women with a BMI of less than 35.
Obesity is a topic that, for obstetrics, has increased greatly in relevance in the past few decades. In a study from our center over a 20-year period from 1980 to 1999, the obesity rate in the obstetric population (BMI 30 or higher before pregnancy) more than doubled from 16% to 36%.24 Since then, the obesity rate among reproductive-aged women in the United States has continued to increase. In 1999, the rate was 28%; it was 34% in 2008.3 The rate at our center currently exceeds 40%. Reversing this trend of an increasing prevalence of obesity would be ideal. Short of that goal, we should at least try to minimize the likelihood of complications in such patients. Whether the association between obesity and fetal acid-base status exists in other populations and whether obesity causes an increased propensity to spinal anesthesia-related hypotension should be evaluated. Likewise, because the rate of cesarean delivery has increased dramatically in recent years and because obesity is associated with an increased likelihood of cesarean delivery, the advantages and disadvantages of various approaches to cesarean delivery in obese women (eg, type of abdominal incision) should be investigated further. Such analyses should include fetal acid-base status among the outcomes evaluated.
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© 2013 by The American College of Obstetricians and Gynecologists.
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