Excessive weight gain in pregnancy has been associated with multiple adverse perinatal outcomes, including macrosomia and increased rates of cesarean delivery.1–5 It is well established that macrosomia is a risk factor for cesarean birth.6–11 However, some investigators have suggested that excessive gestational weight gain is an independent risk factor for cesarean and that the association between excessive weight gain and cesarean is not entirely explained by higher birth weight.12,13 Our primary objective in this study was to determine to what degree the relationship between weight gain and cesarean birth is explained by macrosomia. Our hypothesis was that although infant birth weight would comprise part of the causal mechanism for the association between increased maternal weight gain and cesarean delivery, that even when controlling for birth weight, maternal weight gain would still be associated with increased risk for cesarean delivery. As our methods for predicting and diagnosing macrosomia improve, it is useful to know whether excessive weight gain, independent of birth weight, predicts cesarean birth. If a patient gains excessive weight and yet does not have a macrosomic fetus, is her risk of cesarean still increased?
Gestational weight gain is typically measured by subtracting prepregnancy weight from final pregnancy weight or weight at delivery. Because women carrying a macrosomic fetus will have a higher maternal weight due in part to the excessive weight of that fetus, some investigators have suggested that we should subtract the weight of the products of conception from total maternal weight gain.14 In this study, we examine the relationship between maternal weight gain, birth weight, and cesarean birth using both a traditional definition of weight gain and a second measure that subtracts infant weight and placental weight from total maternal weight gain.
Although gestational and pregestational diabetes are well-recognized causes of macrosomia, the vast majority of macrosomic infants are born to nondiabetic mothers.15 Because the relationships between weight gain, macrosomia, and cesarean birth are likely to be different in diabetic women compared with nondiabetics and because most macrosomic fetuses and cesarean births occur in nondiabetics, we focused on women without diabetes for these analyses.
It is also crucial to consider maternal prepregnancy body mass index (BMI) in these analyses, because prepregnancy BMI has been shown to be an important modifier of the relationship between gestational weight gain and a variety of perinatal outcomes.16–23 We sought to examine the relationship between gestational weight gain, macrosomia, and cesarean birth specifically in term (and postterm), singleton, nondiabetic pregnancies, controlling for, as well as stratifying by, prepregnancy BMI.
MATERIALS AND METHODS
We designed a retrospective cohort study of singleton, term (37 completed weeks or greater), nulliparous women delivering at the University of California, San Francisco Medical Center. Exclusion criteria were as follows: multiple gestation, preterm birth (less than 37 completed weeks), birth weight less than 2,500 g, multiparity, noncephalic presentation, gestational or pregestational diabetes, placenta previa, active herpes at delivery, abdominal cerclage, and fetal anomaly requiring cesarean delivery. Subjects were also excluded if data on maternal prepregnancy BMI or gestational weight gain were missing. The University of California, San Francisco Medical Center is a teaching hospital with a broad ethnic and socioeconomic patient mix. The labor and delivery unit is staffed with obstetric residents who are supervised by University of California, San Francisco obstetric faculty. The faculty attend every delivery and make decisions regarding obstetric management including need for cesarean birth. This study received institutional review board approval from the Committee on Human Research at the University of California, San Francisco.
Gestational weight gain was categorized according to the Institute of Medicine's (IOM) recommended range for that woman's prepregnancy BMI (Table 1). 24 The weight gain variate was categorical and had 3 values: weight gain above, below, or within IOM recommendations. The IOM guidelines do not set an upper limit for weight gain for obese women (BMI more than 29). For the purposes of our weight gain variate, we used the IOM's upper limit for weight gain for the “high” BMI category (more than 25 lb) for obese women as well. A second weight gain measure, absolute weight gain, was created. Absolute weight gain was calculated by subtracting infant birth weight and placental weight from total maternal weight gain. A categorical variate was created for absolute weight gain by subtracting an ideal birth weight, 3,500 g, and an ideal placental weight, 700 g, from the IOM weight gain recommendations. Macrosomia was defined 2 ways: either as birth weight 4,000 g or more or as birth weight 4,500 g or more (results are reported for both definitions).
Results were analyzed with Stata 7 (Stata Corporation, College Station, TX). Statistical methods included the χ2 test to compare proportions and multivariate logistic regression. In the logistic regression model we controlled for the following variables: maternal race or ethnicity, maternal age, maternal prepregnancy BMI, gestational age at delivery, infant sex, maternal cigarette use (cigarettes smoked per day), and date of delivery. In regression models using total maternal weight gain (as opposed to absolute weight gain) we also controlled for infant birth weight.
There were 9,788 patients fulfilling our inclusion criteria. Twelve percent of otherwise eligible subjects were excluded because of missing weight gain data. The cesarean rate for subjects excluded for missing weight gain data was identical to that of subjects with weight gain data (P = .98, χ2 test). The rate of birth weight 4,000 g or greater in the cohort was 11%; 2% of infants weighed at least 4,500 g. The cesarean rate in this cohort of nulliparous women was 17%. Descriptive statistics for this cohort are listed in Table 2.
The IOM issued its weight gain guidelines in 1990, and our cohort includes data on patients between 1976 and 2001. Before 1990, 47% of women in the cohort gained above the IOM guidelines, 36% of women gained within the guidelines, and 17% gained below the guidelines. After and including 1990, 49% of women gained above, 34% gained within, and 17% gained below the IOM guidelines (P 0.16, χ2 test comparing before 1990 to after 1990).
Women gaining above IOM guidelines had higher rates of cesarean and macrosomia than women gaining within the guidelines (Table 3, P < .001). Even among women delivering infants weighing less than 4,000 g, excessive weight gain was associated with a statistically significant increased risk of cesarean birth compared with women gaining within the IOM guidelines.
When we examined weight gain stratifying by prepregnancy BMI, we found that for women with low or normal prepregnancy BMI, the cesarean rates were not statistically different between those gaining below and those gaining within IOM guidelines (χ2 test, P = .5 and P = .8, respectively). However, in women gaining above the IOM guidelines, the cesarean rates are greater than those gaining at or less than IOM guidelines (P < .001). For women with a BMI above 26, cesarean rates are similar between those gaining within and above the guidelines (25% and 29%, P = .3), but rates decrease to 16% (P = .02) for those gaining below the guidelines. These patterns held even when we excluded women delivering infants weighing 4,000 g or more.
Among subjects undergoing cesarean birth, the most common indication recorded was “failure to progress” (50%). Before 1990, 44% of cesareans in the cohort were performed for “failure to progress,” and after and including 1990, this number rose to 62% (P < .001, χ2 test). Before 1990, 19% of cesareans were performed for “cephalopelvic disproportion, not in labor,” and this indication dropped to 1% after and including 1990 (P < .001, χ2 test).
In the multivariate analysis (Table 4) we controlled for potential confounders, including maternal age, maternal race or ethnicity, maternal prepregnancy BMI, infant sex, birth weight, date of delivery, gestational age, and smoking. Women gaining excessive weight had an adjusted odds ratio (AOR) for cesarean birth of 1.40 (95% confidence interval [CI] 1.22–1.59) when the standard weight gain measure was used and birth weight was included in the model. When deliveries of infants weighing 4,000 g or more were excluded from the model, the AOR for cesarean associated with excessive gain was 1.36 (95% CI 1.18–1.57). When birth weight was removed from the model, the AOR for cesarean associated with excessive weight gain went up to 1.60 (95% CI 1.40–1.82). The AOR for cesarean associated with excessive absolute weight gain was 1.17 (95% CI 1.01–1.35). When women delivering infants weighing 4,000 g or more were excluded from the analysis, the AOR for cesarean related to excessive absolute weight gain was 1.19 (95% CI 1.01–1.40). Women gaining excessive weight had an AOR of 2.26 (95% CI 1.91–2.68) for birth weight of 4,000 g or more and an AOR of 2.28 (95% CI 1.46–3.55) for birth weight of 4,500 g or more; however, when absolute weight gain was used, the AORs dropped to 1.23 (95% CI 1.04–1.45) and 1.70 (95% CI 1.14–2.53). When we stratified by prepregnancy BMI, the results mirrored those of the bivariate analyses: the AOR for cesarean birth among women of low or normal BMI who gained excessive weight was elevated, 1.93 (95% CI 1.45–2.53) and 1.26 (1.06–1.50), controlling for birth weight in the model. However, among women with BMI more than 26, there was no elevated risk of cesarean associated with gain above the guidelines compared with women gaining within the guidelines. For women with BMI more than 26 who gained below the guidelines, there was a trend toward lower risk of cesarean birth, but the AOR was only statistically significant when absolute weight gain was used.
In this population of nulliparous, nondiabetic women, both macrosomia and excessive gestational weight gain were associated with increased risk for cesarean birth. Furthermore, excessive weight gain itself was associated with increased rates of macrosomia. Although macrosomia is a stronger predictor of cesarean than is excessive weight gain, excessive weight gain alone is an independent predictor of cesarean delivery. Of note, in this cohort excessive weight gain was much more common than macrosomia. The percentage of women in our cohort gaining excessive weight (according to IOM guidelines) actually increased after the IOM guidelines were issued in 1990 (although the trend did not reach statistical significance). This is consistent with national data that also show an increase in the proportion of women gaining excessive weight.25
The Healthy People 2010 objectives issued by the Department of Health and Human Services advocate a reduction in the rate of cesarean delivery.26 As compared with many other risk factors for cesarean birth, excessive weight gain is potentially modifiable. Based on our findings, if the percentage of nulliparous women gaining excessive weight were to drop from 48% to 0%, based on the odds ratio of 1.6 for cesarean birth, of the approximately 288,000 primary cesareans performed in nulliparas in 2002 in the United States,25 we estimate that 64,000 (or 22%) would be prevented. Of those 64,000 primary cesareans prevented, 52,000 would have been performed in women with infants weighing under 4,000 g.
From a methodologic standpoint, we determined that findings vary depending on whether one uses absolute weight gain or total weight gain as the measure for gestational weight gain. Using absolute weight gain lessens the association between weight gain and cesarean birth as well as weight gain and macrosomia. However, these associations are still statistically significant even using absolute gain, suggesting that excessive pregnancy gain is a risk factor for cesarean birth, independent of birth weight. Why would weight gain alone predispose a woman to cesarean birth, with a normal weight infant? Potential mechanisms include hormonal (estrogen produced by adipose tissue), mechanical (excess maternal soft tissue), and iatrogenic (clinician concern) factors. Future research should examine how these and other possible causes may affect rates of cesarean delivery.
Limitations of our study include the use of secondary data, which may be more susceptible to inaccuracies and miscoding. However, the standard of a randomized clinical trial would not be feasible to address the question of weight gain in pregnancy. Rates of various complications in our cohort were consistent with other reports in the literature.19,27 Another limitation arises from the fact that this cohort is drawn from 1 institution, and therefore may not be generalizable to the entire U.S. population. However, the maternity patients at University of California, San Francisco are racially, ethnically, and socioeconomically diverse. Nearly 50% of nulliparous patients at University of California, San Francisco gained above IOM recommendations for gestational weight gain. This rate is consistent with other studies in the literature,28,29 which generally indicate a higher rate of excessive weight gain among nulliparous women compared with those who are multiparous. Although BMI-specific gestational weight gain data are not available for the entire U.S. population, in 2002 19.2% of U.S. parturients (nulliparous and multiparous combined) gained 40 pounds or more during pregnancy.25
Our study was also limited to nulliparous patients, because the risk of primary cesarean is higher among nulliparous women. However, to assess the effect of excessive weight gain in multiparous women, we also ran our multivariate model with the subcohort of multiparous women who met our other inclusion criteria. Among these women, the adjusted odds ratios for cesarean birth associated with excessive weight gain were substantially lower than those for nulliparous women. However, excessive weight gain did increase the risk of cesarean birth among multiparous women, even among those with infants weighing less than 4,000 g (adjusted odds ratio 1.25, 95% CI 1.01–1.54).
Our findings may be useful to clinicians providing prenatal care. Women who gain excessive weight during pregnancy are at increased risk of cesarean birth, even in the absence of macrosomia. Clinicians should advise women of this risk before pregnancy and at the first prenatal visit, and should monitor and discuss weight gain throughout gestation. Excessive weight gain during pregnancy has been associated with other adverse outcomes, including future obesity,30 and these findings may provide further motivation for women to aim for weight gain within IOM guidelines. Pregnancy is an opportunity for clinicians to teach patients about a healthful, balanced diet that can continue for a lifetime.
Our findings also call into question whether the guidelines established by the IOM are too high for women who are overweight or obese. Among women with a prepregnancy BMI more than 26, those gaining below the IOM guidelines had a 35% decrease in their risk of cesarean birth as compared with women gaining within the IOM guidelines. Prospective trials are needed to determine whether the potential risks of lower weight gain (such as low birth weight) in obese patients overcome the benefit. Cesarean birth is more prevalent27 as well as more risky31,32 in obese women, so it is all the more important to identify modifiable risk factors for cesarean birth in this group.
Further research is necessary to determine the relationship between maternal behaviors (nutrition, exercise), gestational weight gain, and pregnancy outcome. The risks associated with inadequate weight gain must be balanced against those from excessive gain. Given the current obesity epidemic in the United States and other industrialized nations, women in these countries may be better served by a public health strategy designed to prevent excessive weight gain compared with a strategy focused on the prevention of inadequate gain.
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