Obstetrics
Ultrasound
Biparietal diameter (BPD)
Varies with GA
Normal Value
Standard plane: thalami and cavum septi pellucidi. Plane/Technique: Axial
Head circumference (HC)
Varies with GA
Normal Value
Same plane as BPD. Plane/Technique: Axial
Abdominal circumference (AC)
Varies with GA
Normal Value
Level of stomach and umbilical vein. Plane/Technique: Axial
Femur length (FL)
Varies with GA
Normal Value
Exclude epiphysis. Plane/Technique: Straight long axis view
Cerebellum transverse diameter
≈ GA in weeks (e.g., 20 mm at 20 weeks)
Normal Value
Useful to estimate GA. Plane/Technique: Posterior fossa
Cisterna magna
2–10 mm
Normal Value
Enlarged in Dandy-Walker; small in Chiari II. Plane/Technique: Posterior fossa view
Lateral ventricle
<10 mm
Normal Value
Measured at atrium level. Plane/Technique: Axial
Umbilical Artery: Systolic/Diastolic (S/D) Ratio
≤4.0 (approx. 95th percentile)
Normal Value
S/D ratio decreases with GA; abnormal if >95th percentile
Umbilical Artery: Pulsatility Index (PI)
<1.4 (approx. 95th percentile)
Normal Value
Measured in free loop; abnormal if persistent elevation
Umbilical Artery: Resistance Index (RI)
<0.7
Normal Value
Decreases with gestational age
Umbilical Artery: Absent/Reversed Diastolic Flow
Absent: abnormal; Reversed: severe
Normal Value
Associated with IUGR and fetal compromise
Middle Cerebral Artery: Pulsatility Index (PI)
1.5 ± 0.3 (mean ± SD)
Normal Value
Falls in hypoxia; brain-sparing effect
Middle Cerebral Artery: Peak Systolic Velocity (PSV)
<1.5 MoM
Normal Value
Elevated in fetal anemia
Middle Cerebral Artery: Resistance Index (RI)
~0.8 (approx. 50th percentile)
Normal Value
Decreases with GA
Cerebroplacental Ratio: MCA PI / UA PI
>1.08 (5th percentile)
Normal Value
Low CPR indicates fetal hypoxia risk; abnormal <5th percentile
Mean Uterine Artery: Pulsatility Index (PI)
<1.45 (approx. 95th percentile)
Normal Value
Mean of left and right uterine arteries; elevated PI associated with preeclampsia/IUGR
Mean Uterine Artery: Notching
Should be absent
Normal Value
Persistence beyond 24 weeks is abnormal
Gestational sac
2–3 mm at 4.5–5 weeks
Normal Value
Intrauterine location confirms early pregnancy. Plane/Technique: Transvaginal
Yolk sac
≤6 mm
Normal Value
Seen by 5.5 weeks; abnormal if >6 mm or misshapen. Plane/Technique: Transvaginal
Crown-Rump Length (CRL)
45 mm at ~11.5 weeks
Normal Value
Best for dating in 1st trimester. Plane/Technique: Midline sagittal
Heart rate
110–160 bpm
Normal Value
Starts ~6 weeks at ~100 bpm, increases with gestation. Plane/Technique: M-mode
Estimated Fetal Weight (EFW)
Within 10–90th percentile
Normal Value
Use Hadlock formula (BPD, HC, AC, FL). Plane/Technique: Multiple biometric planes
Amniotic Fluid Index (AFI)
5–24 cm
Normal Value
Sum of four quadrant pockets. Plane/Technique: Longitudinal
Single Deepest Pocket (SDP)
2–8 cm
Normal Value
Alternative to AFI. Plane/Technique: Vertical depth in any quadrant
Umbilical artery S/D ratio
<3.0 (at term)
Normal Value
Abnormal if absent/reversed diastolic flow. Plane/Technique: Color Doppler, free loop
EFW
10–90th percentile
Normal Value
Assess for IUGR or macrosomia. Plane/Technique: Multiple biometric planes
Cerebroplacental ratio (CPR)
>1.08
Normal Value
MCA PI / UA PI. Plane/Technique: Doppler of MCA and UA
Placental thickness
~1 mm per week GA (e.g. 35 mm at 35 weeks)
Normal Value
Abnormal if <2 cm or >4–5 cm. Plane/Technique: Sagittal placental view
Nuchal translucency (NT)
<3.0 mm (11–13+6 weeks)
Normal Value
Measured in mid-sagittal plane; CRL should be 45–84 mm. Plane/Technique: Mid-sagittal, neutral neck
CRL
45–84 mm
Normal Value
Gestational age 11–13+6 weeks. Plane/Technique: Mid-sagittal
Nasal bone
Present
Normal Value
Absent or hypoplastic in trisomy 21. Plane/Technique: Tangential view of nose
Ductus venosus PI
<95th percentile
Normal Value
Used in risk assessment for aneuploidy. Plane/Technique: Color Doppler at ductus venosus