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 cmDo 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 weeksMPD < 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 CircumferenceTrans-ventricular Planechoroid plexus
posterior horn
falx cerebri
CSP
anterior horn
Trans-ventricular Plane
Measurement CriteriaBPD & 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
UV
stomach
spine
Ao
Abdominal Circumference
AC Sources of Error
• Oblique Section (Salami Effect)• Too high • Too low• Abdominal compression (oligo-
hydramnios)• 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)