Obstetric Ultrasound
Scanning Technique
Harry Gee
(teaching materials provided
by David Cole, Lecturer at City
University, Birmingham)
2nd & 3rd Trimester Scans
• Identify fetus
– Orientation
– Fetal heart beat
• Placental site
• Biometry
–
–
–
–
Bi-Parietal Diameter (BPD)
Head Circumference (HC)
Abdominal Circumference (AC),
Femur length (FL)
• Liquor Estimation
2nd & 3rd Trimester Scans
• Place probe longitudinal on lower
abdomen superior to symphysis pubis
• Slide probe superiorly using sweeping
movements to locate fetus
2nd & 3rd Trimester Scans
• Slide probe superiorly - sweeping side
to side up to uterine fundus.
FH + Orientation
Fetal Orientation
• Lie
– longitudinal
– Transverse
– Oblique
• Presentation
– cephalic
– breech
Cephalic
Breech
Breech legs extended
Transverse lie
Placental localisation
•
•
•
•
Full bladder
Midline sagittal section
Placental position anterior or posterior
Low placenta - measure distance
between lower edge and internal os *
Fundal placenta
Major placenta praevia
Minor placenta praevia
Liquor volume
• Subjective assessment
• Maximum deepest pool (MPD)
• Amniotic fluid index
Subjective Assessment
Normal liquor volume
Maximum deepest pool
Deepest pool in cm
Do not include cord or fetal limbs
Amniotic Fluid Index
Sum of the deepest pools
in each of four quadrants
Oligohydramnios
MPD < 3 cm up to 36 weeks
MPD < 2 cm 36 weeks - term
Polyhydramnios
MPD > 8 cm
Fetal Biometry
•
•
•
•
Biparietal diameter BPD
Head circumference HC
Abdominal circumference AC
Femur length FL
BPD Measurement
ROCK
Biparietal diameter BPD
BPD Errors
• Few (best inter-observer error)
• Oblique section (increases
measurement)
• Not at BPD (decreases measurement)
Head Circumference
Trans-ventricular choroid
Plane
plexus
anterior horn
posterior horn
CSP
falx cerebri
Trans-ventricular Plane
Measurement Criteria
BPD & HC
•Landmarks: CSP, Falx, thalami
•Alignment: symmetrical –midline
no orbits, no cerebellum
•Calipers: BPD widest distance 90o
midline.
•HC bony perimeter of skull
Abdominal Circumference
•
•
•
•
Long Axis of fetus (Spine)
Fetal trunk parallel to transducer
Rotate transducer at right angles
Move up and down until landmarks
identified.
• Repeat to consistency
Abdominal Circumference
spine
Ao
UV
stomach
Abdominal Circumference
AC Sources of Error
•
•
•
•
Oblique Section (Salami Effect)
Too high
Too low
Abdominal compression (oligohydramnios)
• Fetal Breathing
FL Measurement
• Identify femur close to fetal
bladder/pelvis.
FL Measurement
• Slide probe so that femur is central
FL Measurement
• Rotate probe to obtain full length
image of femur.
Femur Length
Source of Error
• Oblique image (shortens
measurement)