For normal anatomy and scanning protocol




3rd trimester evaluation is primarily to assess appropriate growth and foetal/maternal well-being. Other than the heart, most structural assessment is best performed in the 2nd trimester.



FETAL BIOMETRY (Growth Measurements)

BPD                                                                                                  HC

 Biparietal Diameter  BPD and HC
The correct plane for the measurement of the head circumference (HC) and bi-parietal diameter (BPD)must include the cavum septum pellucidum, thallamus and choroid plexus in the atrium of the lateral ventricles. BPD: Measure outer table of the skull to the inner table.
HC: Meaure around the outer table of the skull.




 Abdominal Circumference  AC
Abdominal Circumference
The abdominal circumference is taken with a transverse image to include the stomach, portal vein and the spine in a true tranverse plane.



 Femur Length  FL
Femur Length
The Femur length should only be measured when the femur is horizontal (beam is perpendicular) and shadows evenly- at least from both ends.


 Cervix  Uterine Contraction
The echogenic mucous plug is readily visible in the cervix.
Measure the length of the cervix. It should be at least 30mm, and contain no fluid.
Anterior Placenta appears to be low lying because of the uterine contraction.


The placenta will mature as the pregnancy progresses. Lacunae (lakes) and calcification outlining the cotyledons will be increasingly visible. 

 Fundal Placenta  Vascular Placental Bed
  • Locate the placenta (anterior V's posterior or lateral)
  • Ensure there is a myometrial rim of 3mm or more under the placenta (otherwise suspect placenta percreta/accreta).

The placental bed may be extremely vascular.


 AFI  Amniotic Fluid Index
Click on the image to enlarge for printable quality.
Greater than the 95th centile =POLYhydramnios.
Less than the 5th centile=OLIGOhydramnios.
Amniotic Fluid index (AFI)
Measure the deepest vertical pocket (with no foetal content) in each quadrant and add them together.


 Umbilical Artery Resistive Index  Umbilical Artery Doppler
Click on the image to enlarge for printable quality.
Measure the Systolic-Diastolic (SD) ratio or the resistive index of the umbilical artery. Both of these are different forms of the same infomation.
Umbilical Artery Doppler




 Middle Cerebral Artery Doppler  MCA Doppler
MCA Doppler Middle cerebral artery doppler gives an indication of the "Brain sparing effect".
Measure the pulsatility index.
Another useful ratio is: MCA RI / UA RI < 1 (normally > 1).




 Fetal Circulation  Ductus Venosus Doppler
Ductus venosus doppler:
Oxygen rich blood from the maternal circulation enters to the foetus via the Umbilical vein. The umbilical vein ascends the foetal abdomen and drains into the left portal vein and the IVC via Ductus venosus.
DV is being flagged as a potentially earlier predictor of adverse pregnancy outcome than Umbilical artery dopplers.
Ductus venosus doppler
  • Is recognisable as a small high velocity structure laying superiorly in the liver, adjacent to the IVC.
  • Should be constant forward flow throughout the cardiac cycle.




Breech Types
Breech Types



For uterine artery assessment





  • Fetal breathing should occur regularly in the 2nd and 3rd trimesters but will not be constant.
  • It is a reassuring sign of fetal wellbeing. It does not exclude pathology but absence of any fetal breathing movements in the 3rd trimenster is concerning, particularly in a small for dates foetus.


  • It is accepted common practice perManning et al who suggested that: 1 or more episodes of ≥20sec of breathing should be observed in a healthy fetus within 30 minutes of scanning.

Fetal breathing movements can be seen with

  • Chest/abdominal motion. (Occasionally this can be documented with m-mode)
  • Amniotic fluid movement in/out of the nostrils (profile view or the 'nose/lips' coronal view), and documented using power doppler.
  • Reflected in fetal dopplers such as umbilical artery traces (see image below).

2 videos demonstrating fetal breathing using ultrasound.

Video 1: B-mode of the abdomen and chest movement.

Video 2: Using power colour Doppler to watch fluid movement through the nose with expiration.



ultrasound doppler umbilical artery with fetal breathing

ultrasound power doppler demonstrating fetal breathing through the nose.

The regular arrhythmia seen as a consequence of fetal breathing.

Calculations of resistive index, pulsatility index or SD ratio should not be taken whilst fetal breathing is occurring.

Use of power doppler demonstrating fetal breathing by detecting oscillation of amniotic fluid through the nose.



3rd Trimester Ultrasound - Protocol

Role of Ultrasound

Ultrasound is essentially used for assessing fetal growth and maternal wellbeing.Ultrasound is a valuable diagnostic tool in assessing the following indications:

  • Follow up of previously identified, or suspected, abnormality.
  • Previous obstetric history of abnormality
  • Suspected or known low placental position
  • Bleeding, fluid loss or pain
  • Altered maternal health (eg hypertension or proteinuria)
  • Decreased foetal movements
  • Small for dates (SFD)or Small for Gestational Age (SGA) or Large for dates (LFD) or Large for Gestational Age (LGA)


Patient History

  • Gravidity
  • Parity (Miscarriage, Termination of Pregnancy (T.O.P))
  • Previous scan results
  • Date of Last Menstrual Period
  • Other pregnancy History
  • Gynaecological History


Equipment Selection

  • Modern ultrasound unit
  • Curved linear probe approx 3-7 MHz depending upon maternal factors
  • Transvaginal probe approx 5-9 MHz (Use of non-latex cover is advised)
  • Ensure patient comfort and privacy.
  • Warm gel, clean towels etc
  • Select "Obstetric" preset for appropriate power levels and measurement packages

Use a curvilinear probe (3.5-6MHZ) with low power to reduce potential risk of bio-effects.

Patient Preparation

The patient does not need to drink a lot of water at this stage. A Transvaginal of the cervix should be done if there is suspicion of a shortening cervix. It is too uncomfortable for the patient to drink large amounts of fluid .


Scanning Technique




  • Cervix - assess if closed and measure length between internal and external os
  • Assess placental location and distance from internal os.
  • Check for retroplacental haemorrhages, placental masses etc
  • Maternal adnexae (if indicated, also maternal kidneys)
  • Confirm heart beat & rate
  • Foetal lie: ( eg cephalic, spine to maternal left) If breech, describe the 'type' of breech.
    • Frank
    • Complete
    • Footling
  • Head:
    • Shape
    • Symmetry/falx
    • Cerebellum
    • Cavum septum pellucidum
    • Ventricles
  • Chest:
    • Heart~ rate (check for arrhythmia) position & orientation (4 chambers, outflow tracts)
    • Diaphragm
    • Lungs (homogenous & echogenic relative to liver)
  • Abdo     
    • stomach
    • kidneys
    • bladder
    • anterior abdo wall & cord insertion
  • Limbs:
      • 12 long bones
      • Position of hands/feet
      • Movement & tone
  • Spine: Symmetry from C spine to the sacral taper and an intact posterior skin edge


Standard Measurements

  • Cervical length
  • Placenta to internal Os distance(>3cm)
  • Biparietal diameter(BPD)
  • head circumference(HC)
  • Abdominal circumference(AC)
  • femur length(FL)
  • Foetal heart rate (FHR)
  • Amniotic fluid index (AFI)
  • Umbilical artery: resistive index (UA RI)
  • If UA RI is abnormal.Check the Ductus venosus and Middle cerebral artery: pulsatility index(MCA PI)

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(For utero-placental vascular insufficiency)

  • Look for foetal movements such as leg, hand flexing and diaphragmatic movements.
  • Assess foetal tone and posture.
  • Biophysical Score is a combination of the following assessments giving them a mark out of 8 in total.
    • Foetal breathing 2/2
    • Foetal limb/body movements 2/2
    • Foetal posture 2/2
    • AFI 2/2

If the score is below 7 then this is a concern which will need close follow up. The assessment must span a minimum of a 30minute period before a negative report is suggested.


Common Pathology

Intra-uterine Growth Retardation (IUGR)

  • Large for Gestational age : weight (> 4000 grams)
  • >90th percentile
  • Associated with diabetic mothers, maternal obesity, diabetes, history of previous LGA baby, maternal weight gain, pregnancy >40 weeks, advanced maternal age and multiparity.

IUGR (Intrauterine Growth Restriction)

  • Small for Gestational Age weight
  • <10th percentile
  • Causes include:
    • placental insufficiency
    • maternal hypertension
    • poor maternal health (drug abuse)
    • collagen vascular disease CMV (cytomegalovirus)
    • chromosomal abnormality
  • 2 Types
    1. Symmetrical- these babies are in proportion but reduced in size
    2. Asymmetrical-these babies have a smaller abdomen compared to limbs and head.


Basic Hard Copy Imaging

A 3rd trimester series should include the following minimum images;

  • Cervix -Longitudinal measurement
  • Placental distance
  • Placenta longitudinal and transverse
  • Placenta and cord insertion
  • Foetal lie with head and spine and body marker labelling occiput and spine position
  • BPD
  • HC
  • AC
  • FL
  • Heart rate
  • 4chamber heart
  • RVOT
  • LVOT
  • Kidney trans with PUJ measurement
  • Kidney lengths
  • Bladder
  • Cord insertion
  • Amniotic Fluid Index
  • Diaphragm
  • Profile if visible (depends on foetal position)
  • watch limbs move
  • Cord assessment with S/D and PI ratios
  • Repeat BPD,HC,AC and FL
  • Ductus venosus if IUGR
  • MCA if IUGR
  • Report (summary of measurements)
  • Trend graphs
  • Maternal adnexae
  • Maternal kidneys if mother has pain
  • Maternal gallbladder if clinically indicated
  • Document the normal anatomy. Any pathology found in 2 planes, including measurements


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