Fetal Biometry: BPD vs. Femur Length for Gestational Dating

Biparietal diameter (BPD)—the transverse width of the fetal skull—is a primary biometric parameter for gestational age estimation in the second trimester. Femur length (FL) is a supplementary measure, not used alone for dating.

Standard Biometric Set (2nd Trimester)

• BPD (most reproducible for dating 14–28 wks)
• Head Circumference (HC) – less affected by head shape
• Abdominal Circumference (AC) – best for fetal weight estimation
• Femur Length (FL) – longest bone; correlates with gestational age but higher variability

Why Multiple Parameters?

• Individual biometrics have ±1–2 week variability.
• Combined algorithms (e.g., Hadlock) reduce error to ±5–7 days (2nd tri) vs. ±10–14 days (3rd tri).
• FL is valuable for detecting skeletal dysplasias, growth asymmetry, and confirming dates when BPD/HC/AC are discordant.

Clinical Practice

• Dating scan (ideally 8–14 wks): CRL (crown-rump length) is gold standard (±3–5 days).
• If dating scan missed: 2nd tri quad-screen (BPD, HC, AC, FL) for GA estimation.
• FL alone never used to assign due date.

Accurate dating optimizes obstetric management (induction timing, growth assessment, preterm labor decisions).