Suspected fetal macrosomia — structured SAQ (15 marks)

(a) Interpreting the scan, including limitations (3 marks)

One mark per point, maximum three. ^[5]

• State that the EFW is an estimate, not a measurement. Ultrasound formulas (Hadlock) carry a roughly 15 percent error margin — a 4100 g estimate could represent a baby of roughly 3500 to 4700 g.^[5]
• Quote the diagnostic performance. An EFW over 4000 g (or 90th centile) has a positive likelihood ratio of roughly 8.74 (95% CI 6.84 to 11.17) for predicting birthweight macrosomia, but only 2.12 (95% CI 1.34 to 3.35) for predicting shoulder dystocia — strong for a big baby, weak for shoulder dystocia.^[5]
• Acknowledge the prediction problem. Most shoulder dystocia occurs in non-macrosomic fetuses, so antenatal prediction will always miss the majority of events.^[5]

(b) Maternal and fetal risks, with the diabetes effect (3 marks)

• Fetal risks: shoulder dystocia, brachial plexus injury (Erb's palsy), clavicular and humeral fracture, birth asphyxia, neonatal hypoglycaemia (especially of a diabetic mother), NICU admission.^[1][6]
• Maternal risks: postpartum haemorrhage (atony from overdistension), severe perineal trauma, operative vaginal birth, caesarean, wound complications.^[1]
• The diabetes differential: diabetes amplifies shoulder dystocia risk at every birthweight, because the fetus grows disproportionately large shoulders and trunk relative to the head — which is why the caesarean threshold is lower in diabetes. Nesbitt 1998 showed roughly 35 percent shoulder dystocia at 4750 to 5000 g in diabetic women versus roughly 14 percent in non-diabetic women.^[6]

(c) Options for timing and mode of birth (6 marks)

Two marks each for induction, caesarean, and the balancing counsel. ^[1][2][4]

• Induction at 37 to 39 weeks. The Boulvain 2015 trial (822 women, EFW over 95th centile) found induction reduced the composite of shoulder dystocia and associated morbidity (RR 0.32, 95% CI 0.15 to 0.71) without increasing caesarean and with more spontaneous vaginal births (RR 1.14, 1.01 to 1.29). The Cochrane 2023 review confirmed a reduction in shoulder dystocia (RR 0.60) and fracture (RR 0.20) without a caesarean difference. The Gardosi Big Baby 2025 trial (2893 women) found no significant difference in shoulder dystocia in intention-to-treat (RR 0.75, 0.51 to 1.09) but a reduction in per-protocol analysis (RR 0.62, 0.41 to 0.92).^[2][3][4]
• Planned caesarean — the ACOG thresholds. ACOG PB 216 advises that prophylactic caesarean may be offered for an EFW over 4500 g in diabetic women and over 5000 g in non-diabetic women, reflecting the steep rise in permanent brachial plexus injury at these weights. Her EFW of 4100 g is below the diabetic threshold of 4500 g, so caesarean is not recommended on the estimate alone.^[1]
• Expectant management is defensible below the thresholds, especially with a previous uncomplicated vaginal birth. The counselling must state the 15 percent error margin and that shoulder dystocia prediction is weak. The recommendation, given her gestational diabetes and previous vaginal birth, would be either induction at 38 to 39 weeks (to reduce shoulder dystocia) or a closely-surveilled trial of labour — an individualised decision documented with the woman.^[1][5]

(d) Intrapartum and postnatal plan for trial of labour (3 marks)

• Intrapartum: senior presence, alert the neonatal team, active third stage to prevent atonic postpartum haemorrhage, and a rehearsed shoulder dystocia team — recognising the turtle sign and beginning manoeuvres (McRoberts, suprapubic pressure, rotational manoeuvres, posterior arm) without delay; never apply fundal pressure.^[1][6]
• Neonatal: early glucose monitoring for hypoglycaemia (especially as the infant of a diabetic mother), trauma assessment, and early feeding.^[1]
• Postnatal: document the birth accurately for governance, counsel on recurrence risk and pre-pregnancy weight optimisation, and support breastfeeding.^[1]