Suspected fetal macrosomia

It is 36 weeks and the growth scan returns an EFW on the 95th centile — 4050 g. The midwife asks whether you will book a caesarean. The honest answer is that the scan is a starting point, not an endpoint: it carries a 15 percent error, it predicts shoulder dystocia poorly, and the evidence on what to do with it is balanced. The fellowship-grade answer is to counsel first and decide second. This topic is that conversation.^[1][5]

Overview and definition

Macrosomia is defined by birthweight, but you will always be working with a suspected macrosomia from an ultrasound estimate. Keep the two ideas separate in your head and in your counselling.^[1]

• Macrosomia — a birthweight over 4000 g; large-for-gestational-age (LGA) — a birthweight over the 90th centile for gestation.^[1]
• Two thresholds matter. 4000 g flags increased risk of operative birth and shoulder dystocia; 4500 g flags substantially higher shoulder dystocia and birth trauma risk, and is the ACOG caesarean threshold in diabetic women.^[1][6]
• Suspected macrosomia — an ultrasound EFW (Hadlock or comparable formula) above the 90th centile or over 4000 g. This is the clinical state you counsel on antenatally; it is not the same as confirmed macrosomia at birth.^[1][5]

One number anchors the whole topic: ultrasound EFW prediction has a positive likelihood ratio around 8 to 9 for predicting birthweight macrosomia but only around 2 for predicting shoulder dystocia — because shoulder dystocia is multifactorial and most cases occur in non-macrosomic fetuses.^[5]

References12ShowHide

1. [1]American College of Obstetricians (ACOG) Macrosomia: ACOG Practice Bulletin, Number 216 Obstet Gynecol, 2020.PMID 31856124
2. [2]Boulvain M, Senat MV, Perrotin F, Winer N, Beucher G, Subtil D, et al. Induction of labour versus expectant management for large-for-date fetuses: a randomised controlled trial Lancet, 2015.PMID 25863654
3. [3]Boulvain M, Thornton JG Induction of labour at or near term for suspected fetal macrosomia Cochrane Database Syst Rev, 2023.PMID 36884238
4. [4]Gardosi J, Ewington LJ, Booth K, Bick D, Bouliotis G, Butler E, et al. Induction of labour versus standard care to prevent shoulder dystocia in fetuses suspected to be large for gestational age in the UK (the Big Baby trial): a multicentre, open-label, randomised controlled trial Lancet, 2025.PMID 40319899
5. [5]Moraitis AA, Shreeve N, Sovio U, Brocklehurst P, Heazell AEP, Thornton JG, Robson SC, Papageorghiou A, Smith GCS Universal third-trimester ultrasonic screening using fetal macrosomia in the prediction of adverse perinatal outcome: A systematic review and meta-analysis of diagnostic test accuracy PLoS Med, 2020.PMID 33048935
6. [6]Nesbitt TS, Gilbert WM, Herrchen B Shoulder dystocia and associated risk factors with macrosomic infants born in California Am J Obstet Gynecol, 1998.PMID 9731856
7. [7]Magro-Malosso ER, Saccone G, Chen M, Navathe R, Di Tommaso M, Berghella V Induction of labour for suspected fetal macrosomia at term in non-diabetic women: a systematic review and meta-analysis of randomized controlled trials BJOG, 2017.PMID 27921380
8. [8]Al-Hawash S, Whitehead CL, Farine D Risk of recurrent shoulder dystocia: are we any closer to prediction? J Matern Fetal Neonatal Med, 2019.PMID 29587540
9. [9]Metzger BE, Lowe LP, Dyer AR, et al. Hyperglycemia and adverse pregnancy outcomes N Engl J Med, 2008.PMID 18463375
10. [10]Dakwar Shaheen J, Hershkovitz R, Mastrolia SA, Charach R Estimation of fetal weight using Hadlock's formulas: Is head circumference an essential parameter? Eur J Obstet Gynecol Reprod Biol, 2019.PMID 31678760
11. [11]Gherman RB, Ouzounian JG, Satin AJ, Goodwin TM A comparison of shoulder dystocia-associated transient and permanent brachial plexus palsies Obstet Gynecol, 2003.PMID 12962939
12. [12]King JR, Korst LM, Miller DA, Ouzounian JG Increased composite maternal and neonatal morbidity associated with ultrasonographically suspected fetal macrosomia J Matern Fetal Neonatal Med, 2012.PMID 22439605