Antenatal surveillance

Fetal size is assessed by measurements of the head, abdomen and limbs. Assessment of size requires reference ranges and knowledge of gestational age

• Head size: Biparietal diameter (BPD), head circumference (HC), cerebellar diameter (TCD)

• Body size: Abdominal circumference (AC)

• Skeletal size: Femur length (FL), humerus length (HL)

• Estimated fetal weight: Composite variable based on head, body and skeletal size

Abnormal fetal growth is subdivided into the asymmetric and symmetric types based on the head to abdomen ratios: (HC/AC, TCD/AC), head to femur ratio (HC/FL) and abdomen to femur ratio (AC/FL):

Asymmetric type:

• Fetal growth restriction due to impaired placentation or triploidy (high HC/AC)
• Skeletal dysplasia (high HC/FL and AC/FL)
• Microcephaly (low HC/AC and low HC/FL), maternal diabetes-related fetal macrosomia (low HC/AC and HC/FL)

Symmetric type (normal HC/AC):

• Severe fetal growth restriction due to impaired placentation
• Aneuploidies, genetic syndromes, congenital infections