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)