Fetal Biometry Methods

Overview

This lecture covers methods for fetal biometry in the second and third trimesters, focusing on head, abdominal, and femur measurements to estimate gestational age and assess fetal growth.

Fetal Head Measurements

• Key head measurements: biparietal diameter (BPD), occipital frontal diameter (OFD), and head circumference (HC).
• All head measurements are taken on an axial image at the level of thalami, third ventricle, and cavum septum pellucidum.
• BPD is measured from the outer edge near the transducer to the inner edge on the far side of the calvarium.
• Incorrect imaging plane or caliper placement causes errors in estimating gestational age and fetal weight.
• OFD: calipers placed at the midline of frontal and occipital bones.
• HC: measured using elliptical calipers or calculated as HC = 1.62 × (BPD + OFD).
• Standard OFD/BPD ratio is 1.265; correction formula: corrected BPD = √(BPD × OFD / 1.265).
• Cephalic index = (BPD / OFD) × 100; normal mean ≈78, range 74–83.
• Low cephalic index suggests dolichocephaly, high suggests brachycephaly.

Fetal Abdomen Measurements

• Abdominal circumference (AC) is measured on an axial image at the stomach and intrahepatic umbilical vein level.
• Two AC measurement methods: directly with ellipse calipers or calculated from anteroposterior (AP) and transverse diameters.
• AP diameter: anterior to posterior skin surface; transverse diameter: perpendicular to AP, both on outer skin.
• AC can be calculated as AC = 1.57 × (AP diameter + transverse diameter).
• Abdomen image must be round, include the entire abdomen, and not show kidneys to ensure accuracy.
• Incorrect positioning or caliper placement leads to inaccurate fetal age and weight estimates.

Fetal Femur Measurement

• Only the femoral diaphysis is measured, as it is the ossified part during most of pregnancy.
• Femur should be as perpendicular as possible to the ultrasound beam (45–90° angle acceptable).
• Calipers are placed at each end of the ossified diaphysis, excluding the epiphysis.

Practical Points

• Obtaining standard images can be difficult in overweight women, with uterine contractions, or if the fetus is in a prone position.
• Sufficient time should be allocated for measurements; repositioning the mother may help obtain better images.

Key Terms & Definitions

• Biparietal Diameter (BPD) — distance between the two sides of the fetal head.
• Occipital Frontal Diameter (OFD) — length from the frontal to the occipital bone of the fetal head.
• Head Circumference (HC) — measured around the outer edge of the fetal skull.
• Cephalic Index — ratio of BPD to OFD, indicating fetal head shape.
• Abdominal Circumference (AC) — circumference measurement around the fetal abdomen.
• Femoral Diaphysis — shaft of the femur bone, excluding the ends (epiphyses).

Action Items / Next Steps

• Review the suggested related videos on fetal biometry for further understanding.