5. Ultrasound in Obstetrics

Question 1

What is the definition of ultrasonic?

Answer 1

All sound waves with a frequency above the range of human hearing (>20KHz).

Typical range of medical ultrasonic devices = 2-10Mhz

Question 2

How are ultrasonic waves generated? How do they form an image?

Answer 2

Piezo-electric crystal located in the probe.
Sound waves reflected at tissue interfaces.
Echo results and is converted into display on CRT.

Dense tissues (e.g. Spine) give increase sound reflections.

Question 3

Can see the baby move in real time?

Answer 3

Surface rendered image.

Question 4

What is U/S used for in Obstetrics?

Answer 4

• Imaging
• Movement detection (M-mode)
• Blood flow monitoring (Doppler)
• Tissue heating (Physiotherapy)

Question 5

What conditions can be picked up?

Answer 5

Congenital conditions e.g. Cleft lip

Question 6

What are the main indications for Obs U/S

Answer 6

• Confirm viability
• Estimate gestational age/ calculate EDD (at booking/1st appointment)
• Diagnose multiple pregnancy (2nd/3rd tri)
• Estimate fetal weight/size/growth (gestational age, EDD) (Repeat 2nd/3rd tri if suspicious)
• Assess fetal well-being/oxygenation (2nd/3rd tri)
• Locate placenta
• Detect fetal anomalies (Major/Minor)
• Confirm presentation (Cephalic/Breach)
• Check Blood Flow Through Major Vessels + Doppler.

Question 7

What is the schedule for routine U/S in pregnancy?

Answer 7

• Dating scan = 12 weeks (Booking visit, conf viability, congential abnormalities e.g. Down syn – maternal option)
• Anomaly scan = 20 weeks (18-22)
• Fetal ‘size’ scan* = 34 weeks (not alway indicated)

Question 8

What information is usually determined by U/S during the ‘Dating Scan’?

Answer 8

• Is the pregnancy viable?
• Singleton or multiple?
• Biometry to confirm gestational age & estimate EDD
• Assessment of uterus and adnexae

Question 9

How is Gestational age estimated using U/S?

Answer 9

Measure something simple as early as possible in the pregnancy (to ensure minimal biological variation).

Can be:

• Mean Gestation Sac Diameter (MGSD)
• Crown to Rump Length (C.R.L)

Combination of these final 3 used after 14 weeks

• Biparietal Diameter (B.P.D)
• Head Circumference (H.C.)
• Femur Lenght (F.L.)

Used to alter menstrual data, or if the menstrual data is irregular, for example due to conceiving on the pill etc

Question 10

What is the Anomaly Scan AKA?

Answer 10

(Structural survey scan)

Question 11

What detection rates are achieved for common congenital abnormalities during the anomaly scan?

Answer 11

Eurofetus Study

Anencephaly 99%
Spina bifida + hydrocephalus 95%
Isolated spina bifida 66%

Major heart anomalies 39% ((if risk usually bring back at 32wks, Things like aortic stenosis can be v.difficult to diagnose)
Cleft lip and palate 22%
Isolated cleft palate 1.4%

Question 12

How is fetal size/growth assessed using U/S?

Answer 12

Fetal Size = Single Scan

Fetal Growth Interval = 2x Scans (3 ideal)

Deviation From Normal Growth = 3x Scans Needed

Question 13

How is Fetal Weight estimated using U/S?

Answer 13

Combination of Head + Abdominal Size (± Femur Size)

Accurate to ± 10-15%

Question 14

Define the abdominal circumference?

Answer 14

Circumference of the abdomen, perpendicular to the trunk at the level of the stomach.

Question 15

Discuss the assessment of patterns of umbilical blood flow?

Answer 15

In the cord there is one artery and 2 veins.

If reduction of blood flow during systole it usually means placental dysfunction.

If the blood flow is reversed then it is major placental dysfunction, and the baby is at risk of hypoxia and acidosis

Question 16

What is the Gradual Hypoxia Concept?

Answer 16

Increasing hypoxia = Progressive depression of CNS mechanisms regulating biophysical variables.

i.e. The Gradual Hypoxia Concept

Question 17

What is cardiotochography

Answer 17

Cardiotochography –> assess fetal heart rate over 30mins, looking for changes with normal fetal activity suggesting health para/symp

Question 18

What is the biophysical profile? NB?

Answer 18

Assessment of 5 biophysical variables over a period of 30 minutes.

1. Fetal Breathing Movements (FBMs)
2. Gross Fetal Body Movements (Fms)
3. Fetal Tone (FT)
4. Fetal Heart Rate Reactivity (CTG)
5. Liquor Volume (AFI)

Question 19

What is the relationship between the biophysical profile of a fetus and mortality?

Answer 19

<6/10 = 8-9/1000

Question 20

Why is the biophysical profile a good test for fetal hypoxia?

Answer 20

Cortex, brainstem, hypothalamus all regulate the 4 biophysical processes and all will be affected by hypoxia.

Question 21

How is the biophysical score calculated?

Answer 21

30 Minute Scoring

Normal = 2 Abnormal = 0 (Different criteria for each category)

Question 22

What is are the normal and abnormal criteria for Fetal Breathing Movements?

Answer 22

Normal = 1 episode ≥30secs
Abnormal = None or <30secs

Question 23

What is are the normal and abnormal criteria for Gross Fetal Body Movements?

Answer 23

Normal = ≥ 3 Discrete FMs
Abnormal = < 3 Fms

Question 24

What is are the normal and abnormal criteria for Fetal Tone?

Answer 24

Normal = One or more episodes of active extension and flexion of a fetal extremity OR opening and closing of the hand.

Abnormal = Slow extension with no return or slow return to flexion of a fetal extremity OR no fetal movement

Question 25

What is are the normal and abnormal criteria for Fetal Heart Rate Activity?

Answer 25

Normal = 2 or more Accelerations >15bpm
Abnormal = <2 accelerations

Question 26

What is are the normal and abnormal criteria for Liquor Volume?

Answer 26

Normal = Pocket of 2x2cms 
Abnormal = No pocket or <2x2cms

Question 27

What are the disadvantages of U/S?

Answer 27

Possibility of Adverse Bioeffects
Variable Accuracy (false -/+)
Expensives (Machine 200k 7-8yrs , Operator Dependent, Training)

Question 28

What affects the accuracy/usefullness of U/S images?

Answer 28

Problems Related


• Getting good images

• Range v resolution

• Attenuation

• Refection

• Baby in right position 

• Mothers Size
• 
Defects may resolve between scans