What are the definition, risk factors, diagnostic criteria, and management recommendations for fetal macrosomia?

Fetal Macrosomia: Clinical Management Guide

Definition and Diagnostic Criteria

Fetal macrosomia is defined as birth weight ≥4,000 g (8 lb 13 oz), with critical thresholds at 4,500 g and 5,000 g that guide delivery decisions. 1, 2

• Ultrasound estimation of fetal weight performs no better than clinical palpation using Leopold's maneuvers for diagnosing macrosomia 3
• The 4,500 g threshold is particularly important in diabetic pregnancies, while 5,000 g is the key decision point for non-diabetic women 1, 3

Risk Factors

Maternal Factors

• Diabetes mellitus (pregestational and gestational) is the most significant modifiable risk factor, with untreated gestational diabetes carrying up to 20% risk of macrosomia 1
• Maternal obesity and excessive gestational weight gain are strongly associated with macrosomia 1, 3, 2
• Previous delivery of a macrosomic infant significantly increases recurrence risk 2
• Grand multiparity, maternal age <17 years, and postmaturity (>40 weeks gestation) 1, 4
• Positive 50-g glucose screen with negative 3-hour glucose tolerance test 1
• Male fetus, maternal birth weight, maternal height, and ethnicity 1

Diabetic vs Non-Diabetic Populations

• In diabetic pregnancies, 15-45% of newborns may be macrosomic compared to 12% in non-diabetic pregnancies 5
• Approximately 26% of mothers delivering macrosomic infants have abnormal glucose tolerance testing 6

Complications and Risks

Maternal Complications

• Cesarean delivery risk doubles when estimated fetal weight exceeds 4,500 g 1
• Postpartum hemorrhage, vaginal lacerations, and operative delivery complications 6, 5
• Higher rates of gestosis in pregnancies with macrosomic infants 6

Fetal/Neonatal Complications

• Shoulder dystocia is the most serious complication, occurring in 1.4% of all vaginal deliveries but escalating dramatically with increasing birth weight 1:
  • 9.2-24% risk when birth weight >4,500 g in non-diabetic pregnancies 1, 3
  • 19.9-50% risk when birth weight >4,500 g in diabetic pregnancies 1, 7, 8
• Brachial plexus injury risk increases 18-21 fold when birth weight exceeds 4,500 g 1, 7
• Clavicular fracture risk increases approximately 10-fold 1, 7
• Increased risk of childhood obesity and type II diabetes later in life 5
• Higher neonatal intensive care unit admission rates 5

Prevention Strategies

Glycemic Control

• In diabetic pregnancies, dietary intervention with insulin for early macrosomia (29-33 weeks gestation) may decrease likelihood of birth weight >4,500 g 1
• Tight glycemic control is essential, with particular attention when fasting glucose exceeds 105 mg/dL 7

Weight Management

• Excessive gestational weight gain is linked to increased macrosomia risk, though no data support dietary restrictions in obese non-diabetic women 1, 3
• Exercise during pregnancy has been shown to reduce macrosomia risk 2

Timing Considerations

• For diabetic pregnancies, delivery at 38 weeks prevents progressive macrosomia without increasing cesarean rates 7
• No data support delivery before 38 weeks in gestational diabetes without maternal or fetal complications 7
• Post-term gestation (>40 weeks) increases macrosomia risk and warrants intensified fetal monitoring 7, 8

Management Algorithm

Non-Diabetic Pregnancies

Estimated Fetal Weight <5,000 g:

• Trial of labor is safe and recommended 3
• Large cohort studies confirm safety of vaginal delivery for estimated weights >4,000 g 1, 3
• Prophylactic cesarean delivery is not cost-effective below this threshold 3, 9

Estimated Fetal Weight ≥5,000 g:

• Prophylactic cesarean delivery should be considered 1, 3
• Cost-effectiveness analyses support this threshold for cesarean consideration 3, 9

Diabetic Pregnancies

Estimated Fetal Weight <4,500 g:

• Trial of labor is reasonable 3

Estimated Fetal Weight ≥4,500 g:

• Prophylactic cesarean delivery is advisable due to markedly higher shoulder dystocia rates (19.9-50%) 3, 7, 8

Induction of Labor

Do NOT induce labor solely for suspected macrosomia 1, 3, 8, 2

• Induction doubles the cesarean delivery risk without reducing shoulder dystocia or improving neonatal outcomes 3, 7
• Randomized trials show similar cesarean rates between induction (19.4%) and expectant management (21.6%) 1, 3
• Induction before 39 weeks carries additional neonatal respiratory complications 3

Intrapartum Management

During Active Labor:

• If second stage is prolonged or arrest of descent occurs with estimated fetal weight >4,500 g, convert to cesarean delivery 3
• Avoid mid-pelvic operative vaginal delivery except in extreme emergencies; cesarean is indicated for mid-pelvic arrest with suspected macrosomia 1, 3, 4

Cesarean Technique:

• Ensure uterine incision is sufficiently large to facilitate delivery and avoid difficult extraction 3

Vaginal Birth After Cesarean (VBAC):

• Suspected macrosomia does not contraindicate trial of labor after cesarean 3

Critical Counseling Points

• Inform patients that shoulder dystocia can occur unpredictably even in normal birth weight infants 1
• Discuss the 10-fold clavicular fracture risk and 18-21-fold brachial plexus injury risk when birth weight exceeds 4,500 g 1, 7
• Teach mothers to monitor fetal movement during the last 8-10 weeks and report any reduction immediately 7
• Explain that ultrasound weight estimation has significant limitations and is no more accurate than clinical examination 3

Common Pitfalls to Avoid

• Do not rely solely on ultrasound for weight estimation; clinical palpation is equally accurate 3
• Do not induce labor before 39 weeks for suspected macrosomia alone 3, 2
• Do not perform elective cesarean for estimated weights <5,000 g (non-diabetic) or <4,500 g (diabetic) based on cost-effectiveness data 3, 9
• Do not attempt mid-pelvic operative vaginal delivery when macrosomia is suspected 1, 3