Obstetric Ultrasound Imaging in the Patient with Obesity | Article

This chapter should be cited as follows:
Chung K, Han CS, Glob. libr. women's med.,
ISSN: 1756-2228; DOI 10.3843/GLOWM.419113

The Continuous Textbook of Women’s Medicine Series – Obstetrics Module

Volume 18

Ultrasound in obstetrics

Volume Editors: Professor Caterina M (Katia) Bilardo, Amsterdam UMC, Amsterdam and University of Groningen, Groningen, The Netherlands
Dr Valentina Tsibizova, PREIS International School, Florence, Italy

Chapter

Obstetric Ultrasound Imaging in the Patient with Obesity

First published: August 2023

AUTHORS

Kathleen Chung, MD

Department of Obstetrics and Gynecology, David Geffen School of Medicine at UCLA, USA

Christina S Han, MD

Associate Clinical Professor, Division of Maternal-Fetal Medicine, Department of Obstetrics and Gynecology, David Geffen School of Medicine at UCLA, USA

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INTRODUCTION: THE PROBLEM OF OBESITY

Obesity is one of the most common diagnoses among reproductive age patients, and the prevalence is continuing to increase. In 2019, 29% of individuals had obesity prior to becoming pregnant in the United States, which is an 11% increase from 2016. This trend is noted across all maternal ages, races, and educational levels.¹^,² The estimated annual medical cost of obesity in the United States was nearly $173 billion in 2019 dollars.³ The global mean body mass index (BMI) also continues to increase, with the mean BMI reaching the overweight category by 2016.⁴

Obesity in adults is defined as a BMI greater than or equal to 30 kg/m² (Table 1). This standard measurement has been adopted by the NIH and WHO for white, Hispanic, and black individuals, though the threshold for obesity in Asian individuals is likely lower. Although BMI is easily measured and has been shown to be correlated with percentage of body fat and body fat mass,⁵ concerns exist regarding its ability to accurately predict cardiometabolic health sequelae.⁶

Classification of body mass index in adults.⁷

Body mass index (kg/m²)	Weight status
Below 18.5	Underweight
18.5–24.9	Normal Weight
25.0–29.9	Overweight
≥30.0 30–34.9 35–39.9 ≥40	Obese Class 1 Class 2 Class 4

Obesity is associated with higher rates of pregnancy complications.⁸^,⁹^,¹⁰ In the first trimester, obesity is independently associated with higher rates of miscarriage.¹¹ Associated maternal pregnancy complications in the second and third trimester include increased rates of hypertensive disorders of pregnancy, gestational diabetes, obstructive sleep apnea, cardiac dysfunction, nonalcoholic fatty liver disease, thromboembolic disease, and the need for cesarean section birth.⁹^,¹²^,¹³^,¹⁴^,¹⁵^,¹⁶^,¹⁷ Fetal risks include increased risks for macrosomia, congenital fetal anomalies, and stillbirth.¹⁸^,¹⁹^,²⁰

A major challenge in management of individuals with obesity during pregnancy is the ability to accurately, effectively, and atraumatically utilize ultrasound in prenatal diagnosis. The excess adipose tissue can result in degraded acoustic windows (DAWs), suboptimal visualization of the intrauterine contents and repetitive stress injury on the workforce. The goal of this chapter is to review the following topics related to pregnancy with obesity:

Pregnancy complications associated with obesity.
Fetal complications associated with obesity.
Indications for ultrasound in obesity.
Ultrasound challenges in patients with obesity.
Image acquisition techniques and alterations.
Image optimization.
Discrimination, bias, and social determinants of health in management of obesity in pregnancy.

PREGNANCY COMPLICATIONS ASSOCIATED WITH OBESITY

Obesity is an independent risk factor for multiple pregnancy complications in otherwise healthy patients with the potential for long-term sequelae for the children. In the first trimester, obesity is independently associated with first-trimester recurrent pregnancy loss.¹¹ The etiology for recurrent pregnancy loss is unclear, but proposed mechanisms include an unfavorable hormonal environment from the excess adipose tissue affecting endometrial receptivity²¹ or the effects of low-grade chronic inflammation in the setting of polycystic ovary syndrome.²²

Beyond the first trimester, obesity is associated with dysregulation of inflammatory, metabolic, and vascular pathways. This results in increased risk for hypertensive disorders of pregnancy, hypothesized to be through abnormal placental growth and function.¹⁵ High maternal weight is also associated with a significantly higher risk of gestational diabetes and fetal macrosomia.¹⁶ Maternal pre-pregnancy BMI independent of maternal gestational diabetes is a strong predictor of childhood obesity. Children born to obese individuals are at increased risk of developing metabolic syndrome and obesity in childhood.²³^,²⁴

Higher pre-pregnancy BMI is also a risk factor for developing obstructive sleep apnea in pregnancy, even in patients who otherwise do not have a history of sleep disordered breathing.¹² The consequences of obstructive sleep apnea in pregnancy include hypertensive disorders of pregnancy, low birth weight, preterm delivery, cardiomyopathy, and pulmonary embolism.¹³^,¹⁴

Obesity in pregnancy and its sequelae often lead to medically indicated preterm birth. This is primarily due to hypertension and diabetes. A systematic review and meta analysis found that pregnant individuals who are obese have an increased risk of induced preterm birth compared to those with a normal BMI, and the risk increased with increasing weight.²⁵

FETAL COMPLICATIONS ASSOCIATED WITH OBESITY

Obesity is also associated with a range of fetal structural anomalies. A systematic review found that maternal obesity is associated with an increased risk for fetal anomalies (Table 2). Concordant with the understanding that a risk factor for gastroschisis is being underweight, the risk of gastroschisis was significantly lower in individuals with high BMI (OR, 0.17; 95% CI, 0.10–0.30).¹⁹ While the relative risk for these anomalies is increased, the absolute risk remains low.

Fetal structural anomalies associated with obesity.¹⁹

Anomaly	OR (95% CI)
Neural tube defects Spina bifida	1.87 (1.62–2.15) 2.24 (1.86–2.69)
Cardiovascular anomalies Ventricular septal defect	1.30 (1.12–1.51) OR, 1.20 (1.09–1.31)
Cleft palate Cleft lip and palate	1.23 (1.03–1.47) OR, 1.20 (1.03–1.40)
Anorectal atresia	1.48 (1.12–1.97)
Hydrocephaly	1.68 (1.19–2.36)
Limb reduction anomalies	1.34 (1.03–1.73)

Furthermore, obesity may interfere with the ability to perform non-invasive prenatal screening. Cell-free placental DNA screening results in greater rate of no-calls for obese pregnant individuals due to greater cell turnover, apoptosis of adipose tissue or a dilutional effect due to an increased maternal blood volume.²⁶ This limitation in screening may result in delayed or missed diagnoses of aneuploidy fetuses.

Accurate dating in pregnancies in the individual with obesity is also challenging due to the high rates of anovulation and delayed pregnancy diagnoses. Combined with the increased risk of prematurity,

There is also an increased risk for stillbirth in obese patients compared to non-obese patients. The risk increases in a dose-dependent fashion with increasing BMI: class I (adjusted hazard ratio 1.3; 95% CI 1.2–1.4); class II (adjusted hazard ratio 1.4; 95% CI 1.3–1.6) and extreme obesity (adjusted hazard ratio 1.9; 95% CI 1.6–2.1; P for trend <0.01). Obese black mothers experienced more stillbirths than their white counterparts (adjusted hazard ratio 1.9; 95% CI 1.7–2.1 compared with adjusted hazard ratio 1.4; 95% CI 1.3–1.5). The black disadvantage in stillbirth widened with increase in BMI, with the greatest difference observed among extremely obese black mothers (adjusted hazard ratio 2.3; 95% CI 1.8–2.9).²⁰

INDICATIONS FOR ULTRASOUND IN OBESITY

In general, routine ultrasound surveillance in pregnancy includes an initial ultrasound examination to determine gestational age, fetal number, viability, and placenta location. A second trimester detailed anatomy ultrasound is performed to screen for structural abnormalities.²⁷ Additional ultrasound examination may be indicated in the setting of other obesity-related diagnoses such as gestational diabetes or macrosomia. The types of ultrasounds, trimesters, and rational and indications are shown in Table 3.

Types of ultrasounds that may be indicated in pregnancies with obesity.

Type of ultrasound	Trimester	Rationale and indications
Early viability	1	Risk of miscarriage and recurrent pregnancy loss Dating due to risk of premature delivery
First-trimester anatomy	1	Risk of severe anomalies, including neural tube defects No-call or low-fetal fraction on non-invasive prenatal screening
Detailed anatomy	2*	Increased risk of developing fetal anomalies Increased risk of missing anomalies
Fetal echocardiogram	2	Concomitant pregestational diabetes
Growth ultrasound	3	Inability to assess fundal height Diabetes or gestational diabetes Hypertensive disorders of pregnancy
Biophysical profile, full, or modified	Late 3rd	Risk of stillbirth
Limited ultrasound	Late 3rd	To assess fetal presentation due to inability to perform Leopold’s maneuver

ULTRASOUND CHALLENGES IN PATIENTS WITH OBESITY

Given the significant risks for maternal and fetal complications in obesity, ultrasound plays an integral role in the management of the gravida with obesity throughout gestation. One of the primary challenges of obesity in pregnancy are DAWs secondary to two factors in individuals with central obesity: (1) increased depth of insonation; and (2) absorption of ultrasound waves by adipose tissue. As ultrasound waves travel over greater distances, more energy is absorbed and dispersed in the tissue. This leads to a weaker signal with greater background noise distorting the images.

The DAWs result in potential gaps in prenatal diagnosis throughout gestation. In the first trimester, obese women have higher failure rates of nuchal translucency and nasal bone measurements and require more time to complete the examination.²⁸ The sensitivity of ultrasound to detect fetal anomalies varies ranging from about 15–80%, though has an overall sensitivity of about 40%.²⁷^,²⁹ This is significantly decreased in obesity. In a retrospective cohort study of pregnancies 18–24 weeks, detection of fetal anomalies was found to decrease with increasing BMI with at least 20% decreased detection rate when comparing obese patients to patients with normal BMI. This detection rate was decreased further in mothers with pregestational diabetes.³⁰ Detection of fetal anomalies is improved with the increased training and skills utilized in targeted or detailed ultrasound (Table 4).

Detection of fetal anomalies.³⁰

BMI	Standard ultrasonography	Targeted ultrasonography
Normal	66%	97%
Overweight	49%	91%
Class 1 obesity	48%	75%
Class 2 obesity	45%	88%
Class 3 obesity	22%	72%

OPTIMIZATION OF IMAGING

Three variables can be controlled by the operator to improve image quality: (1) patient factors; (2) hardware or probe selection; and (3) software or post-processing capabilities.

Timing of ultrasound

Gestational age at ultrasound examination plays an integral role in optimization of images in the obese gravida. Early anatomy examination via transvaginal ultrasound is one option to capitalize on the higher resolution of the transvaginal ultrasound probe and bypass abdominal adiposity for greater image clarity. In a prospective study of 1144 women with singleton pregnancies at 11–14 weeks, early anatomy ultrasound was found to be feasible via transvaginal ultrasound, which included visualization of the skull, brain, face, spine, four-chamber, and three-vessel cardiac views, stomach, abdominal wall, kidneys, bladder, and extremities.³¹ Guidelines have been published to standardize first-trimester ultrasound scans.³²

Performing a transabdominal anatomy ultrasound at 18–20 weeks is feasible in obese patients, but there is a higher likelihood of suboptimal imaging. A prospective study with a single operator compared the feasibility of completing a second-trimester ultrasound in one setting and comparing the image quality between obese and non-obese patients. In the obese group, a complete scan was achieved in one session in 70% of patients as opposed to 80% in the non-obese group, though this was not statistically significant. Significant factors that were associated with completing the scan were having 10 additional minutes, moving the patient so the fetus’ back was in a posterior or lateral position, sonographer experience, and thinner maternal abdominal wall thickness. Image quality was significantly worse in the obese group.³³

In the event of suboptimal ultrasonographic visualization, repeat ultrasound imaging at a later gestational age dramatically reduces the rate of suboptimal imaging. A retrospective database study looked at the utility of repeated antenatal ultrasound in improving fetal cardiac visualization in obese and non-obese populations. Cardiac anatomy was found to have a 11% rate of suboptimal views with significant association with maternal obesity in a dose-dependent fashion. The rate of suboptimal imaging was 1.5% for non-obese patients, 12% for class I obesity, 17% for class II obesity, and 20% for class III obesity (p < 0.0001). Repeated ultrasound at a later gestational age was found to significantly reduce the rate of suboptimal imaging, though obese patients had a higher rate of persistent suboptimal views.³⁴

Deferring transabdominal anatomy ultrasounds until after 20 weeks for obese patients may reduce the need for repeat imaging for suboptimal views. Another study found that during a routine anatomy scan at 18–20 weeks' gestation, maternal obesity increases the rate of suboptimal images by about 50% for fetal cardiac structures and about 30% for craniospinal structures. This study suggests that perhaps deferring the transabdominal anatomy survey after 20 weeks to allow these structures to grow in size may help obtain adequate images for more accurate prenatal diagnosis.³⁵

Another option to optimize imaging in the setting of significant obesity is to place the transducer periumbilically when the patient has a full bladder. At 20 weeks, a full bladder can help displace the uterus superiorly and the periumbilical areas can offer a thinner window to try to get more detailed views of smaller structures such as cardiac anatomy.³⁶ The transvaginal probe can also be inserted into the umbilicus to improve visualization.³⁷

Patient positioning

In ultrasonography, a general principle to optimize visualization is to reduce the distance between the transducer and the organ of interest, or the fetus in obstetrical ultrasound. In patients with central obesity or a panniculus, this can prove to be challenging. The increased depth of insonation required and the increased absorption of energy by the adipose tissue can result in a decreased signal-to-noise ratio and worse overall image quality.³⁸

The abdominal panniculus is thickest between the pubic symphysis and the umbilicus. Scanning from above or below the panniculus can decrease the distance between the transducer and the fetus. When scanning from above, the provider or patient can compress the panniculus to decrease the distance between transducer and fetus. The patient can also sit upright, with the transducer placed at the fundus.³⁹ A full bladder can also help displace the uterus cephalad. When scanning from below the panniculus, the provider or the patient can help lift the pannus cephalad to decrease strain on the operator’s arm.³⁹ Turning the patient to left lateral decubitus can help with visualization as the panniculus falls away, leaving a thinner window through the flank with the transducer pointed medially.³⁸^,³⁹

Ultrasound probe selection

Proper probe selection can help optimize imaging, especially in the setting of obesity. Lower-frequency transabdominal ultrasound probes can achieve deeper levels of penetration, but generally have lower resolution. In contrast, high-frequency probes, such as transvaginal transducers, achieve higher resolution but with lower penetration. Proper probe selection to reflect the depth of the target tissue can help optimize image quality, and the use of different probes on the same patient may be needed to allow for more flexibility with resolution and penetration. Adjusting the gain and the focal zone to reflect the target depth can also create a clearer image.²⁸

Pre- and post-processing techniques

Modern ultrasound machines are equipped with pre- and post-processing techniques that improve acoustic output and on-screen visualization. Three such imaging techniques are as follows: (1) tissue harmonic; (2) compound imaging; and (3) speckle reduction.

1. Tissue harmonic sonography

Tissue harmonic sonography is one method that helps decrease background noise to produce a sharper image. Tissue harmonics relies on the principle that sound waves experience nonlinear propagation after passing through tissue and this distortion increases with depth. The high pressure portion of the wave travels faster than the low-pressure portion, and the change in the wave produces harmonics, or multiples of the fundamental transmitted frequency. Therefore, by setting the receiver to interpret waves at a multiple of the frequency, for example emitting 2 MHz and reading 4 MHz frequencies, this improves image contrast and lateral resolution of the image as compared to conventional sonography.⁴⁰ Harmonic imaging has been shown to improve image quality in obese patients and has been found to improve the visualization of cardiac anatomy.³⁶

2. Compound imaging

Traditional imaging relies on ultrasound signals sent in a single line perpendicular to the transducer. In contrast, compound imaging is an ultrasound technique that takes images from several overlapping angles and forms a compound image in real time. Compounding imaging results in decreased acoustic artifacts for a clearer image and can improve contrast and tissue differentiation.⁴¹

3. Speckle reduction imaging

Speckle reduction is a technique to reduce speckle artifacts and improve contrast resolution. Speckle reduction is a post-processing method that filters the resulting image to identify weaker images or “speckles.” This noise is removed and the stronger signals are amplified to produce a clearer image.³⁶

MUSCULOSKELETAL INJURY IN THE SONOGRAPHER

Ultrasonography in patients with obesity carries additional risk for work-related musculoskeletal injury in the sonographer. According to the National Institute for Occupational Safety and Health (NIOSH), work-related musculoskeletal disorder (MSD) is caused by repetitive motions, forceful or awkward movements, duration of pressure, overuse, poor posture and improper positioning, excessive force and strain, and vibration.⁴²^,⁴³ There are three stages of MSD: (1) aching and fatigue that resolve with rest and do not affect work performance; (2) symptoms occur earlier in the day and may affect work performance; and (3) reduced performance in work and leisure activities, disturbed sleep, and lasts for months to years.⁴⁴ Some examples of MSD include carpal tunnel, tennis elbow (lateral epicondylitis), golfer’s elbow (medial epicondylitis), DeQuervain’s tenosynovitis, trigger finger, shoulder bursitis, back and neck pain, thoracic outlet syndrome. Factors that contribute to MSD in sonographers include number of scans per month, scan time, transducer/system design, chair position, exam table position, and pushing system.

There are several positions that are risk factors for developing MSD. Thoughtful and ergonomic approach to ultrasonography may help reduce the risk for injury over time, including assessment of the sonographer–patient distance, the examination bed position, and the sonographer’s body mechanics. It is important for the sonographer to position themselves close to the patient, position the monitor for minimal turning of the head, and either stand with weight distributed evenly between the feet or sit upright with the shoulders and hips aligned, engaging abdominal muscles to support the trunk.

One risk factor is abduction of the arm at the level of the shoulder when navigating the probe. This can be the result of inadequate adjustment of the chair or exam table or too much distance to the patient. The strain of abduction can be mitigated by reducing the angle of the shoulder to 30 degrees or less. This can be achieved by relaxing the shoulder, lowering the exam table, elevating the chair, or standing and positioning yourself close to the patient, at the level of the hip. For transvaginal scans, the scanning arm may be supported by the exam table footrests to help relax and further adduct the shoulder. Reaching, and twisting of neck and/or trunk may also cause MSD over time.

Regarding hand positioning, the probe should be held with the entire grip rather than pinching with fingertips. Gloves may also provide additional friction to allow for decreased grip intensity. The deviation at the wrist should never be greater than 15 degrees on the radial side or 25 degrees on the ulnar side.

SOCIAL DETERMINANTS OF HEALTH, BIAS AND DISCRIMINATION

Obesity is a stigmatized condition, and patients with obesity are often met with discrimination, judgments, and reduced quality of care. Healthcare providers have been found to hold strong negative views and uphold stereotypes about people with obesity, and these negative attitudes influence personal perceptions, judgment, behavior, and decision-making.⁴⁵ Ultimately, these biases influence the care provided to patients with obesity. Consequently, patients with obesity may experience or expect poor treatment. The experience of stress and mistrust of healthcare providers, and resultant avoidance of care or poor adherence to therapy overall leads to compromised care for this population.⁴⁵ It is important to be mindful of the experience of patients with obesity and create an accepting and welcoming clinical environment where these patients can feel comfortable and confident that they are receiving high-quality care.

CONCLUSION

Ultrasonography in obese pregnant patients can be challenging at any point in pregnancy, but it is particularly important due to the increased risk for obesity-associated anomalies and abnormal fetal growth. This can have implications for pregnancy management and delivery planning. Given the higher rates of suboptimal imaging and decreased detection rates, it is important to utilize various techniques and approaches to optimize patient positioning, probe selection, and pre- and post-processing tools to help optimize prenatal diagnosis and guide management for these patients.

PRACTICE RECOMMENDATIONS

Obesity defined as BMI >30 kg/m² is one of the most common diagnoses among reproductive age patients and the prevalence is rising.
Obesity is associated with higher maternal and fetal complications in pregnancy and ultrasound is a critical tool for prenatal diagnosis and management.
Beyond routine ultrasounds in prenatal care, additional ultrasounds may be indicated for patients with obesity especially in the setting of comorbidities, such as a fetal echocardiogram, growth ultrasounds, or assessment of presenting presentation.
Early anatomy examination via transvaginal ultrasound is a good opportunity to capitalize on the higher resolution of the transvaginal probe and decreased distance between transducer and fetus bypassing abdominal adiposity.
Significant factors associated with completing a second-trimester ultrasound include having additional time, changing patient position, sonographer experience, and thinner maternal abdominal wall thickness.
In the event of suboptimal ultrasonographic visualization, repeat ultrasound imaging at a later gestation can be performed.
Deferring transabdominal anatomy survey after 20 weeks to allow fetal structures to grow may help obtain adequate images for prenatal diagnosis.
Placing the transducer periumbilically, scanning above or below the panniculus, or placing the patient in left lateral decubitus can improve image quality.
Pre- and post-processing techniques, such as tissue harmonic sonography, compound imaging, and speckle reduction imaging, can help increase image resolution and contrast and decrease background noise.
Ultrasounds on patients with obesity pose additional risk for workplace injury in the sonographer. This can be mitigated by adjusting both patient and provider positioning for a more ergonomic approach to scans.
Obesity is a stigmatized condition, and provider bias combined with patient mistrust can lead to worse care. It is important to create a safe and welcoming environment where these patients can receive high-quality care.

CONFLICTS OF INTEREST

The author(s) of this chapter declare that they have no interests that conflict with the contents of the chapter.

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