Module 9 ACOG Macrosomia

ACOG Practice Bulletin on Macrosomia

The American College of Obstetricians and Gynecologists (ACOG) has published a practice bulletin (Number 216) addressing the clinical management of suspected macrosomia. This replaces Practice Bulletin Number 173 from November 2016. This document aims to quantify risks, address the accuracy of fetal weight estimation methods, and suggest clinical management strategies.

Background and Definition

Two terms are used to describe excessive fetal growth:

Large for Gestational Age (LGA): Birth weight at or above the 90th percentile for a given gestational age. This definition is based on a statistical distribution within a population, where newborns are considered LGA if their weight exceeds that of 90% of babies at the same gestational age. It's a relative measure, indicating that the baby is larger than expected for its gestational age but doesn't necessarily imply any pathology.
Macrosomia: Growth beyond an absolute birth weight, typically 4,000 g or 4,500 g, regardless of gestational age. However, a universally accepted definition is lacking, and the threshold at which macrosomia is diagnosed can vary among institutions and clinical guidelines. Macrosomia represents an absolute measure of fetal size, indicating that the baby has achieved a substantial weight, regardless of its gestational age. This condition may raise concerns about potential delivery complications and neonatal morbidities.

A study using the 2011 U.S. Live Birth File from the National Center for Health Statistics provides a reference for birth weight based on the best obstetric estimate of gestational age for over 3.2 million births. This reference is crucial for interpreting birth weights in the context of gestational age and identifying LGA or macrosomic newborns.

The risk of morbidity for infants and women increases when birth weight is LGA or between 4,000 g and 4,500 g and rises sharply above 4,500 g. A study using U.S. Vital Statistics from 2011 to 2013 showed increased maternal and infant morbidity for newborns with a birth weight at or above the 90th percentile but less than 4,000 g, delivered between 37–39 weeks. This finding highlights that adverse outcomes are not solely associated with very high birth weights but can also occur in newborns classified as LGA within a specific gestational window.

A large cohort study of 8.3 million births analyzed live-birth and infant death files in the United States. It demonstrated:

Increased labor abnormalities and newborn complications (e.g., Apgar score less than 4 at 5 minutes, assisted ventilation >30 minutes, birth injuries) within the 4,000–4,499 g birth weight category. Labor abnormalities may include protracted labor, arrest of dilation, or arrest of descent, potentially leading to interventions such as cesarean delivery. Newborn complications encompass a range of issues, including low Apgar scores, need for resuscitation, birth injuries like fractures or nerve damage, and metabolic disturbances.
Further increase in newborn morbidity within the 4,500–4,999 g category. As birth weight increases, the risk of newborn morbidity escalates, with a higher likelihood of requiring intensive care, experiencing respiratory distress, and developing infections.
Increased newborn mortality with birth weights exceeding 5,000 g. Extremely high birth weights are associated with a substantial increase in newborn mortality, reflecting the challenges in managing these deliveries and the potential for severe complications.

Another large cohort study of over 6 million birth and infant death records showed that perinatal outcomes were similar in the 4,000–4,499 g group compared to those weighing less than 4,000 g. However, morbidity and mortality, including stillbirth, significantly increased in newborns weighing 4,500 g or more, with a greater increase at 5,000 g or more. This research emphasizes that the most pronounced risks are observed in newborns with the highest birth weights.

The risks associated with increasing birth weight exist on a continuum. Macrosomia is often categorized into three groups based on differing risk levels:

4,000–4,499 g: This category represents the lower end of the macrosomia spectrum, with a modest increase in associated risks.
4,500–4,999 g: Newborns in this category face a higher risk of complications compared to the previous group, including shoulder dystocia and birth injuries.
More than 5,000 g: This represents the most severe form of macrosomia, with a substantial increase in morbidity and mortality risks.

Frequency of Occurrence

National Center for Health Statistics data indicate that 7.8% of live-born newborns in the United States weigh 4,000 g or more. Only 1% weigh more than 4,500 g, and 0.1% weigh more than 5,000 g. The rate of newborns weighing at least 4,000 g has decreased from 10% in 1996. This decline may reflect improved management of maternal diabetes and obesity, as well as changes in obstetric practices.

Women with gestational diabetes mellitus (GDM) or obesity have higher rates of LGA newborns. A study of nearly 10,000 women showed the following LGA rates:

Normal-weight women without GDM: 7.7%
Obese women without GDM: 12.7%
Normal-weight women with GDM: 13.6%
Obese women with GDM: 22.3%

Risk Factors for Macrosomia

Maternal factors that predispose a newborn to macrosomia include:

Constitutional factors: Maternal height and pre-pregnancy weight are significant determinants of fetal growth. Taller and heavier women tend to have larger newborns.
Preexisting diabetes and GDM: Poorly controlled diabetes leads to fetal hyperglycemia, resulting in excessive fetal growth and fat deposition.
Maternal prepregnancy obesity: Obese women have higher levels of circulating glucose and insulin, which can promote fetal growth.
Excessive gestational weight gain: Gaining more weight than recommended during pregnancy increases the risk of macrosomia.
Abnormal fasting and postprandial glucose levels: Elevated glucose levels, even in the absence of diagnosed diabetes, can contribute to fetal overgrowth.
Dyslipidemia: Abnormal lipid levels can affect insulin sensitivity and glucose metabolism, impacting fetal growth.
Prior macrosomic newborn (weight > 4,000 g): Women who have previously delivered a macrosomic newborn are at higher risk of recurrence.
Postterm pregnancy: Prolonged gestation allows for continued fetal growth, increasing the likelihood of macrosomia.

The interplay of these factors is complex and varies by prepregnancy BMI, race, and ethnicity. Genetic predisposition, environmental influences, and access to healthcare also play a role in determining macrosomia risk.

Gestational age influences birth weight and macrosomia risk. The risk of birth weight > 4,500 g increases from 1.3% at 39 weeks to 2.9% beyond 41 weeks of gestation (2014 US data). This highlights the importance of optimal timing of delivery to minimize the risk of macrosomia.

Maternal hyperglycemia increases macrosomia risk. Glucose transfer across the placenta leads to fetal hyperglycemia, causing fetal insulin release, insulin-like growth factors, and growth hormone, leading to increased fetal fat deposition and size. Fetal insulin acts as a growth hormone, promoting the synthesis of protein, fat, and glycogen, resulting in increased fetal size.

The Hyperglycemia and Adverse Pregnancy Outcomes (HAPO) study showed a linear relationship between maternal glucose concentration and LGA fetuses, fetal adiposity, and fetal hyperinsulinemia. This landmark study established the continuum of risk associated with maternal glucose levels and adverse pregnancy outcomes.

A meta-analysis showed an association between macrosomia (weight > 4,000 g) and maternal glucose levels in non-diabetic women, particularly fasting glucose levels. Even in the absence of diagnosed diabetes, elevated maternal glucose levels can significantly impact fetal growth.

In women with GDM, the risk of macrosomia increases twofold to threefold even with treatment. This underscores the importance of vigilant glucose control in women with GDM to mitigate the risk of fetal overgrowth.

Fetal Body Composition and Maternal Glucose Intolerance

Anthropometric studies suggest that macrosomia resulting from maternal glucose intolerance differs from macrosomia due to other factors. Newborns of mothers with glucose intolerance tend to have:

More total body fat: Increased fat deposition is a hallmark of macrosomia associated with maternal glucose intolerance.
Larger shoulder and upper-extremity circumferences: Disproportionate growth of the shoulders and upper extremities is characteristic of this type of macrosomia.
Higher upper-extremity skin-fold measurements: This indicates increased subcutaneous fat in the upper extremities.
Smaller head-to-abdominal-circumference ratios: In these newborns, the abdominal circumference tends to be larger relative to the head circumference.

This altered body shape may explain the higher incidence of shoulder dystocia in newborns of women with diabetes. The disproportionate growth of the shoulders increases the risk of impaction during vaginal delivery.

Regardless of birth weight, newborns of women with diabetes have an increased risk of shoulder dystocia, clavicular fracture, and brachial plexus palsy. This highlights that even in the absence of macrosomia, maternal diabetes increases the risk of these complications.

GDM and hyperglycemia often occur with prepregnancy obesity and excessive gestational weight gain, making it hard to distinguish their individual contributions to macrosomia. These conditions share metabolic characteristics like insulin resistance, hyperglycemia, and hyperinsulinemia. These factors create a synergistic effect, further increasing the risk of macrosomia.

Birth weight generally increases with maternal BMI. However, several issues confound this observation:

Obese women are more likely to have diabetes.
Excess weight gain during pregnancy is a risk factor for excessive fetal growth.
The risk of newborn macrosomia (>4,000 g) associated with excessive maternal weight gain is higher for obese women than for nonobese women.

Multiple studies show that GDM, obesity, and excess gestational weight gain are independently associated with macrosomia and their effects appear to be synergistic. A multivariate analysis of nearly 106,000 pregnancies showed that any of obesity, GDM, or excess weight gain increased the odds of LGA newborns by 2–2.5. When any two were present, the odds ratio (OR) was 3.5–5, and when all three were present, the OR was 5–11. This highlights the exponential increase in risk when multiple factors are present.

Due to the increasing prevalence of maternal obesity compared with diabetes, maternal obesity plays a larger role in macrosomia at a population level. With the rising rates of obesity, its contribution to macrosomia has become increasingly significant.

Other Maternal Factors

A woman with a history of giving birth to a newborn weighing more than 4,000 g is 5–10 times more likely to have a newborn weighing more than 4,500 g. A history of macrosomia is the strongest individual risk factor, even when controlling for other factors. This emphasizes the importance of considering prior obstetric history when assessing macrosomia risk.

Maternal birth weight may predict newborn weight. Women whose birth weights exceeded approximately 3,600 g are twice as likely to have newborns weighing more than 4,000 g than women whose birth weights were between approximately 2,700 g to 3,500 g. This suggests a potential genetic or familial component to fetal growth.

Two cohort studies show that multiparity and grand multiparity increase the risk of macrosomia. With each subsequent pregnancy, the risk of macrosomia may increase slightly.

Genetic factors, such as parental phenotype, also affect newborn birth weight. Tall women (≥80th percentile) have a higher risk of macrosomia than short women (≤20th percentile), even when controlled for weight. Parental height is a significant determinant of fetal growth potential.

Male newborns typically weigh more than female newborns at any gestational age and constitute a larger proportion of newborns with birth weights exceeding 4,500 g. This is likely due to hormonal and genetic factors that influence fetal growth.

Diagnosis

Accurate diagnosis of macrosomia can only be made after birth by weighing the newborn. Prenatal prediction of newborn birth weight is imprecise. Despite advancements in prenatal assessment, accurately predicting birth weight remains a challenge.

Published formulas for estimating fetal weight correlate with birth weight, but the variability of the estimate is up to 20% with most formulas. These formulas rely on biometric measurements obtained through ultrasound, but inherent limitations in accuracy exist.

Ultrasonography allows direct measurement of fetal body parts, but its accuracy in predicting macrosomia is poor. While ultrasound provides valuable information, its ability to predict macrosomia is limited by factors such as fetal position, maternal body habitus, and operator skill.

A meta-analysis found a sensitivity of 56% and specificity of 92% for predicting birth weight > 4,000 g. Ultrasound accuracy decreases with increasing fetal weight beyond 4,000 g. This indicates that ultrasound is more effective at ruling out macrosomia than ruling it in, and its accuracy diminishes as fetal weight increases.

An ultrasound-estimated fetal weight of more than 4,500 g accurately predicts birth weight > 4,500 g in only 33–44% of cases. Longitudinal ultrasound examinations or individual growth-curve modeling does not improve macrosomia prediction. Customized growth curves are no better than population-based growth curves. This suggests that routine or serial ultrasounds do not significantly improve the accuracy of macrosomia prediction.

Small studies of three-dimensional ultrasonography have shown mixed results. Formulas using biacromial diameter and macrosomic-specific formulas have shown high accuracy rates but are from single institutions with no validation studies. Further research is needed to validate these findings and assess their clinical utility.

Magnetic resonance imaging has higher sensitivity and specificity than ultrasonography but is limited by cost, discomfort, and size limitations for obese women. MRI offers improved accuracy but is not routinely used due to practical limitations.

Studies comparing ultrasound accuracy with physical examination have had inconsistent findings. Parous women seem able to predict the weight of their newborns as well as clinicians using ultrasonography or clinical palpation maneuvers. This highlights the value of incorporating patient input into the assessment of fetal weight.

Risks Associated with Macrosomia

Maternal Morbidity

The primary maternal risk is an increased risk of cesarean birth. Studies show that with birth weights > 4,500 g, the risk of cesarean birth for women attempting vaginal delivery is at least double that of controls. Macrosomia can lead to labor dystocia, necessitating cesarean delivery.

Labor protraction and arrest disorders are more frequent with macrosomia, and almost all of the increased risk of cesarean birth is attributed to labor abnormalities. This suggests that labor abnormalities are a primary driver of the increased cesarean birth rate in macrosomic pregnancies.

Inaccurate ultrasonographic prediction of macrosomia predisposes women to the diagnosis of labor abnormalities and cesarean birth independent of actual birth weight. Overestimation of fetal weight can lead to unnecessary interventions, including cesarean delivery.

Macrosomia was responsible for 10% of the overall increase in cesarean birth rates over a 7-year study period, despite no change in the true rate of newborn macrosomia (birth weight > 4,500 g) during that time. This underscores the impact of suspected macrosomia on cesarean birth rates.

The risks of postpartum hemorrhage, chorioamnionitis, and significant vaginal lacerations are elevated with macrosomia. Macrosomia can increase the risk of these complications during and after delivery.

A multivariate analysis showed that birth weight > 4,500 g was associated with significantly increased risks of chorioamnionitis (OR 2.4), shoulder dystocia (OR 7.1), third-degree or fourth-degree lacerations (OR 1.7), and postpartum hemorrhage (OR 3.1). This quantifies the increased risk of specific complications associated with macrosomia.

The risk of third-degree and fourth-degree lacerations is increased twofold to threefold with macrosomia, especially if delivery is complicated by shoulder dystocia. Shoulder dystocia significantly increases the risk of severe perineal lacerations.

Fetal Morbidity and Mortality

Macrosomia increases the risk of shoulder dystocia, which occurs in 0.2–3.0% of all vaginal deliveries. The risk increases to 9–14% when birth weight is more than 4,500 g. Shoulder dystocia is a significant obstetric emergency that can lead to fetal injury.

In the presence of maternal diabetes, a birth weight of 4,500 g or more has been associated with rates of shoulder dystocia from 20% to 50%. Maternal diabetes further increases the risk of shoulder dystocia in macrosomic pregnancies.

The fetal injuries most commonly associated with macrosomia and shoulder dystocia are fracture of the clavicle and damage to the nerves of the brachial plexus, specifically at vertebrae C5 and C6, which can produce Erb–Duchenne paralysis. These injuries can result in long-term morbidity for the newborn.

Fracture of the clavicle complicates 0.4–0.6% of all births. For macrosomic newborns, the risk of clavicular fracture is increased approximately 10-fold. Macrosomia significantly increases the risk of clavicular fracture.

Most instances of shoulder dystocia occur unpredictably among newborns of normal birth weight, and most macrosomic newborns do not experience shoulder dystocia. While macrosomia increases the risk, shoulder dystocia can occur in newborns of any size.

The rate of neonatal brachial plexus palsy is low, with an incidence of both transient and persistent neonatal brachial plexus palsy of 1.5 per 1,000 total births. Brachial plexus palsy can result in weakness or paralysis of the affected arm.

Case–control studies demonstrate that the risk of brachial plexus palsy among newborns delivered vaginally is increased 18-fold to 21-fold when birth weight exceeds 4,500 g, with absolute rates between 2.6% and 7%. Macrosomia is a significant risk factor for brachial plexus palsy.

Brachial plexus palsy can occur without shoulder dystocia or with cesarean birth. While shoulder dystocia is a major risk factor, brachial plexus palsy can also occur in its absence.

Most cases of brachial plexus palsy resolve without permanent disability (80–90% by age 1 year). Persistent injury is more common with higher birth weights and, in particular, birth weights more than 4,500 g. The prognosis for brachial plexus palsy is generally good, but the risk of permanent injury increases with macrosomia.

Macrosomia increases the risks of depressed 5-minute Apgar scores, hypoglycemia, respiratory problems, polycythemia, meconium aspiration, and increased rates of admission and prolonged admission (more than 3 days) to a neonatal intensive care unit. Macrosomia can lead to a variety of neonatal complications requiring specialized care.

Macrosomic newborns are more likely than normal-weight newborns to be overweight and obese later in life. Macrosomia may have long-term implications for the child's metabolic health.

Clinical Considerations and Recommendations

Accuracy of Clinical Estimates of Fetal Weight

The prediction of birth weight is imprecise via ultrasonography or clinical measurement. For suspected macrosomia, the accuracy of estimated fetal weight using ultrasound biometry is no better than that obtained with abdominal palpation. Despite technological advancements, clinical acumen remains valuable in assessing fetal weight.

Clinical palpation alone, irrespective of the obstetrician’s level of training, predicts macrosomia as accurately as any reported ultrasound method. Maternal obesity may have no effect or lead to overestimation of birth weight. Data in women with morbid obesity (BMI ≥ 40) is limited. This highlights the challenges in accurately assessing fetal weight in obese women.

Measurement of the symphysis to fundal height is commonly used to detect size discrepancies for referral to ultrasonography, but fundal height alone is a poor predictor of macrosomia. Fundal height measurement is a simple clinical tool, but its accuracy in predicting macrosomia is limited.

Retrospective studies suggest its sensitivity is 20–70%, but the specificity is > 90%, indicating it is more effective for ruling out macrosomia than ruling it in. Fundal height measurement is more useful for identifying women who are unlikely to have macrosomic newborns.

Prospective studies report sensitivity of 16–68%, specificity of 90–99%, and positive predictive values between 38% and 80% for abdominal palpation maneuvers. Clinical palpation can provide valuable information but is subject to inter-observer variability.

An algorithm of ultrasonography performed on women with clinically estimated fetal weights of 3,700 g or more did not improve positive predictive value. This suggests that routine ultrasound in women with suspected macrosomia may not significantly improve predictive accuracy.

Extrapolated estimated fetal weight based on an earlier ultrasonogram was similarly accurate to clinically estimated fetal weight and more accurate than ultrasonography in labor. Earlier ultrasound estimates may be as useful as those performed closer to delivery.

Clinical estimates of macrosomia are as predictive as those derived with ultrasonography, even in women with diabetes. This suggests that clinical assessment remains valuable even in the presence of maternal diabetes.

A parous woman’s ability to predict birth weight > 4,000 g may be as accurate as that of clinicians using clinical palpation maneuvers alone. This highlights the importance of incorporating patient input into the assessment of fetal weight.

Accuracy of Ultrasonographic Measurement in Predicting Macrosomia

Most commercially available ultrasound units have estimated fetal weight equations programmed into the system software. However, most regression formulas have significant errors when the newborn is predicted to be macrosomic. These formulas are based on biometric measurements and may not accurately predict weight in macrosomic newborns.

For example, the Hadlock formula has a mean absolute percent error of 13% for newborns weighing > 4,500 g, compared with 8% for nonmacrosomic newborns, and the absolute error increases with increasing estimated fetal weight. The Hadlock formula is a commonly used ultrasound formula, but its accuracy decreases in macrosomic newborns.

Ultrasonography is less effective at identifying newborns with birth weights more than 4,500 g. The sensitivity of ultrasound decreases as fetal weight increases.

Among women without diabetes, ultrasound biometry used to detect birth weight > 4,500 g has a sensitivity of only 10–45%, a specificity of 57–99%, a positive predictive value of 11–44%, and a negative predictive value of 92–99%. Ultrasound is more effective at ruling out macrosomia than ruling it in.

When birth weight exceeds 4,500 g, only 40–60% of newborns weigh within 10% of the ultrasonography-derived estimate. Ultrasonography estimates tend to be less accurate in macrosomic newborns.

Estimations derived from ultrasonography most commonly overestimate the actual birth weight. Overestimation of fetal weight can lead to unnecessary interventions.

Key sources of inaccuracy include large intraobserver and interobserver variability or technical difficulties obtaining accurate fetal measurements in late gestation. Accurate ultrasound measurements can be challenging to obtain, particularly in late gestation.

No single formula based on ultrasound biometry performs significantly better than others for the detection of macrosomia > 4,500 g. No single ultrasound formula consistently outperforms others in predicting macrosomia.

Estimates based on three or four biometric parameters performed better than estimates based on the abdominal circumference alone. Incorporating multiple biometric parameters may improve accuracy.

Ultrasonographic determination of fetal weight appears to predispose women to the diagnosis of labor abnormalities and cesarean birth independent of actual birth weight. Overestimation of fetal weight can lead to unnecessary interventions.

A prospective study showed that the cesarean birth rate was doubled when ultrasonography-derived estimated fetal weight was > 4,000 g versus < 4,000 g, although actual birth weight of 4,000 g or more was seen in 56% and 30% of individuals, respectively. This highlights the impact of suspected macrosomia on cesarean birth rates.

A retrospective cohort study showed an adjusted OR of cesarean birth of $2.1$ (95% CI, 1.06 to 4.3) in women who received an ultrasound examination within 1 month of delivery versus those who did not. This suggests that late-term ultrasound may increase the risk of cesarean birth.

A Maternal-Fetal Medicine Units Network analysis showed an adjusted OR of cesarean birth of $1.44$ (95% CI, 1.31 to 1.58) for women with an ultrasonography-derived estimated fetal weight. The OR increased to $2.15$ (95% CI, 1.55 to 2.98) when the ultrasonography-derived estimated fetal weight was > 4,000 g versus < 3,500 g. This quantifies the association between ultrasound-estimated fetal weight and cesarean birth risk.

A randomized controlled trial demonstrated that routine ultrasound examinations at 18 weeks of gestation and an additional ultrasound examination at 33 weeks of gestation resulted in a slight reduction in induction of labor and scheduled cesarean birth for suspected macrosomia, but this study failed to demonstrate any significant differences in perinatal outcomes. Routine ultrasound may not significantly improve perinatal outcomes in suspected macrosomia.

Similar to clinical estimates of fetal weight, ultrasonography can be used most effectively as a tool to rule out macrosomia, which may help avoid maternal and fetal morbidity. Ultrasound is more useful for identifying women who are unlikely to have macrosomic newborns.

Effective Interventions for Treating or Preventing Suspected Macrosomia

Interventions shown to reduce macrosomia include exercise during pregnancy, a low glycemic diet in women with GDM, and prepregnancy bariatric surgery in women with class 2 or class 3 obesity. These interventions address modifiable risk factors for macrosomia.

A meta-analysis of 28 randomized clinical trials found a decreased risk of macrosomia or LGA newborns (OR 0.69; 95% CI, 0.55 to 0.86) with supervised prenatal exercise, without an increase in small for gestational age (SGA) or preterm delivery. Supervised prenatal exercise can reduce the risk of macrosomia without increasing other adverse outcomes.

Women randomized to exercise gained less weight and had a 20% lower rate of cesarean birth. Exercise during pregnancy can have multiple benefits, including reduced weight gain and cesarean birth risk.

Most studies included normal-weight women without GDM. The benefits of exercise may be most pronounced in normal-weight women without GDM.

A newer meta-analysis of 15 high-quality randomized controlled trials found that exercise-only interventions reduced macrosomia by 39% (OR 0.61; 95% CI, 0.41–0.92). Exercise alone can significantly reduce the risk of macrosomia.

Combining more than one type of exercise further reduced the odds of macrosomia (OR 0.46; 95% CI, 0.29–0.73); SGA and preterm delivery were not increased. Combining aerobic and strength-conditioning exercises may be particularly beneficial.

Women without contraindications should be encouraged to engage in aerobic and strength-conditioning exercises during pregnancy. Exercise is a safe and effective intervention for reducing macrosomia risk.

Dietary interventions without exercise have shown modest-to-no benefit in preventing macrosomia in women without diabetes. Dietary interventions alone may not be sufficient to prevent macrosomia in women without diabetes.

However, in a subgroup analysis of overweight women or women with GDM, combined diet plus exercise resulted in a 15% reduced risk of macrosomia (RR 0.85; 95% CI, 0.73 to 1.00). Combined diet and exercise may be more effective in preventing macrosomia in certain populations.

Control of maternal hyperglycemia reduces the risk of macrosomia; therefore, maternal glucose management is recommended for pregnancies complicated by diabetes. Effective glucose management is crucial for reducing macrosomia risk in diabetic pregnancies.

One clinical trial suggests adding insulin to diet therapy may benefit women at risk of LGA newborns diagnosed between 29 weeks of gestation and 33 weeks of gestation. Insulin therapy may be beneficial in women with poorly controlled glucose levels.

The addition of insulin therapy decreased the likelihood of birth weight more than the 90th percentile from 45% among those treated with diet only to 13% among those receiving insulin (P < .01). Insulin therapy can significantly reduce the risk of LGA newborns in women with diabetes.

Trials have confirmed that controlling maternal hyperglycemia is important in preventing macrosomia among women with gestational diabetes. Effective glucose control is essential for preventing macrosomia in women with GDM.

In the Australian Carbohydrate Intolerance Study in Pregnant Women trial, the risk of a birth weight > 4,000 g was reduced from 21% to 10% (RR, 0.47; 95% CI, 0.34 to 0.64; P≤.001). This trial demonstrated the effectiveness of glucose control in reducing macrosomia risk.

Similarly, in a large multicenter randomized trial of treatment of mild GDM, the risk of a birth weight > 4,000 g was reduced from 14.3% to 5.9% (RR, 0.41; 95% CI, 0.26–0.66; P≤.001). This trial further supports the benefits of glucose control in preventing macrosomia.

A meta-analysis compared low glycemic diets to usual care in 300 women with GDM and found a 73% decrease in macrosomia (OR, 0.27; 95% CI, 0.10 to 0.71). Low glycemic diets can be effective in reducing macrosomia risk in women with GDM.

Diets that included additional dietary fiber further decreased the risk. Dietary fiber may further enhance the benefits of a low glycemic diet.

For women with class 2 or class 3 obesity (BMI > 35 or > 40, respectively), having had bariatric surgery before pregnancy is associated with decreased odds of GDM (OR 0.31 and 0.47, respectively) and LGA newborns (OR 0.40 and 0.46, respectively) in meta-analyses of observational studies. Prepregnancy bariatric surgery can reduce the risk of GDM and LGA newborns in obese women.

However, previous bariatric surgery also was associated with an increase in SGA newborns and a possible increase in preterm delivery. Bariatric surgery may also increase the risk of SGA newborns and preterm delivery.

The largest study showed lower rates of LGA newborns among those who had undergone bariatric surgery (4.2% versus 7.3%; OR 0.6), but the rate of SGA newborns was higher (5.2% versus 3.0%: OR 2.0) as was preterm birth (9.7% versus 6.1%: OR 1.7). This study highlights the potential benefits and risks of bariatric surgery in pregnancy.

Among 670 women who had bariatric surgery, there were lower rates of GDM (1.9% versus 6.8%; OR 0.25) and LGA newborns (8.6% versus 22.4%; OR 0.33). Bariatric surgery can significantly reduce the risk of GDM and LGA newborns.

Rates of preterm birth were not significantly different. However, there was an increase in SGA newborns (15.6% versus 7.6%; OR 2.20) and a nonsignificant increase in stillbirth (1.7% versus 0.7%; P=0.06). Bariatric surgery may increase the risk of SGA newborns and stillbirth.

Given the health benefits, particularly for pregnancy outcomes, prepregnancy counseling of morbidly obese patients regarding the benefits and risks of bariatric surgery is recommended. Prepregnancy counseling is essential for women considering bariatric surgery.

Role for Induction of Labor in the Management of Term Patients with Suspected Macrosomia

Evidence from retrospective cohort studies examining induction of labor for suspected macrosomia is inconsistent. The evidence regarding induction of labor for suspected macrosomia is conflicting.

Some reports show that induction increases the risk of cesarean birth without reducing shoulder dystocia or newborn morbidity. Induction may increase the risk of cesarean birth without clear benefits.

Others suggest a slight decrease or no effect on the risk of cesarean birth and no difference in shoulder dystocia rates with induction. Some studies suggest that induction does not increase the risk of cesarean birth or shoulder dystocia.

Two randomized clinical trials have examined induction of labor at term for ultrasonography-derived estimated fetal weight >90th percentile. These trials provide the most reliable evidence regarding induction of labor for suspected macrosomia.

One trial randomized 273 women at ≥38 weeks of gestation with estimated fetal weights between 4,000 g and 4,500 g to either planned induction or expectant management. Cesarean birth rates and shoulder dystocia were similar. This trial found no significant difference in cesarean birth rates or shoulder dystocia with induction.

In a second European trial, 822 women with estimated fetal weights above the 95th percentile at 37–38 weeks were randomized to induction within 3 days or expectant management. This trial provides further evidence regarding induction of labor for suspected macrosomia.

Induction reduced the risk of shoulder dystocia from 4% to 1% (RR, 0.32; 95% CI, 0.12 to 0.85). Cesarean birth rates were similar. This trial found that induction reduced the risk of shoulder dystocia without increasing cesarean birth rates.

Two meta-analyses, including these trials and smaller unpublished trials involving 1,190 women with suspected macrosomia, have been published. These meta-analyses provide a comprehensive overview of the evidence regarding induction of labor for suspected macrosomia.

Compared with expectant management, induction reduced the risk of shoulder dystocia (RR, 0.60; 95% CI, 0.37 to 0.98) and any type of fracture (RR, 0.20; 95% CI, 0.05 to 0.79) with no change in the risk of cesarean birth (RR 0.91; 95% CI, 0.76 to 1.09) or instrumental delivery (RR, 0.86; 95% CI, 0.65 to 1.13). These meta-analyses suggest that induction may reduce the risk of shoulder dystocia and fracture without increasing cesarean birth rates.

The American College of Obstetricians and Gynecologists recommends avoiding delivery before 39 0/7 weeks of gestation unless medically indicated. This guidance emphasizes the importance of avoiding early delivery unless there is a clear medical indication.

Whether intervention is better than expectant management for suspected LGA fetuses and the gestational age at which delivery should be performed are unclear. The optimal management of suspected LGA fetuses remains uncertain.

Suspected macrosomia or LGA fetus is not an indication for induction of labor before 39 0/7 weeks of gestation because there is insufficient evidence that benefits of reducing shoulder dystocia risk would outweigh the harms of early delivery. ACOG does not recommend induction of labor for suspected macrosomia or LGA fetus before 39 weeks of gestation.

When to Consider Scheduled Cesarean Birth for Suspected Macrosomia

The goal of scheduled cesarean birth for suspected macrosomia is to reduce fetal or maternal morbidity. Scheduled cesarean birth may be considered to reduce the risk of complications associated with macrosomia.

Though morbidity increases with birth weights > 4,000 g, most births of macrosomic newborns are uncomplicated. Most macrosomic newborns are delivered without complications.

In a Norwegian study, shoulder dystocia occurred in 2.6% of newborns weighing 4,000–4,500 g, 6.7% of newborns weighing 4,500–5,000 g, and 15% of newborns weighing > 5,000 g. The risk of shoulder dystocia increases with increasing birth weight.

Transient brachial plexus palsy is estimated to occur in 1–17% of births complicated by shoulder dystocia, with 3–33% persisting at