Chapter 5. Fetal Growth II: Macrosomia

This chapter covers the fetal growth condition known as macrosomia, or large fetus. Detecting larger fetuses is crucial because of potential complications for both mother and baby.

Objectives

By the completion of this chapter the participant will be able to:

Define and differentiate LGA (large for gestational age) and macrosomia

List the risk factors for development of macrosomia

List the perinatal risks associated with macrosomia

Describe current recommendations for management of LGA and macrosomia

Macrosomia is associated with an elevated risk of labor dystocia, delivery complications, birth trauma affecting both mother and infant, as well as increased morbidity and mortality.^{1. 2} Shoulder dystocia during delivery may lead to neonatal injury (such as brachial plexus palsy). The likelihood of caesarean section or assisted delivery due to fetal distress or failure to progress is two to three times higher. Maternal complications are more frequent with the delivery of larger infants. Postnatally, affected newborns may experience impaired physiological adaptation and altered glucose regulation.

Definitions – Large babies

There are two terms for excessive fetal growth:^{2, 3}

Figure 1. Definitions used to describe large babies.

· LGA (Large for Gestational Age): Birth weight at or above the 90th percentile for gestational age, which can include preterm infants with weights exceeding normal for their gestation. Biometric measures and estimated fetal weight (EFW) both consistently track higher percentiles.

· Macrosomia: Defined as birth weight over 4000g or 4500g, independent of gestational age. Unlike LGA, macrosomia often suggests a pathological cause. Definitions differ (ACOG, ISUOG), and the terms are sometimes used interchangeably despite the technical difference.^2,3

Perinatal morbidity and Mortality

Figure 2. Perinatal morbidity and mortality associated with macrosomia.

Implications for LGA

LGA is defined as birth weight above the 90th percentile, determined post-delivery. While LGA increases the risk of shoulder dystocia, this can also occur in babies of normal size. Garchar's study found that EFW above the 90th percentile correlates with dystocia but has low predictive value, likely because of differences in maternal pelvic anatomy.⁴

Figure 3. General implications for LGA babies.

Neonatal complications of fetal macrosomia

Macrosomia is generally defined as birth weight over 4000 g or above the 95th percentile on growth charts, though definitions vary. In the United States, it occurs in over 10% of pregnancies; prevalence differs internationally. Macrosomia is linked to gestational diabetes, particularly when poorly managed. While higher maternal weight gain correlates with repeat C-section after planned VBAC, studies have not shown a direct link between increased maternal weight gain and higher macrosomia rates.^1,5

Macrosomia Overview

Figure 4. Macrosomia overview; prevalence and risk factors.

Additonal risk factors for macrosomia:⁵

Figure 5. Additional risk factors for macrosomia.

· Gestational diabetes, particularly elevated fasting plasma glucose after 30 weeks, increases the risk.

· Greater maternal weight gain is linked to higher rates of repeat C-section in planned VBAC (vaginal birth after Ceserean section) cases.

· While maternal obesity is an independent risk factor for fetal macrosomia, it does not always correlate with gestational diabetes, nor are all women with GDM obese.

· A study (n=882) by Poehlmann found no difference between 32-week and 36-week ultrasounds in predicting LGA, regardless of maternal obesity.⁶

Table 1. Comparison of clinical risk factors and outcomes for macrosomia.

This chart highlights risk factors for macrosomia and their links to shoulder dystocia. The top predictors of macrosomia are a previous macrosomic infant and maternal weight gain. Fetal macrosomia has a 16.1 odds ratio (16.1 higher risk than normal sized babies) for causing shoulder dystocia. ⁵

Predicting Macrosomia

Fetal macrosomia may be predicted through clinical assessment and ultrasonography. However, the accuracy of ultrasound-derived estimated fetal weight (EFW) decreases as gestation advances toward term, resulting in a higher incidence of false-positive results. Ultrasound detects macrosomia in uncomplicated pregnancies with a post-test probability of 15–79%, while palpation yields 40–52%. Ultrasound is more likely to identify macrosomia, though accuracy varies widely, while palpation predicts large fetuses about half the time. In diabetic patients, both methods have approximately 60% accuracy. There is no reliable method to predict shoulder dystocia, which can occur even with normal birth weight and without risk factors. ^{1, 4, 5.}

Figure 6. Considerations for predicting macrosomia.

Macrosomia Prevention

Preventive approaches to macrosomia include:⁵

· Managing nutritional habits, physical activity, and body mass index (BMI)

· Reviewing family history for diabetes mellitus (DM) and gestational diabetes mellitus (GDM)

· Meta-analyses indicate that GDM is independently associated with macrosomia

· Higher placental levels of insulin growth factor are correlated with increased risk of macrosomia

Figure 7. List of some prevention strategies for macrosomia.

Ultrasound detection of macrosomia

Various ultrasound techniques have been used to detect and predict fetal macrosomia. Assessments of fetal soft tissue adiposity in regions such as the shoulder, abdomen, thigh, and peri-orbital area have not demonstrated superiority over conventional biometric parameters. Tests evaluating fetal well-being, except in cases of gestational diabetes mellitus (GDM), are generally considered invalid for predicting macrosomia with the exception of AFI measurement. Combined use of amniotic fluid index (AFI) with estimated fetal weight (EFW) may enhance the accuracy of macrosomia prediction.⁵

Figure 8, Image 1. Methods for ultrasound detection of macrosomia.

Figure 9. Efficacy of ultrasound monitoring for macrosomia

Inaccuracies associated with ultrasound detection of macrosomia

Ultrasound may not always provide precise estimates of EFW due to potential inaccuracies in prediction formulas and technical measurements. The limitations of an ultrasound exam can include factors such as maternal obesity, oligohydramnios, equipment constraints, and the experience level of the POCUS provider.

Figure 10. Inaccuracies of determining EFW to detect macrosomia.

Avoiding ultrasound inaccuracy in detecting macrosomia

The most reliable method for detecting macrosomia is the evaluation of fetal abdominal circumference (AC). The maximum random error for ultrasound weight prediction is 100g/Kg (10%). Measurements should follow these specific criteria:⁵

· Kidneys are not visible.

· Only one pair of ribs is shown.

· Three ossification points of the vertebrae are present.

· The abdominal view includes the stomach and the umbilical vein at the level of the portal sinus.

· Measurements are taken along the outer skin using an ellipse.

· Place the initial caliper at the skin line near the vertebra.

Complications of macrosomia

Fetal macrosomia can lead to complications such as fetal trauma, shoulder dystocia, failed assisted delivery, and increased risk of C-section.

Figure 11. Images illustrating complications of macrosomia at birth.

Shoulder Dystocia

Shoulder dystocia is a true obstetric emergency, offering the birth provider only minutes to deliver the fetal trunk once the head emerges. It is a significant cause of fetal morbidity and mortality. It is for this reason that detecting macrosomia can be useful for prevention. Although shoulder dystocia can occur in normal size babies for reasons other than large size, it is critical for the provider to recognize the potential before or during labor to have appropriate emergency back-up readily available.

Shoulder Dystocia Risks:^1,5

· 0.2% risk in average-sized fetuses

· Risk rises to 5% for fetuses weighing 4000–4500g, and 30% above 4500g

· Clavicle fracture is a risk factor

· Brachial plexus injury occurs in 4% to 40% of cases

· Hypoxic-ischemic brain injury happens in 0.5% to 23% of cases, especially if head-to-body delivery time exceeds 5 minutes

· Fetal mortality is seen in 0.4% of cases, mainly when all maneuvers fail

Figure 12. Implications of shoulder dystocia.

Macsrosomia _ Maternal complications

Maternal complications during delivery of a macrosomic infant include: ^1,5

· OASIS (Obstetric Anal Sphincter Injury), leading to rectovaginal fistula

· Postpartum hemorrhage

· Labor dystocia, often requiring oxytocin

· Higher risk of fourth-degree perineal tears and vaginal lacerations

· Hemorrhage due to uterine atony

Figure 13. List of maternal complications associated with macrosomia.

Macrosomia delivery timing

The optimal delivery method for fetal macrosomia is debated. Some groups question the benefit of elective C-section, but ACOG advises prophylactic C-section if estimated fetal weight exceeds 5000 grams (or 4500 grams in diabetic pregnancies).⁵

Figure 14. Macrosomia management.

Induction of labor for macrosomia

Labor induction has been suggested to limit fetal growth (about 280g/week) and potentially reduce dystocia, birth trauma, and C-section rates, though evidence is inconclusive.⁵A primary reason for this is what has been described earlier in this chapter as the relative inaccuracy of determining fetal weight closer to term in pregnancy. Since induction of labor has its own risks, these risks must be compared to the risk of either overestimating fetal weight by ultrasound (unnecessary induction) or not inducing a truly macrosomic baby before it grows even larger.

Figure 15. Induction of labor for macrosomia: Cochrane review findings by Boulvain.⁷

A Cochrane review by Boulvain et al in 2023 reported that induction of labor shortly before term reduces shoulder dystocia and neonatal fractures for macrosomic babies. The evidence did not find that induction of labor increases the risk of C/Section or instrumental delivery (as had previously been thought), but may, in fact, reduce these.⁷

SUMMARY

One of the primary reasons for performing ultrasound in pregnancy is to monitor fetal growth and well-being.

Ultrasound is useful in identifying those babies who are at increased risk for stillbirth and neonatal mortality/morbidity.

Large babies are at increased risk for neonatal morbidity and mortality, as are mothers who give birth to large babies.

Early recognition is important to more carefully monitor fetal growth patterns.

Appropriate delivery timing should be offered to those at risk for significant a macrosomic baby.

NOTE: Although shoulder dystocia can occur with normal sized babies, the midwife or obstetric provider should always be prepared for shoulder dystocia, especially if macrosomia is suspected, and ensure adequate assistance is available.

Women's Point-of-Care Ultrasound: Fetal Biometry and Growth

Chapter 5. Fetal Growth II. Macrosomia