Obesity in Pregnancy
Tania Roman
Patrick S. Ramsey
Introduction
Pregnancy is a time of significant change in a woman’s life. The pregnant body undergoes a myriad of physiological changes not seen simultaneously in any other medical condition. As more women delay childbearing and new public health concerns gain strength, the care for the pregnant woman becomes more complex and challenging. One of those compelling public health issues is the rising obesity epidemic taking place around the world, specifically in developed countries. It is for this reason that physicians should be aware of the potential issues surrounding women with obesity in order to address them preconceptually and throughout the pregnancy and puerperium. This is a collaborative task requiring a multidisciplinary approach in the realm of obstetrics, public health, and general medicine.
Definition
Obesity in the United States has reached unprecedented rates affecting women of reproductive age and leading to significant maternal and perinatal complications. The Centers for Disease Control and Prevention defines adult obesity as a body mass index (BMI) ≥ 30 kg/m2, which is calculated as weight in kilograms divided by height in meters squared. It is further stratified by class: class I (BMI 30.0-34.9 kg/m2), class II (BMI 35.0-39.9 kg/m2), and class III (BMI ≥ 40 kg/m2).1 The caveat to this standard definition is that it does not account for the physiological changes in pregnancy such as increased blood volume and fetal weight accrued over a short period of time. Thus, classification of obesity in pregnancy is based on prepregnancy BMI.
Epidemiology
Over the past 3 decades, obesity rates in the United States have increased. Based on data collected over a 1-year period, from 2015 to 2016, more than one-third of adults (39.8%) were obese, and almost one-fifth of all youths (18.5% aged 5-18 years) were obese. A breakdown by gender shows that 41.1% of women aged 20 years and older and 36.5% of women aged between 20 and 39 years were obese (Figure 32.1).2 These staggering numbers do not seem to be on the decline, as these rates have been steadily increasing over the last 10 years.
The most recent data have also shown differences in obesity prevalence by race and ethnicity. Non-Hispanic black (54.8%) and Hispanic (50.6%) women have the highest rates of obesity. This was followed by non-Hispanic white (38.0%) and non-Hispanic Asian (14.8%) women. These differences reflect the disparities in maternal and neonatal health seen in the United States (Figure 32.2).2
Pathogenesis
The obesity epidemic affecting pregnant women has a vast array of clinical implications. Pregnant women with obesity have higher rates of severe maternal morbidity (SMM), defined as “unexpected outcomes of labor and delivery that result in significant short- or long-term consequences to a woman’s health.”3 This includes postpartum hemorrhage requiring transfusion; thromboembolic events; and cardiovascular, respiratory, and hematological complications. The excess adipose tissue serves as an active endocrine organ that activates metabolic and inflammatory pathways leading to many pathological processes.4,5 On a molecular level, extracellular vesicles in adipose tissue have been shown to play a role in glucose and lipid metabolism, leading to insulin resistance and metabolic syndrome.6 Given that the amount of excess weight is linearly associated to obstetrical and fetal complications further supports the causative role of obesity in the pathogenesis of these disease processes.7
 Figure 32.1 Age-adjusted prevalence of obesity among adults aged 20 years and older, by sex and race and Hispanic origin: United States, 2015-2016.1Significantly different from those aged 20 to 39 years. (Reprinted from Defining Adult Overweight and Obesity. CDC Division of Nutrition, Physical Activity, and Obesity, National Center for Chronic Disease Prevention and Health Promotion. Page last reviewed on April 11, 2017. Accessed April 4, 2019. https://www.cdc.gov/obesity/adult/defining.html) |
Clinical Presentation
Antepartum
Maternal Considerations
Recent studies have shown that prepregnancy weight (eg, maternal BMI at conception) is a stronger predictor of adverse maternal and infant outcomes than weight gain during pregnancy. A 2017 meta-analysis of 25 pooled cohort studies of 196,670 participants found that the odds ratio (OR) for any maternal or fetal adverse outcome per one standard deviation (1-SD) increase in maternal prepregnancy BMI was 1.28 (95% confidence interval [CI] 1.27-1.29) compared to 1.04 (95% CI 1.03-1.05)
per 1-SD increase in gestational weight gain (P < .001 for comparison).8 These findings emphasized the importance of preconceptual counseling to reinforce and educate women on weight loss prior to conception. However, many women may have limited access to health care or may only seek care during pregnancy or when seeking emergency services. For this reason, a majority of education and counseling regarding obesity and pregnancy will have to occur during prenatal care.
per 1-SD increase in gestational weight gain (P < .001 for comparison).8 These findings emphasized the importance of preconceptual counseling to reinforce and educate women on weight loss prior to conception. However, many women may have limited access to health care or may only seek care during pregnancy or when seeking emergency services. For this reason, a majority of education and counseling regarding obesity and pregnancy will have to occur during prenatal care.
 Figure 32.2 Age-adjusted prevalence of obesity among adults aged 20 years and older, by sex and race and Hispanic origin: United States, 2015 to 2016. 1Significantly different from non-Hispanic Asian persons. 2Significantly different from non-Hispanic white persons. 3Significantly different from Hispanic persons. 4Significantly different from women of same race and Hispanic origin. (Reprinted from Defining Adult Overweight and Obesity. CDC Division of Nutrition, Physical Activity, and Obesity, National Center for Chronic Disease Prevention and Health Promotion. Page last reviewed on April 11, 2017. Accessed April 4, 2019. https://www.cdc.gov/obesity/adult/defining.html) |
Given that earlier studies have shown an association with excess gestational weight gain and adverse maternal neonatal outcomes, the National Academy of Medicine (formerly the Institute of Medicine) provided guidelines regarding gestational weight gain (Table 32.1). Under these recommendations, women with obesity should gain in total 11 to 20 lbs during pregnancy. This is in sharp contrast with women in the normal range (BMI 18.5-24.9 kg/m2) who can gain 25 to 35 lbs during their pregnancy. Of note, the guidelines also addressed women with obesity who had multiple fetuses and recommended a weight gain of 25 to 42 lbs.9
Maternal Complications
Spontaneous Abortion
Obesity has been found to modestly increase the risk of early pregnancy loss. In a 2011 systematic review, including six retrospective studies and a total of 28,538 women (3800 obese [BMI ≥ 28 or 30 kg/m2], 3792 overweight [BMI 25-29 kg/m2], and 17,146 normal weight [BMI < 25 kg/m2]), the percentages of women with ≥1 miscarriage were 16.6% for those in the group with obesity, 11.8% for those in the group with overweight, and 10.7% for those in the normal weight group.10 The odds of having ≥1 miscarriage increased for women with obesity (OR 1.31, 95% CI 1.18-1.46) and women with overweight (OR 1.11, 95% CI 1.00-1.24) when compared with women with normal BMI. Furthermore, a 2018 systematic review and meta-analysis11 of recurrent pregnancy loss (RPL), defined as three or more miscarriages, found that women with obesity had a higher risk of RPL compared to women with overweight. The meta-analysis included two prospective cohort studies of 1742 women with a history of RPL. The results showed that obesity (BMI ≥ 30 kg/m2) was associated with RPL (OR 1.75, 95% CI 1.24-2.47, and P = .001), while overweight was not associated with RPL.
Table 32.1 National Academy of Medicine Weight Gain Recommendations for Pregnancy | ||||||||||||||||||||||||||||||||||||||||||||||
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Although aneuploidy is the main cause of first trimester pregnancy loss in the general population, women with obesity have been observed to have an excess loss of normal karyotype embryos.12 One possible explanation for this is a hostile uterine environment related to the inflammatory pathways triggered by excess adipose tissue.13 This is further supported by observations that patients with obesity who have polycystic ovary syndrome (PCOS) have a miscarriage rate of 20% to 40% higher than the baseline in the general obstetric population. Low-grade chronic inflammation commonly seen in patient with PCOS worsens during gestation and may contribute to the excess risk of miscarriage.14
Diabetes
Pregnancy is characterized by increased insulin resistance and decreased insulin sensitivity, both worsen with advanced gestation. These natural physiologic changes due greatly to placental
hormones are accentuated by obesity. Excess adipose tissue contains extracellular vesicles that have been shown to increase insulin resistance.4,6
hormones are accentuated by obesity. Excess adipose tissue contains extracellular vesicles that have been shown to increase insulin resistance.4,6
The prevalence of gestational diabetes mellitus (GDM) is significantly higher in women with obesity than in the general obstetrical population, and the risk increases with increasing maternal weight and BMI. In a systematic review of studies on prepregnancy BMI and risk of GDM, the prevalence of GDM increased by 0.92% for every 1 kg/m2 increase in BMI.15 Pregnant women with obesity are both more likely to have diabetes at conception or develop it during pregnancy. The American College of Obstetricians and Gynecologists recommends that women with obesity who have another risk factor (ie, physical inactivity, first-degree relative with diabetes, high-risk ethnicity, history of macrosomia, previous GDM, hypertension, dyslipidemia, PCOS, hemoglobin A1C ≥5.7%, or history of cardiovascular disease) be screened when initiating prenatal care for GDM with a 1-hour glucose challenge test and screened again at 24 to 28 weeks of gestation, even if the result is normal.16 See detailed discussion in Chapter 30.
Hypertensive Disorders
Maternal obesity has been consistently identified as an independent risk factor for gestational hypertension and preeclampsia. In a systematic review of 13 cohort studies comprising nearly 1.4 million women, the risk of preeclampsia doubled with each 5 to 7 kg/m2 increase in prepregnancy BMI.17 This relationship persisted in studies that excluded women with chronic hypertension, diabetes mellitus, or multiple gestations, and after adjustment for other confounders. Cohort studies of women who underwent bariatric surgery suggest that weight loss significantly reduces the occurrence of preeclampsia.18 Given this increased risk of gestational hypertension and preeclampsia, it is important to closely monitor vital signs during every clinic visit to detect any trend in increased blood pressures from baseline. If there is a high suspicion of developing hypertensive disorder, the patient should be counseled on the symptoms of preeclampsia. A home blood pressure kit should be considered for patients to measure their blood pressure at home or work.
Hepatic Conditions
Pregnant women with obesity are more likely to be affected by nonalcoholic fatty liver disease (NAFLD) when compared to their normal weight counterparts. Up to 80% of patients with NAFLD are obese, and the severity of the disease is directly correlated with increasing BMI.19 Most patients with NAFLD are asymptomatic and demonstrate no biochemical abnormalities. This condition is most commonly found incidentally when women have laboratory work for other causes (eg, nausea, vomiting, abdominal pain, preeclampsia panel). NAFLD is something to consider when there is an isolated mild elevation (less than two times the upper limit of normal) of liver enzymes with no other clinical symptoms or signs. Because some women may only seek routine health care during pregnancy, it is important to inform them of this finding so that they are aware of lifestyle modifications (eg, low-fat diet, exercise, weight loss) that can slow down the progression of NAFLD.
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