Large for Gestational Age (Macrosomia)
Summary
Large for Gestational Age (LGA) is defined as a weight >90th centile for gestation. Macrosomia is defined by an absolute birthweight, typically >4000g or >4500g. The primary driver is Maternal Hyperglycaemia (Diabetes), which drives fetal hyperinsulinaemia and somatic overgrowth (Pedersen hypothesis). The major clinical risk is Shoulder Dystocia (where the bisacromial diameter exceeds the pelvic outlet), leading to Brachial Plexus Injury (Erb's Palsy) and Hypoxic Ischaemic Encephalopathy. [1,2]
Clinical Pearls
The "Diabetic Shoulder": In non-diabetic macrosomia (e.g., genetic/post-dates), the head and shoulders grow proportionately. In diabetic macrosomia, insulin-driven fat deposition occurs preferentially on the trunk and shoulders. Thus, a 4kg diabetic baby is at much higher risk of shoulder dystocia than a 4kg non-diabetic baby.
Turtle Sign: The pathognomonic sign of Shoulder Dystocia. The head delivers but then retracts back against the perineum (like a turtle withdrawing into its shell) because the neck is tethered by the impacted shoulders.
Beckwith-Wiedemann Syndrome: Consider this in a neonate with LGA + Macroglossia (big tongue) + Abdominal Wall defects (Omphalocele) + Ear creases. They are at risk of Wilms' tumour.
Demographics
- Incidence: 10% of all pregnancies. Rising due to obesity epidemic.
- Risk Factors:
- Maternal Diabetes (RR 3.0).
- Maternal Obesity (BMI >30).
- Excessive Weight Gain during pregnancy.
- Post-dates pregnancy.
- Multiparity.
- Male Fetus.
The Pedersen Hypothesis (Hyperglycaemia-Hyperinsulinaemia)
- Glucose Transport: Maternal glucose crosses the placenta via facilitated diffusion.
- Insulin Barrier: Maternal insulin does not cross the placenta.
- Fetal Response: The fetus produces its own insulin in response to high glucose load.
- Growth: Insulin is a potent anabolic growth factor (like IGF-1). It drives:
- Lipogenesis (Fat storage).
- Glycogen deposition (Liver/Heart).
- Somatic growth.
- Neonatal Crash: At birth, the glucose supply is cut (cord clamped), but the fetal pancreas is still pumping out high insulin -> Profound Neonatal Hypoglycaemia.
Antenatal
Intrapartum
- Leopold's Maneuvers: Palpating a large fetus. High head in late pregnancy.
- Ultrasound:
- Abdominal Circumference (AC): The most sensitive biometric marker for macrosomia/diabetes effect.
- EFW: Estimated Fetal Weight. Note: Error margin is +/- 15% (huge). A 4.0kg estimate could be 3.4kg or 4.6kg.
Screening
- OGTT (Oral Glucose Tolerance Test): Check for Gestational Diabetes (GDM) at 24-28 weeks, or earlier if risk factors present.
- Ultrasound: Growth scans at 28, 32, 36 weeks if GDM.
Management Algorithm
PREDICTED MACROSOMIA (EFW >90th)
↓
SCREEN FOR DIABETES (OGTT / HbA1c)
┌─────────┴─────────┐
DIABETIC MOTHER NON-DIABETIC
↓ ↓
GLYCAEMIC CONTROL DISCUSS RISKS
(Metformin/Insulin) (Shoulder Dystocia)
↓ ↓
DELIVERY PLANNING DELIVERY PLANNING
• EFW < 4.5kg: • EFW < 5.0kg:
- IOL at 38w - Vaginal Birth
- Be wary of
Instrumental
• EFW > 4.5kg: • EFW > 5.0kg:
- Elective CS - Consider CS
Obstetric Management
- Induction of Labour (IOL): The Big Baby Trial (2023) showed that IOL at 38 weeks reduces the risk of shoulder dystocia and fractures compared to expectant management.
- Elective C-Section:
- Diabetic: Offered if EFW > 4,500g.
- Non-Diabetic: Considered if EFW > 5,000g.
- Vaginal Delivery:
- Senior obstetrician present.
- McRoberts maneuver ready.
- Avoid Mid-Cavity Instrumental delivery (high risk of impaction).
Neonatal Management
- Hypoglycaemia Protocol: Early feeding (within 30 mins). Capillary blood glucose monitoring pre-feeds.
- Trauma Check: Check clavicles (crepitus) and arm movement (Erb's).
Maternal ("The Passage and Powers")
- PPH: Atonic uterus from overdistention.
- Trauma: 3rd/4th degree perineal tears. Vaginal lacerations.
- Uterine Rupture: Rare.
Fetal ("The Passenger")
- Shoulder Dystocia:
- Brachial Plexus Injury: Erb's Palsy (C5/C6 - Waiter's Tip). Klumpke's (C8/T1 - Claw hand). 10% are permanent.
- Fractures: Clavicle or Humerus.
- Hypoxia: Compression of cord during impaction.
- Metabolic: Hypoglycaemia, Polycythaemia (Hyperviscosity -> Jaundice), Hypocalcaemia.
- Neonatal: Most trauma resolves. Permanent Erb's palsy is a devastating medico-legal outcome.
- Long Term: LGA infants have increased risk of Obesity, Metabolic Syndrome, and Type 2 Diabetes in later life ("Fetal Programming").
Key Guidelines
| Guideline | Organisation | Key Recommendations |
|---|---|---|
| Shoulder Dystocia | RCOG Green-top | McRoberts -> Suprapubic Pressure -> Internal Maneuvers. |
| Diabetes | NICE NG3 | Induction timing and CS thresholds. |
Landmark Evidence
1. The Big Baby Trial (Lancet 2023)
- Large RCT comparing IOL at 38 weeks vs Expectant Care for suspected macrosomia. Found IOL reduced shoulder dystocia and fractures significantly, without increasing C-section rates. Changed practice towards earlier induction.
What is a Macrosomic baby?
It means a "Big Baby", usually defined as weighing more than 4kg (8lb 13oz) or 4.5kg (9lb 15oz).
Why is my baby big?
Often it is genetics (big parents make big babies). However, the most medically important cause looks at sugar. If a mother has diabetes or high sugar, the baby eats that sugar and stores it as fat, especially around the shoulders and tummy.
Is it dangerous for the birth?
The main worry is Shoulder Dystocia. This is when the baby's head is born, but the broad shoulders get stuck behind your pelvic bone. It is an emergency. It can cause nerve damage to the baby's arm or bleeding for the mother.
Should I have a C-Section?
- If you have diabetes and the baby is predicted to be over 4.5kg, we recommend a C-section to avoid the risks.
- If the baby is big but you don't have diabetes, we often discuss inducing labour a week or two early (at 38 weeks) so the baby doesn't grow any bigger.
What about the baby after birth?
Because the baby is used to high sugar levels inside the womb, their sugar can drop dangerously low after birth (Hypoglycaemia). We will need to feed them immediately and check their blood sugar levels carefully.
Primary Sources
- Boulvain M, et al. Induction of labour at or near term for suspected fetal macrosomia. Cochrane Database Syst Rev. 2016.
- RCOQ. Shoulder Dystocia (Green-top Guideline No. 42). 2012.
- The Big Baby Trial. Induction of labour for predicted macrosomia. Lancet. 2023.
Common Exam Questions
- Protocol: "First maneuver for Shoulder Dystocia?"
- Answer: McRoberts Maneuver (Flex hips to abdomen).
- Pathology: "Cause of neonatal hypoglycaemia?"
- Answer: Fetal Hyperinsulinaemia.
- Complication: "Waiter's Tip position of arm?"
- Answer: Erb's Palsy (C5/C6 injury).
- Threshold: "C-section weight threshold for diabetic mother?"
- Answer: EFW > 4,500g.
Viva Points
- Why AC is better than BPD: The head size (BPD) is often normal in diabetic macrosomia. The Abdominal Circumference (AC) reflects the liver/fat deposition which is the true pathology.
- Polycythaemia: Why does it happen? Fetal Hyperinsulinaemia increases metabolic rate -> increased oxygen demand -> relative hypoxia -> increased Erythropoietin -> more Red blood cells.
Medical Disclaimer: MedVellum content is for educational purposes and clinical reference. Clinical decisions should account for individual patient circumstances. Always consult appropriate specialists.