fetal macrosomia

see also:

• Obstetrics
• labour

• fetal macrosomia is arbitrarily defined as a birth weight over 4000 g – 4500 g, or a fetus >90th centile estimated fetal weight (EFW) for gestation or abdominal circumference (AC)
• morbidity increases sharply beyond birth weights of 4,500g

• Race
• Parental stature (height)
• High BMI
• Previous macrosomic infant
• Matrilineal tendency to give birth to macrosomic babies
• Maternal diabetes
• Maternal impaired glucose intolerance
• Multiparity
• Advanced maternal age
• Postterm pregnancy
• Excessive maternal weight gain during pregnancy (>20 kg)

• Ensure that the EDD is accurate by reviewing early ultrasound scans
• Consider early Glucose Tolerance Test (GTT) if fetal weight >90% centile on routine ultrasound. If normal, repeat GTT at 28 weeks gestation.
• Routine antenatal care/tests in pregnancy

• assessment of Symphysial Fundal Height (SFH):
  • the volume of amniotic fluid, the size and configuration of the uterus and maternal body habitus complicate estimation of the size of the fetus by palpation through the abdominal wall. Several studies have documented mean errors of about 300 g.
  • Consider ultrasound if:
    • SFH > 3cm above dates persistently
    • SFH > 40 cm at 37 weeks and beyond
• third trimester USS:
  • The value of ultrasound in assessing EFW is also unpredictable due to the limited sensitivity and specificity.
  • Typical mean error ranges from 300 g – 550 g.
  • Sonography is most accurate when performed near the time of delivery in singleton, cephalic presenting, non diabetic pregnancies.
  • A single estimation at 29 to 34 weeks of gestation has very poor predictive value for birth weight at term.
  • EFW at 29 to 34 weeks can significantly underestimate birth weight, because of accelerated growth in the latter part of the third trimester.
  • Hadlock's formula (encompassing head circumference [HC], abdominal circumference [AC] and femur length [FL] measurements) has the highest predictive value in the nondiabetic population.

• screen for GDM by GTT if not already done
• suspected fetal macrosomia in iteself is not a contraindication to attempt vaginal birth after 1 previous caesarean section.
• ultrasound at 37-38 weeks to ensure EFW thresholds for possible LUSCS are not reached:
  • non-diabetics:
    • elective caesarean section should be considered ONLY if the EFW > 5 kg.
    • take maternal habitus and past history into account.
  • diabetics:
    • Planned caesarean section should be considered in women with diabetes and suspected fetal macrosomia where the EFW > 4500 g
    • Macrosomic infants of diabetic mothers have larger shoulders and greater amounts of body fat, decreased head-to-shoulder ratio, and increased skin folds in the upper extremities. Therefore they are at the greatest risk of shoulder dystocia.