Clinical overview
The macrosomic fetus is the large baby whose size threatens both itself and its mother — chiefly through obstructed and complicated labour, shoulder dystocia, birth trauma, perinatal asphyxia, and, on the maternal side, postpartum haemorrhage and severe perineal injury. The clinical problem is not the size itself but the mismatch between fetal dimensions and the maternal pelvis and soft tissues, played out under the time pressure of labour. Macrosomia therefore sits at the intersection of antenatal surveillance, intrapartum decision-making, and emergency obstetric drill.
The central tension is prediction versus prevention. We cannot measure fetal weight precisely before birth — clinical estimation and ultrasound both carry wide error margins — yet we are asked to make high-stakes decisions (induction, elective caesarean, mode and place of delivery) on those imperfect estimates. Over-call it and you generate unnecessary caesareans, inductions, and maternal morbidity in a system where theatre access and blood are finite. Under-call it and you risk a shoulder dystocia in a district facility without the team or skills to manage it. In the South African setting — where undiagnosed and poorly controlled diabetes is common, where many women book late, and where the gradient between district, regional and tertiary care is steep — the macrosomic fetus is a recurring and high-consequence problem. The clinical task is to suspect it, assess it honestly, plan delivery, and be drilled for the emergency it most threatens: shoulder dystocia and PPH.
Core knowledge
Definitions
There is no single universal threshold; two definitions are in common use, each with its own rationale.
- Macrosomia classically refers to an absolute birth weight above a fixed cut-off, most commonly ≥4000 g, with ≥4500 g marking a higher-risk category, irrespective of gestational age. Some authorities add a ≥5000 g tier where morbidity rises steeply.
- Large for gestational age (LGA) is a relative definition: birth weight above the 90th centile for gestational age (and ideally for sex and population). A preterm infant can be LGA without being macrosomic by absolute weight.
These are not interchangeable. Absolute thresholds matter because the risks of shoulder dystocia and birth trauma climb with absolute fetal dimensions; centile-based definitions matter for identifying the pathological growth trajectory (especially the diabetic fetus) that warrants surveillance. Note the contrast with the small fetus covered in Intrauterine growth restriction: both are growth-disorders, but macrosomia is over-growth with mechanical and metabolic consequences.
Pattern of growth — why the diabetic baby is different
Not all large babies are large in the same way, and the distinction is clinically important.
- Constitutionally large / symmetrical macrosomia: proportionate enlargement, often in tall parents or with grand multiparity and post-term pregnancy. Risk is mechanical (dystocia, trauma) but proportionate.
- Diabetic / asymmetrical macrosomia: driven by fetal hyperinsulinaemia. Maternal hyperglycaemia crosses the placenta; the fetal pancreas responds with insulin, which is a potent fetal growth factor acting on insulin-sensitive tissues. The result is disproportionate deposition of fat and glycogen in the trunk, shoulders and interscapular region, raising the shoulder-to-head and chest-to-head ratios. This is precisely the geometry that causes shoulder dystocia, which is why the diabetic macrosomic fetus carries a disproportionately higher dystocia risk at any given weight than the non-diabetic large baby.
Figure J22.1 — Macrosomia risk map showing absolute and centile definitions, diabetic asymmetrical growth, EFW uncertainty, and maternal-neonatal consequences.
Risk factors
- Maternal diabetes — pre-existing type 1/2 and gestational diabetes (GDM); the strongest modifiable driver, mediated by glycaemic control.
- Maternal obesity and excessive gestational weight gain.
- Previous macrosomic infant — strongly predictive of recurrence.
- Post-term pregnancy (continued growth beyond term).
- Multiparity and advanced maternal age; tall, heavy parents (genetic potential); male fetus.
- Rarely, fetal overgrowth syndromes (e.g. Beckwith–Wiedemann) — consider where macrosomia is extreme or syndromic features coexist.
Consequences
Fetal/neonatal: shoulder dystocia and its sequelae — brachial plexus injury (Erb's palsy), clavicular/humeral fracture, hypoxic-ischaemic injury and, in the worst case, intrapartum death; neonatal hypoglycaemia (hyperinsulinaemic, in the diabetic baby), polycythaemia, jaundice, and later-life metabolic risk.