3rd trimester normal

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.

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For scanning normal cervix and placenta

Fetal Biometry (Growth Measurements)

Head Circumference (HC) and Bi-Parietal Diameter (BPD)

The correct plane for the measurement of the head circumference (HC) and bi-parietal diameter (BPD)must include:

  1. The cavum septum pellucidum.
  2. The thallamus.
  3. The choroid plexus in the atrium of the lateral ventricles.

The correct plane for the measurement of the head circumference (HC) and bi-parietal diameter (BPD).

Ultrasound image.
BPD: Measure outer table of the skull to the inner table.
HC: Measure around the outer table of the skull.

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

Abdominal Circumference

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

Femur Length (FL)

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

Femur length.

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

Cervix

Ultrasound image- 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.

Ultrasound image- Anterior Placenta appears to be low lying because of the uterine contraction.

Placenta

Ultrasound to 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).

Ultrasound image. The placental bed may be extremely vascular.

Liquid (Liquor) and AFI

Greater than the 95th centile =POLYhydramnios.
Less than the 5th centile=OLIGOhydramnios.

Ultrasound image- Amniotic Fluid index (AFI)
Measure the deepest vertical pocket (with no foetal content) in each quadrant and add them together.

Dopplers in 3rd trimester

Dopplers are routinely obtained from:

  1. Umbilical artery
  2. Ductus venosus
  3. Middle cerebral artery
A schematic of the foetal circulation.
  • Oxygenated, nutrient rich blood placental blood enters the foetus via the umbilical vein.
  • The umbilical vein joins the portal venous system in the liver, some umbilical blood is shunted directly to the IVC and right heart via Ductus Venosus.
  • Blood from the right heart pumps to the lungs through the pumonary artery. However it is already oxygenated so is bypassed through foramen ovale (right atrium to left atrium) and Ductus arteriosus (pulmonary artery to aortic arch).
  • Foetal blood returns to the placenta from the iliac arteries into the umbilical arteries.
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For a detailed page dedicated to obstetric dopplers.

Umbilical Artery
  1. Resistive index
  2. SD ratio
  3. Pulsatility index

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 Ultrasound Image

Middle cerebral artery Artery

MCA Doppler

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

Ductus venosus
  • 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 suggested as a potentially earlier predictor of adverse pregnancy outcome than Umbilical artery dopplers.

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 ultrasound
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.

reference article

Excellent obstetric doppler guidelines

Presentation

  • Cephalic – Foetal head against cervix.
  • Breech – head towards maternal head (see subtypes below)
  • Transverse – Foetal head to maternal left or right

Breech Types

Foetal Lie

  • Refers to whether the baby is laying on it’s back, side or front.
  • At delivery this becomes more important. Preferably the baby is cephalic and face down. The term for face up is “posterior” or “occiput posterior” (OP).

Fetal Breathing Movements

  • 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 trimester is concerning, particularly in a small for dates foetus.
  • It is accepted common practice per Manning 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:

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

Ultrasound image- 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 ultrasound demonstrating fetal breathing by detecting oscillation of amniotic fluid through the nose.

Colour doppler of fetal breathing
Using power doppler to watch amniotic fluid oscillate in and out of the nostrils with foetal breathing movements.
Fetal breathing movements in the fetal torso
Observing the rise and fall of the foetal chest, abdomen and diaphragm on ultrasound during foetal breathing movements.

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)

Limitations

  • BMI >30
  • Fibroids
  • Moving baby
  • Oligohydramnios

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 .

 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 setup

  • 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.

Common Pathology

  • Intra-uterine Growth Retardation (IUGR)
    Macrosomia
  • 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

  • Symmetrical- these babies are in proportion but reduced in size
  • Asymmetrical-these babies have a smaller abdomen compared to limbs and head.

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)


BIOPHYSICAL PROFILE ASSESSMENT
(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.

 

Basic Hardcopy 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