Fetal Macrosomia — Risk, Diagnosis & Delivery Planning
Macrosomia means a fetus is considerably larger than average — commonly defined as a birth weight >4000 g,
with >4500 g the threshold most strongly linked to real complications (definitions vary by guideline).
It raises the risk of a difficult delivery for both mother and baby — but one of the most important things to
know about it is also the most counter-intuitive: most shoulder dystocia happens in babies who were never
flagged as macrosomic at all.
ЁЯОп AT A GLANCE — DEFINITIONS & RISK
Macrosomia: birth weight above an absolute threshold, usually >4000 g
Large for gestational age (LGA): a related but distinct concept — weight above the 90th centile for gestational age, rather than a fixed number
GREEN: EFW <4000 g, no risk factors — routine intrapartum care
AMBER: EFW 4000–4499 g, or risk factors present (diabetes, prior macrosomic baby, obesity, post-term) — individualize counselling
RED: EFW ≥4500 g, especially with maternal diabetes — highest risk for shoulder dystocia and birth trauma; plan delivery carefully
ЁЯзм CAUSES & RISK FACTORS
Maternal diabetes (pre-existing or gestational) is the single most significant, and most modifiable, risk factor — fetal hyperinsulinaemia drives disproportionate growth of the shoulders and trunk relative to the head
Maternal obesity or excessive gestational weight gain
Post-term pregnancy, grand multiparity, a prior macrosomic infant
Constitutional factors (large parents), male fetal sex
Why diabetic macrosomia is different: the asymmetric, trunk-heavy growth pattern from fetal hyperinsulinaemia carries disproportionately higher shoulder dystocia risk than the same estimated weight in a non-diabetic pregnancy.
ЁЯФО ESTIMATING FETAL WEIGHT
Clinical assessment (symphysis-fundal height, Leopold's manoeuvres) is a reasonable screen but imprecise — especially with maternal obesity
Ultrasound biometry (abdominal circumference is the most influential measurement) gives a more objective estimate, but still carries a real margin of error
Ultrasound EFW can be wrong by several hundred grams, even in skilled hands — this imprecision matters, because delivery decisions are often made on an estimate that could be meaningfully off in either direction.
Maternal Risks
Prolonged labour, higher operative vaginal delivery and caesarean rates
Perineal trauma and postpartum haemorrhage from uterine overdistension
Fetal / Neonatal Risks
Shoulder dystocia — with risk of brachial plexus injury or fracture
Neonatal hypoglycaemia (especially with maternal diabetes), birth asphyxia
ЁЯЪи WHY MACROSOMIA SCREENING ISN'T ENOUGH ON ITS OWN
Macrosomia raises the risk of shoulder dystocia — but it is a poor predictor of it
Most individual cases of shoulder dystocia occur in infants of normal birth weight, simply because far more deliveries fall in that range
A "reassuring" estimated fetal weight is not a guarantee that the delivery will be straightforward
Every vaginal delivery deserves shoulder dystocia readiness — not only the ones labelled high-risk
ЁЯЫа️ ANTENATAL MANAGEMENT
Screen and optimize maternal diabetes control — the most effective preventive measure available
Serial growth scans where risk factors are present; counsel on appropriate gestational weight gain
Have a genuine shared decision-making conversation about delivery route — and be explicit that the EFW is an estimate, not a certainty
ЁЯЧУ️ DELIVERY PLANNING
EFW <4500 g, no diabetes: vaginal delivery generally appropriate
EFW <4000 g, with diabetes: vaginal delivery is usually reasonable
EFW ≥4500 g with diabetes, or ≥5000 g without: elective caesarean may reasonably be offered to reduce birth trauma risk
Induction of labour for suspected macrosomia alone (with no other indication) is not routinely recommended by most guidelines — it has not been shown to reliably improve outcomes, and practice varies; discuss with a senior colleague where this is genuinely uncertain.
ЁЯС╢ AT DELIVERY — BEING PREPARED
1
Make sure senior obstetric and neonatal staff are aware and available
2
Have a clear shoulder dystocia drill ready to run, not just discussed in theory
3
Anticipate postpartum haemorrhage — active management of the third stage, uterotonics ready
4
Keep a low threshold to move to caesarean rather than pursue a difficult, prolonged operative vaginal delivery
5
Alert the neonatal team for possible resuscitation and hypoglycaemia monitoring
The Humbling Fact
Most Shoulder Dystocia Happens in Normal-Weight Babies
Macrosomia raises the risk of shoulder dystocia, but it's a poor predictor of it — the majority of cases occur in babies who were never flagged as macrosomic at all. Every delivery deserves readiness, not just the ones we've already labelled high-risk.
Ultrasound EFW can be off by several hundred grams even in skilled hands — a reassuring estimate is not a guarantee
Shoulder dystocia drills and preparedness should be routine for every vaginal delivery, not reserved for cases labelled macrosomic
Elective caesarean for suspected macrosomia alone is reserved for the most extreme estimates, usually only alongside maternal diabetes — it is not a routine recommendation
Document the EFW, the counselling discussion, and the delivery plan clearly, whichever route is chosen
Reminder: prepare for the emergency you can't fully predict — that's the honest lesson of this topic.
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