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Clinical Reviews


Obesity in pregnancy: Risks and interventions by gestational stage

Gestational diabetes, preeclampsia, prolonged hospitalization—these are just a few of the complications that may affect obese gravidas. Here, the authors present a rundown of what to look for when treating this unique population.

September 2003 · Vol. 15, No. 9

KEY POINTS

  • All obese patients have an increased risk of gestational diabetes and preeclampsia.
  • Deep venous thrombosis and its complications—which include maternal mortality—are seen more frequently in the obese patient.
  • Obesity is associated with an increased likelihood of induction of labor and cesarean delivery.
  • Obesity is a specific risk factor for several operative complications, including hemorrhage during surgery, postoperative wound infections, aspiration, and pulmonary embolism.

Specific interventions can help reduce the complications associated with obesity in pregnancy, provided physicians remain vigilant in applying the appropriate preventive measures.

Since one third of American women of childbearing age are overweight, obesity clearly has a major impact on the health of pregnant patients. And, as in the general population, the prevalence of this condition is escalating among gravidas. A 2001 study cited a 20% increase in mean maternal weights between 1980 and 1999.1

In the United States, the prevalence of obesity leaped from 12% to 17.9% between 1991 and 1998.2 Even more alarmingly, each year in this country, 280,000 adult deaths are attributable to obesity.

As health-care providers, it is imperative that we understand the impact this epidemic has on pregnancy and delivery so that we can work to minimize related complications.

What is obesity?

There is no single definition of obesity. In obstetric literature, it has been defined as a maternal weight of more than 90 kg (200 lb), more than 114 kg (250 lb), more than 135 kg (300 lb), and anywhere from 50% to 120% above ideal body weight.

In recent years, clinicians have usually determined obesity according to the body mass index (BMI), a simple mathematical formula (weight in kilograms divided by height in square meters) that correlates height and weight with body fat. This method offers several advantages over a basic weight measurement. For one, weight alone does not correlate well with body fat content; BMI, on the other hand, has a 0.7 to 0.8 correlation. In addition, this definition of obesity correlates with morbidity and mortality.3

Using BMI, the Institute of Medicine developed 4 body-type categories4:

  • under 19.8: lean
  • 19.8 to 26.0: normal
  • 26.1 to 29: overweight
  • over 29: obese

In obstetric patients, BMI is calculated using prepregnancy weight. While the varying definitions of obesity make it difficult to compare and interpret research findings, it is important to note that adverse obstetric outcomes are associated with each classification. The Institute of Medicine also made recommendations on how much weight women in each category should gain during pregnancy5:

  • lean women: 28 lb to 40 lb
  • normal-weight women: 25 lb to 35 lb
  • overweight women: 15 lb to 25 lb
  • obese women: 15 lb or more

While no upper weight-gain limit was set for obese patients, 3 studies recommend 37 lb; researchers found that obese women who gain more than this have increased risk of cesarean delivery and large-for-gestational-age infants.6-8

Preconception: Control hypertension and diabetes

The negative impact that excess weight has on pregnancy begins even before conception (TABLE 1). For example, obese women are more likely to have chronic hypertension and diabetes. In 1 study, researchers reported the incidence of chronic hypertension among obese patients (defined as those weighing 300 lb or more) to be 33%, compared with 5% among controls, while diabetes occurred in 15% of obese patients and 3% of controls.9

Through preconception counseling and management, practitioners can improve pregnancy outcomes among patients with these medical complications. Strict glucose control of pregestational diabetes, for example, decreases the risk of congenital malformations. The 4-fold increase in malformations related to poor glucose control during embryogenesis is diminished if preconceptional glycosylated hemoglobin levels are in the normal range.10

Note that hypertension may be falsely diagnosed in an obese woman if an inappropriately small cuff is used. When taking the blood pressure (BP) of these patients, therefore, clinicians should make sure the length of the cuff is 1.5 times the upper arm circumference or that the inflatable bladder of the cuff encircles at least 80% of the arm.11 For women with an arm circumference of more than 41 cm, use a thigh cuff to ensure an accurate measurement.

In general, any hypertensive woman of childbearing age should take only agents with documented fetal safety. Drugs such as angiotensin-converting enzyme inhibitors should not be used due to their association with oligohydramnios, fetal hypocalvaria, and neonatal renal failure.

TABLE 1

Obstetric concerns among obese patients

Preconception

Pregestational diabetes mellitus

Chronic hypertension

Antepartum period

Gestational diabetes

Preeclampsia

Deep venous thrombosis

Stillbirth

Intrapartum period

Induction

Cesarean delivery

Poor VBAC success

Macrosomia

Postpartum period

Prolonged hospitalization

Cesarean complications

Wound infection

VBAC = vaginal birth after cesarean

Antepartum

Gestational diabetes and preeclampsia. During pregnancy, all obese patients—even those without a history of hypertension or diabetes—have an increased risk of gestational diabetes and preeclampsia. Baeten et al12 recently reported the odds ratios for gestational diabetes, preeclampsia, and eclampsia in the obese nulliparous patient as 5.2, 3.3, and 3.0, respectively.

What are the reasons for this? For one, obesity and pregnancy are both associated with increased insulin resistance. The combination of these 2 conditions can overwhelm the pancreas and unmask any small abnormality in its ability to secrete insulin.

The pathophysiology of preeclampsia is less clearly understood and, therefore, so is its link with obesity. However, Stone et al13 theorized that the relationship between obesity and hyperlipidemia is what leads to preeclampsia. Hyperlipidemia damages endothelial cells through lipid peroxidases. This damage leads to increased vasoconstriction and platelet aggregation.

The obese gravida should undergo early glucose screening along with regular blood pressure measurements.

For the obese patient, clinicians should place increased emphasis on preeclampsia and gestational diabetes screening and prevention. The obese gravida should undergo early glucose screening along with regular BP measurements. Several studies have investigated possible interventions for women at high risk for pregnancy-induced hypertension. In 1 systematic review of 41 randomized controlled trials, aspirin was associated with a 15% reduction in the relative risk of preeclampsia (95% confidence interval, 0.78 to 0.92), with no increase in adverse outcomes.14 Another systematic review found that calcium supplementation (at least 1 g per day) can reduce the risk of preeclampsia by 30%.15 Still, no trials have examined aspirin or calcium supplementation among obese patients; the clinician must therefore weigh the benefits of these prophylactic measures.

Deep venous thrombosis. Along with preeclampsia and gestational diabetes, deep venous thrombosis and its complications—which include maternal mortality—are seen more frequently in the obese patient. One 10-year review in Minnesota looked at weight distributions for mothers who died. Researchers found that 12% of this population, compared with 2% of the control group, had prepregnancy weights greater than 200 lb.16 The leading cause of death among the obese group was pulmonary embolus.

Fetal death. A large, population-based cohort study reported a relationship between maternal obesity and fetal death.17 Among nulliparous women in this study, the risk of late fetal death (stillbirth occurring at 28 weeks’ gestation or later) increased as the BMI rose. The obese woman was 4 times as likely to have a late fetal death as the lean woman. In parous women, the risk was only increased in the obese BMI category—rather than in all classifications of BMI. After excluding women with hypertensive diseases and diabetes, the association persisted. Huang et al18 supported these findings by identifying maternal prepregnancy weight greater than 68 kg as a risk factor for unexplained fetal deaths, even after controlling for maternal diabetes and hypertensive disease.

Intrapartum

Labor induction. Obesity is associated with an increased likelihood of labor induction. Gross et al19 reported that 15% of obese women (over 90 kg) had labor induced, compared with 8% of controls (P<.0001). Ekblad and Grenman20 also showed a significantly higher induction rate in obese patients and those with excessive weight gain during pregnancy.

Cesarean delivery. The effect of obesity on cesarean delivery rates has been debated, but most studies indicate a direct correlation (TABLE 2). Kaiser and Kirby21 showed that even among low-risk patients in a nurse-midwifery service, a BMI above 29 was associated with a 3-fold to 4-fold increase in cesarean delivery. A study by Cnattingius et al17 demonstrated that the effect of BMI on cesarean rates also was influenced by maternal height: Short obese women had the highest cesarean rate (36%), followed by (in decreasing order) short, lean women; tall, obese women; and, finally, tall, lean women.

VBAC. These findings raise a natural follow-up question: What is the success rate of vaginal birth after cesarean (VBAC) among obese parturients? Among 30 women weighing more than 300 lb at conception, Chauhan et al22 noted a VBAC success rate of less than 15%.This is much lower than the general success rate of 60% to 80% quoted in the ACOG practice bulletin on VBAC.23 Grobman et al24 reported that VBAC is cost-effective among women with 1 prior cesarean delivery only if the success rate is above 40%; it is therefore worth pondering whether VBAC should be attempted in overweight patients.

TABLE 2

Obesity and cesarean delivery rates

AUTHORS

NUMBER OF SUBJECTS

OBESITY DEFINED AS

RATE OF CESAREAN DELIVERY

COMMENTS

Baeten et al, 200112

9,817

BMI≥30

Increased

Kaiser and Kirby, 200121

452

BMI≥29

Increased*

Population was low risk without prior cesarean.

Kumari, 200137

188

BMI >40

Increased*

Elective* and emergency cesareans examined.

Steinfeld et al, 200038

168

BMI≥29

Increased*

Excluded elective cesareans and those performed due to fetal malpresentation and previa.

Jensen et al, 199939

163

BMI≥30

Increased

Excluded patients with prior cesarean.

Ranta et al, 199540

53

BMI≥30

Increased

Issacs et al, 19949

117

>300 lb

Increased*

Primary and repeat cesareans examined.

Hood and Dewan, 199325

117

>300 lb

Increased*

Elective and emergency* cesareans examined.

Ekblad and Grenman, 199220

77

≥20%

Increased

Emergency cesareans examined.

Perlow et al, 199235

111

>300 lb

Increased*

Primary* cesareans and those performed due to fetal distress examined.

Pongthai, 199041

741

≥80 kg

Increased*

Primary and repeat* cesareans examined.

Johnson et al, 198730

588

>113.6 kg

Increased*

Primary cesareans examined only.

Garbaciak et al, 198536

1,889

>120%

Increased*

Primary cesareans examined only.

Gross et al, 198019

279

≥90 kg

Increased

Repeat cesareans omitted.

Edwards et al, 197827

208

>50%

Increased

BMI = body mass index

*Significant increase

†Over ideal body weight for height

‡Above standard weight for height on the Metropolitan Life Insurance tables

Postpartum: Longer hospitalization

Although they did not provide the reasons, Hood and Dewan25 linked obesity with prolonged postpartum hospitalization. They found obese patients to have significantly longer hospital stays, regardless of the type of delivery:

  • Following vaginal delivery, postpartum hospitalization was 3.8±2.4 days among overweight patients and 2.9±2.1 days among controls.
  • After cesarean delivery, obese patients were in the hospital for 7.3±5.0 days; nonobese, for 5.4±3.1 days.

One study found the use of a subcutaneous drain or subcutaneous suture decreased the rates of wound infection or separation among obese women undergoing cesarean.

Cesarean complications

Obesity is a specific risk factor for several operative complications, including hemorrhage during surgery, postoperative wound infections, aspiration, and pulmonary embolism. A case-control study by Naef et al26 revealed that a weight of more than 250 lb has an odds ratio of 13.1 (95% confidence interval, 1.7 to 102.7) for hemorrhage (decrease in hematocrit of 10% or greater, estimated blood loss greater than 1,500 mL, or packed red blood cell administration) during abdominal delivery.

Multiple studies have shown obesity to be a risk factor for postoperative wound infections.27-30 For example, Johnson et al30 reported a wound infection rate of 37.6% for the obese parturient and 10.2% for those of normal weight (P<.001).

The link between excess weight and infectious morbidity may be secondary to the increased subcutaneous tissue layer and accumulation of loculated fluid. In 2000, Vermillion et al31 published a study that looked at 140 women who had cesarean deliveries. Initially, a univariate analysis identified the risk factors for wound infection as maternal weight (a mean of 82.8 kg±18.6 kg in the uninfected population versus 99.4 kg±33.3 kg in the infected population), BMI (44.5±2.1 for uninfected versus 49.7±6.3 for infected), and thickness of subcutaneous tissue (2.3 cm±1.2 cm for uninfected versus 4.1 cm±1.8 cm for infected). After a multiple logistic regression analysis, however, subcutaneous tissue thickness was the only significant risk factor confirmed. A potential explanation for this finding is that the blood supply to subcutaneous fat is relatively poor.

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