Maternal Obesity

1. What every clinician should know

BMI = Body mass index

CS = cesarean section

PPH = postpartum hemorrhage

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VTE = venous thromboembolism

The prevalence of obesity has been rising steadily over the last several decades. Body mass index (BMI) is commonly used to quantify the severity of obesity. In 2009-2010, the prevalence of obesity (BMI 30 or greater) in the United States was 30% among childbearing-aged women. The morbidly obese (BMI 40 or more) make up 7.2-8.4% of childbearing-aged women, with numbers as high as 15% in non-Hispanic black females. The presence of obesity during pregnancy has been associated with a multitude of maternal, fetal and neonatal complications.

2. Diagnosis and differential diagnosis

The diagnosis of obesity rests on the calculation of the BMI (weight in Kg/height in meters squared). Online calculators are available (e.g.,

Obesity is classified into class I (BMI between 30 and 35), class II (BMI 35-40) and class III (BMI above 40).

3. Management


Rule out the presence of chronic medical conditions that can be associated with obesity and can increase the risk of perinatal morbidity and mortality. These include chronic hypertension, diabetes, hypothyroidism, non-alcoholic fatty liver disease and sleep apnea. The following tests may be considered: serum TSH, transaminases, fasting glucose level. If co-morbidities are present (e.g., chronic hypertension or diabetes), a 12-lead electrocardiogram and echocardiogram may be considered.

Order sleep studies if obstructive sleep apnea (OSA) is suspected based on snoring (particularly with gasping for air), daytime sleepiness, hypertension, morning headaches and alterations in mood. Frequency of OSA is BMI-dependent and it is underestimated in the majority of women. Since OSA increases the risk of gestational diabetes mellitus (GDM), preeclampsia, indicated preterm delivery, fetal growth restriction and possibly stillbirth, it should be treated.

Dietary counseling is key to minimize weight gain. In 2009 the Institute of Medicine put forth recommendations for weight gain during pregnancy in obese women (11-20 lb total for singletons and 25-42 lb for twins). However, given that most obese women tend to gain in excess of such recommendations, that the gained weight increases risks at delivery, and that it is not easily lost after delivery, the American College of Obstetrics and Gynecology (ACOG 2015) has recommended to minimize any weight gain during pregnancy, provided adequate fetal growth is documented.

Assess for vitamin and elements deficiencies (as obese people are often malnourished: 40% are deficient in iron, 24% in folic acid, and 4% in B12). The frequency and severity of such dietary deficiencies are BMI-related. A history of previous gastric bypass surgery poses additional risk of nutritional deficiencies. Identification of deficiencies can be accomplished by requesting a complete blood cell count, serum ferritin, RBC folate and vitamin B12 level; in the presence of a history of previous bariatric surgery, 25(OH)D should also be requested to rule out vitamin D deficiency.

Because obesity may be associated with menstrual irregularities, a first trimester ultrasound exam for dating is often indicated; it will obviate any uncertainty about fetal age if abnormalities in fetal size are detected later in pregnancy. A first trimester scan also will confirm the viability of the fetus, as obesity increases the risk of early miscarriage by two- to threefold. The risk of recurrent miscarriage also increases with maternal BMI.

As obese women have greater risk of glucose intolerance or undiagnosed type 2 diabetes, order a glucose challenge test at the end of the 1st trimester. If results are abnormal, assume pre-gestational diabetes mellitus: assessment of glycosylated hemoglobin (HgbA1c) will provide an indication of the degree of hyperglycemia during embryogenesis. If HbA1C is elevated, a fetal echocardiogram and targeted ultrasound examination are indicated to assess for fetal anomalies. A conclusive diagnosis of pre-gestational diabetes requires confirmatory glucose challenge testing in the postpartum period. If the glucose challenge test result is normal, a second test should be requested at 26-28 weeks. If GDM is not responsive to diet or oral hypoglycemic agents, insulin should be administered with dosages calculated up to 50% higher than for normal BMI, as obesity is usually accompanied by insulin resistance.

Obesity is associated with an increased risk of hypertensive disorders of pregnancy, with a risk proportionate to BMI. Low dose aspirin prophylaxis has been recommended for obese women if other moderate risk factors are present (e.g., nulliparity, family history of preeclampsia, African American race, low socio-economic status, age ≥35 years, previous adverse obstetric outcome). As recurrent pre-eclampsia is more frequent in obese women, weight loss between pregnancies is recommended as it can decrease such risk. Basal laboratory values of transaminases may provide valuable help in diagnosing HELLP syndrome or acute fatty liver later in pregnancy.

The risk of spontaneous preterm birth is 20% lower in obese than non-obese women. However, the risk of indicated preterm birth increases in a manner proportional to the BMI, and in general it is almost three times higher than in non-obese women.

Obese women have increased risk of major fetal malformations (risk is BMI-related and independent of medical or dietary confounders, such as folate deficiency). In particular, neural tube defects, cardiac defects (particularly ventricular and atrial septal defects) and cleft lips/palate are more frequent with obesity. Additionally, obesity interferes with adequate ultrasonographic visualization of fetal anatomy, particularly of the heart and spine. Adequate visualization of fetal anatomy may require multiple targeted sonograms; alternatively, a transvaginal sonogram at 14 weeks may allow good visualization of fetal anatomy. In class III obesity the risk of fetal cardiac anomalies approaches that of other conditions (e.g., diabetes) for which targeted examination of the fetal heart is indicated.

Obesity is associated with higher rates of macrosomia (birth weight greater than 4500 g) and large for gestational age babies (weight more than 90th percentile). Fundal height may not be accurately assessed during pregnancy due to the presence of pannus; in such cases, fetal growth should be followed with serial sonographic scans (e.g., at 32 and 36 weeks).

Stillbirth risk is increased in a BMI dependent fashion. Indeed, obesity is one of the most prevalent of the identified risk factors for stillbirth. The benefit of fetal testing, such as with fetal non-stress tests (NSTs), for stillbirth prevention has not been adequately evaluated. Women early in their fertile life should be encouraged to lose weight between pregnancies to lower the risk of stillbirth.


Obesity is associated with increased risk of the following complications:

  • Prolonged labor (the dilation curve over time is shifted to the right until a dilation of about 6 cm). Thus diagnosis of protracted or arrest of labor should be made with caution in obese women. It is unclear at present whether obese women require higher doses of oxytocin.

  • Difficult anesthesia: Consultation with an anesthesiologist during the late part of pregnancy or early in labor is prudent for obese women with OSA because they are at increased risk of hypoxemia and sudden death during anesthesia. High classes of obesity are associated with increased rates of failed intubation, failure at epidural placement and complicated epidurals (i.e. dural puncture). Epidurals can be facilitated by ultrasonography (to estimate skin-to-epidural space depth and to identify midline spinous processes) and using sitting position (facilitates identification of anatomic landmarks). Systemic opioids have limited efficacy and are relatively contraindicated as they may cause airway obstruction and hypoxemia.

  • An early epidural may be beneficial as it lowers respiratory effort, reduces pain perception and thus decreases secondary release of catecholamines that affect cardiac work. Lateral decubitus positioning optimizes fetal oxygenation and is indicated.

  • Risk of shoulder dystocia is doubled in obese women.

  • Cesarean delivery (CD): both pre-pregnancy weight and excessive weight gain are independent risk factors for CD. Increased CD rates are independent of antepartum complications, baby’s birth weight and gestational age at delivery, and they show a dose-effect relationship with BMI.

  • Complicated CD are more frequent with obesity, including CD done emergently in labor, with longer time incision-to-delivery, blood loss greater than 1000 mL, and higher rates of infectious complications.

  • Prophylactic antibiotics before CD should be administered at higher dosages (given greater volume of distribution and lower tissue levels of antibiotics in obese women). Broadening the spectrum (e.g., adding azythromycin) can be considered to minimize the risk of endometritis and wound infection.

  • The optimal type of skin incision is probably horizontal above the pannus (to avoid anaerobic areas).

  • Closure of subcutaneous layer has been shown to decrease the risk of wound disruptions and wound seromas.

  • Placement of subcutaneous drainage increases the risk of wound complications and should not be used.

  • Risk of post-partum hemorrhage (PPH) is increased, due to atony. The risk of proportional to the BMI. Consider requesting a type and cross match if BMI is above 40 and additional risk factors for PPH are present.

  • Higher rates of: endometritis, perineal or wound infection, wound breakdown leading to prolonged hospitalization.

  • Higher risk of venous thromboembolism (VTE): the majority of maternal deaths from VTE occur in obese women. Pneumatic compression stockings should be placed on every obese woman during labor (if unable to walk) or CD. Fractionated heparin for postpartum thromboprophylaxis should be administered if thromboembolic score is 3 or greater:

    Score 1: BMI 30-40; age above 35; smoker; varicose veins; family history of thromboembolism; preeclampsia; systemic infection; immobility; elective CD; PPH greater than 1000 mL.

    Score 2: BMI above 40; known thrombophilia; medical co-morbidities; CD in labor.

    Score 3: previous VTE.

Utilization of the above score has been shown in epidemiologic studies to significantly reduce the risk of VTE. The optimal duration of thromboprophylaxis is presently unknown, with some experts recommending continuation for 1 week (since 50% of post-partum VTE occur in the 1st week) and others stopping prophylaxis at ambulation or discharge from hospital.

  • Obesity is associated with poor breastfeeding due to delayed onset of lactogenesis. However, breastfeeding should be encouraged as it is associated with less postpartum weight retention and is compatible with appropriate rates of weight loss (1 lb/wk). The input of a lactation consultant may be beneficial.

4. Prognosis and outcome

  • Because of the increased risk of the above listed complications, pregnancy in obese women is associated with higher risk of maternal morbidity and mortality, longer hospital stays, higher rates of neonatal admission to the neonatal intensive care unit, and greater neonatal morbidity and mortality.

  • The epigenetic effect of obesity leads to significantly higher rates of obesity in the offspring.

  • BMI above 30 is associated with increased risk of neonatal encephalopathy, cerebral palsy and developmental delay in early childhood for those delivered at term or near term.

Women with BMI above 40 or BMI between 35 and 40 with co-morbidities (e.g., chronic hypertension or sleep apnea) who have failed dietary and medical regimens in the past should be informed of the benefits of bariatric surgery. Such benefits are particularly beneficial if patients are planning future pregnancies, as the risk of pregnancy complications are significantly and proportionately reduced with inter-pregnancy decrease in BMI.

5. What is the evidence for specific management and treatment recommendations

Flegal, KM, Carroll, MD, Kit, BK, Ogden, CL. “Prevalence of obesity and trends in the distribution of body mass index among US adults, 1999-2010”. JAMA. vol. 307. 2012. pp. 491-7.

Athukorala, C, Rumbold, AR, Willson, KJ, Crowther, CA. “The risk of adverse pregnancy outcomes in women who are overweight or obese”. BMC Pregnancy Childbirth. 2010. pp. 10-56.

Stothard, KJ, Tennant, PW, Bell, R, Rankin, J. “Maternal overweight and obesity and the risk of congenital anomalies: a systematic review and meta-analysis”. JAMA. vol. 301. 2009. pp. 636-50.

Chu, SY, Kim, SY, Lau, J, Schmid, CH, Dietz, PM. “Maternal obesity and risk of stillbirth: a metaanalysis”. Am J Obstet Gynecol. vol. 197. 2007. pp. 223-8.

Lashen, H, Fear, K, Sturdee, DW. “Obesity is associated with increased risk of first trimester and recurrent miscarriage: Matched case-control study”. Hum Reprod. vol. 19. 2004. pp. 1644-6.

Aasheim, ET, Hofsø, D, Hjelmesaeth, J. “Vitamin status in morbidly obese patients: A cross-sectional study”. Am J Clin Nutr. vol. 87. 2008. pp. 362-9.

Rasmussen, KM, Yaktine, AL. “Committee to Reexamine IOM Pregnancy Weight Guidelines; Institute of Medicine; National Research Council: Weight Gain During Pregnancy: Reexamining the Guidelines”. 2009.

Weiss, JL, Malone, FD, Emig, D. “Obesity, obstetric complications and cesarean delivery rate – A population-based screening study”. Am J Obstet Gynecol. vol. 190. 2004. pp. 1091-7.

Chelmow, D, Rodriguez, EJ, Sabatini, MM. “Suture closure of subcutaneous fat and wound disruption after cesarean delivery: a meta-analysis”. Obstet Gynecol. vol. 103. 2004. pp. 974-80.

“Obesity in pregnancy. American College of Obstetricians and Gynecologists. Obesity in pregnancy. Practice Bulletin No. 156”. Obstet Gynecol. vol. 126. 2015. pp. 1321-22.

“Weight gain during pregnancy. Committee Opinion No. 548. American College of Obstetricians and Gynecologists”. Obstet Gynecol. vol. 121. 2013. pp. 210-2.

“LeFevre MC and the U.S. Preventive Services Task Force. Low dose aspirin use for the prevention of morbidity and mortality from preeclampsia: U.S. Preventive Services Task Force recommendation statement”. Ann Intern Med. vol. 161. 2014. pp. 819-26.