What are the criteria for defining a large for gestational age (LGA) fetus and how is it diagnosed?

Large for Gestational Age (LGA) Fetus: Diagnostic Criteria and Methods

Large for gestational age is defined as an estimated fetal weight (EFW) or abdominal circumference (AC) at or above the 90th percentile for gestational age, diagnosed by ultrasound biometry using population-based growth standards.

Diagnostic Criteria

The 90th percentile threshold is the universally accepted cut-off for defining LGA, mirroring the inverse definition used for fetal growth restriction (10th percentile) 1, 2, 3. This threshold identifies fetuses at increased risk for adverse maternal and neonatal outcomes, though the clinical significance varies based on additional factors 2, 3.

Specific Biometric Parameters

• Estimated fetal weight ≥90th percentile is the primary diagnostic criterion for LGA 2, 3, 4
• Abdominal circumference ≥90th percentile can also be used as a standalone marker, as it reflects fetal adiposity and hepatic glycogen stores 2
• Macrosomia (absolute birth weight >4000 g or >4500 g) represents a related but distinct concept from LGA, which is percentile-based 3, 4, 5

Growth Standard Selection

• Population-based standards (Hadlock curves) are recommended for consistency with fetal growth restriction diagnosis, though they have lower sensitivity (30–50%) for detecting LGA compared to customized standards 6, 1, 3
• The NICHD and Fetal Medicine Foundation (FMF) standards demonstrate superior sensitivity (72–77%) compared to Hadlock (30%) for LGA detection, but at the cost of lower specificity (85–88% vs. 97.5%) and higher false-positive rates 4
• Customized growth charts (adjusting for maternal height, weight, parity, and ethnicity) improve detection rates, particularly in women with low (<18.5 kg/m²) or high (>30 kg/m²) body mass index 3

Diagnostic Methods

Ultrasound Timing and Technique

• Late third-trimester ultrasound (35+0 to 38+0 weeks gestation) provides the most clinically relevant assessment for delivery planning 3
• Comprehensive biometry should include biparietal diameter, head circumference, abdominal circumference, and femur length to calculate EFW using validated formulas 3, 4
• Scans performed specifically for suspected large fetus (based on clinical examination) have higher positive predictive value (50%) compared to scans done for any indication (41%), though with increased false-positive rates (36% vs. 8.6%) 3

Performance Characteristics

The diagnostic accuracy of ultrasound for LGA varies significantly:

• Sensitivity ranges from 30% to 77% depending on the growth standard used and maternal body mass index 3, 7, 4
• Specificity is generally high (85–97%), meaning most fetuses identified as LGA truly are large 3, 7, 4
• Positive predictive value is modest (41–52%), indicating that roughly half of fetuses diagnosed as LGA will actually be LGA at birth 2, 3, 7
• Detection improves with increasing maternal BMI: sensitivity increases from 55.6% in underweight women to 67.8% in obese women 3

Clinical Examination Triggers

• Fundal height >3 cm above gestational age (by McDonald rule) should prompt ultrasound evaluation for possible LGA 1
• Fundal height >90th percentile on customized charts warrants ultrasound assessment 1
• Rapid fundal height increase (>10 mm over 14 days) may indicate accelerated fetal growth 1

Associated Risk Factors and Evaluation

When LGA is diagnosed on second-trimester anatomy scan (18–22 weeks):

• The risk of developing gestational diabetes mellitus increases 2.5-fold (adjusted OR 2.54,95% CI 1.29–5.03), independent of maternal age, obesity, and parity 2
• The risk of LGA at birth increases nearly 7-fold (adjusted OR 6.85,95% CI 3.60–13.05) 2
• Maternal factors associated with both second-trimester LGA and adverse outcomes include advanced maternal age, obesity, and multiparity 2

Maternal Conditions to Assess

• Screen for gestational diabetes with early glucose tolerance testing when LGA is detected, as 6% of women with second-trimester LGA will develop GDM 2
• Evaluate for pre-existing diabetes or impaired glucose metabolism 5
• Assess maternal obesity (BMI >30 kg/m²), which both increases LGA risk and improves ultrasound detection accuracy 2, 3

Critical Clinical Pitfalls

• Do not rely solely on clinical examination (fundal height, Leopold maneuvers) for LGA diagnosis, as ultrasound provides objective biometry despite imperfect accuracy 3, 7
• Recognize that the sonographic diagnosis of LGA—even when incorrect—significantly alters clinical management, increasing rates of diagnosed labor abnormalities (30% vs. 19%), epidural use (74% vs. 57%), and cesarean delivery (53% vs. 32%) 7
• Avoid routine induction of labor based solely on suspected LGA/macrosomia, as current high-quality evidence and guidelines do not support this intervention for reducing maternal or neonatal morbidity 5
• Understand that ultrasound accuracy decreases with increasing fetal size, and estimated fetal weight has a margin of error of ±10–15% 3, 4
• Be aware that false-positive LGA diagnoses lead to unnecessary interventions, including elective cesarean sections, particularly when using more sensitive but less specific growth standards 7, 4