Fetal macrosomia and large for gestational age on ultrasound: definition, limits of estimated weight and reporting

Fetal macrosomia is a weight above a fixed cut-off, usually 4,000 or 4,500 g at term, whereas a large for gestational age (LGA) fetus exceeds a chart limit for its age, typically the 90th percentile of estimated fetal weight or abdominal circumference. Ultrasound raises the suspicion, but confirmation comes from birth weight, so the report should give the estimate together with its margin of error.

Fetal macrosomia or LGA: what is the difference

• LGA: ISUOG describes estimated fetal weight (EFW) or abdominal circumference (AC) above the 90th percentile, noting that the literature also uses the 95th percentile, 97th percentile, +2 SD and Z-scores. The result depends on the chart.
• Macrosomia: a fixed weight cut-off (4,000 or 4,500 g), with no single definition across sources.
• The two do not always overlap: a fetus at 34 weeks can be LGA well below 4,000 g, and one at 40 weeks weighing 4,100 g passes the 4,000 g cut-off yet stays below the 90th percentile of Hadlock (4,234 g) and WHO (4,135 g).

The limits of estimated fetal weight

• ISUOG reports errors of 10 to 15% as common, relatively larger in exactly the small and large fetuses. At 15%, an EFW of 4,000 g corresponds to a range of 3,400 to 4,600 g.
• In a meta-analysis of low-risk and mixed-risk populations (Moraitis 2020), EFW above 4,000 g or the 90th percentile and AC above 36 cm or the 90th percentile had sensitivity above 50% for macrosomia at birth, with positive likelihood ratios of 8.74 and 7.56.
• Performance for shoulder dystocia is far lower: 22% sensitivity and a positive likelihood ratio of 2.12.
• ACOG regards prediction of weight as imprecise, both by ultrasound and clinically.
• Serial scans should generally be at least 3 weeks apart to reduce false positives (ISUOG).

Polyhydramnios and diabetes

SMFM defines polyhydramnios in singleton pregnancy as a deepest vertical pocket of 8 cm or more or an amniotic fluid index of 24 cm or more. The same document names maternal diabetes and fetal anomalies as the two most common pathologic causes, and most mild cases as idiopathic. With a large fetus and increased fluid, describe both findings, review the anatomy and leave the correlation with glycemic assessment to the obstetrician. ACOG lists optimizing glycemic control among the measures related to macrosomia.

Mode of delivery: where the report stops

ACOG states that scheduled cesarean may be beneficial with EFW above 5,000 g without diabetes and 4,500 g or more with diabetes, and that suspected macrosomia is not an indication for induction before 39 weeks. Thresholds vary across sources (Calderon 2006 cites 4,000 to 5,000 g). That decision is obstetric and individual: the report gives the weight and its limits of precision, without indicating mode of delivery.

What the report should include

• Biometric measurements, EFW in grams, percentile or Z-score, formula and chart used.
• AC percentile, when it drives the classification.
• Margin of error of the estimate.
• Amniotic fluid (deepest vertical pocket and/or index), with its classification.
• Comparison with the previous scan, respecting the interval.
• A descriptive conclusion, for example: "estimated fetal weight above the 90th percentile, characterizing a large for gestational age fetus" or "consistent with fetal macrosomia".

Common mistakes

• Using LGA and macrosomia as synonyms.
• Stating the weight as an exact value, with no margin of error.
• Omitting the formula and chart, which prevents comparison between scans.
• Plotting EFW on a newborn weight chart, which ISUOG advises against.
• Indicating mode of delivery or predicting shoulder dystocia in the report.

See also our articles on fetal biometry and estimated weight, amniotic fluid and the third-trimester report.

How Laudário helps

In the second and third trimester obstetric template of Laudário:

• The weight sentence comes with the Hadlock formula used, the measurements and the 15% margin.
• The conclusion can classify the percentile, including LGA and macrosomia (EFW of 4,000 g or more and/or AC above the 95th percentile).
• Amniotic fluid is classified by the index and the deepest pocket, with an option to grade polyhydramnios.

Try it free for 15 days, no credit card required.

References

• Salomon LJ, et al. ISUOG Practice Guidelines: ultrasound assessment of fetal biometry and growth. Ultrasound Obstet Gynecol 2019;53:715-723.
• Macrosomia: ACOG Practice Bulletin No. 216. Obstet Gynecol 2020;135:e18-e35.
• Moraitis AA, Shreeve N, Sovio U, et al. Universal third-trimester ultrasonic screening using fetal macrosomia in the prediction of adverse perinatal outcome. PLoS Med 2020;17:e1003190.
• SMFM Consult Series #46: Evaluation and management of polyhydramnios. Am J Obstet Gynecol 2018;219:B2-B8.
• Calderon IMP, Rudge MVC. Macrossomia fetal: um desafio obstétrico. Rev Bras Ginecol Obstet 2006;28:211.