Macrosomia

Fetal macrosomia refers to an estimated fetal weight above 4000 to 4500 g (thresholds vary by guideline) or above the 90th centile for gestational age.

Major risk factors include maternal diabetes (gestational or pre-existing, the strongest modifiable association), maternal obesity, previous macrosomic infant, post-term pregnancy, and excessive gestational weight gain.

Fundal height greater than expected for gestational age; often identified on routine growth scan. Clinically, the principal concerns relate to labor and delivery, particularly shoulder dystocia (see that entry) risk.

Estimated fetal weight by ultrasound biometry, though this carries a significant margin of error (roughly ±10 to 15%) particularly at higher weights, and clinical correlation with symphysis-fundal height and maternal risk factors remains important.

Screening for and optimizing control of maternal diabetes is central, given how strongly this drives macrosomia and its associated birth complications (asymmetric, truncal fat deposition with disproportionate shoulder and abdominal growth relative to head circumference, which specifically raises shoulder dystocia risk).

• Optimize glycemic control where diabetes is present (see Gestational Diabetes Mellitus entry)
• Induction of labor is considered from around 38 to 39 weeks in confirmed macrosomia, particularly with maternal diabetes, though evidence and practice regarding routine induction purely for suspected macrosomia without diabetes is more mixed and individualized by local protocol and shared decision-making
• Planned cesarean section is considered for estimated fetal weight above roughly 4500 g in a diabetic mother, or above 5000 g in a non-diabetic mother (thresholds vary by guideline), given the substantially elevated shoulder dystocia risk at these weights
• Delivery should occur in a unit with immediate access to staff experienced in shoulder dystocia management, given the elevated risk regardless of planned delivery mode
• Active management of the third stage and vigilance for postpartum hemorrhage, given the elevated risk with macrosomia related uterine overdistension

Referral: obstetric-led care and delivery planning discussion, particularly with coexisting diabetes.