iWNH- Fetal Biometry
Fetal Biometry

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Ultrasound Safety

WSLHD JMO and midwifery staff with limited training (i.e. have not completed their MFM term or have a formal ultrasound qualification such as CMFM, COGU, DDU, DMU) should only undertake:
Limited B-Mode U/S studies, ideally based on ISUOG 6-steps (see refs), involving assessment of
o fetal presentation
o fetal heart motion - using B-mode/M-mode not Doppler
o number of fetuses
o +/- limited placenta location
o amniotic fluid index
o +/- limited biometry
Brief use of colour/power Doppler, if required, to exclude the presence of umbilical cord when assessing the AFI. As far as practicable, the colour/power Doppler box should not overlay the fetus.
JMOs/MWs are NOT credentialed to use spectral/PW Doppler^ for any purpose, and especially not to ‘hear’ the fetal heart. B-mode/M-mode should be used for FH demonstration.

Mid-level registrars who have completed the MFM term, should be guided by their level of experience. If using Doppler, it must be as briefly as possible and with continual monitoring of the Thermal Index (TI).
B-mode/M-mode should be used for FH demonstration. Very rarely, other medical staff with limited credentials may be requested by an MFM consultant to use PW Doppler on the umbilical artery for specific indications.
The reason for the restriction in use of Doppler to those with appropriate training is outlined in this education sheet.
Ultrasound unavoidably heats the tissues it passes through. While B-mode is a low power/intensity mode with low tissue heating capacity, Doppler imaging modes (colour, power and Spectral/PW), used improperly, have capacity to cause thermal injury to the fetus. Greatest vulnerability occurs
In the first & early second trimester
Near bone at any gestation – the fetal brain and spine are especially vulnerable e.g. MCA Doppler (see later for more detail)
B-mode ultrasound scan technology (including 3D)
o Used for routine black and white (‘grey scale’) scanning
o Low power/intensity à extremely low risk of significant fetal heating.
M-mode ultrasound technology
o A single line of B-mode, used for measuring FHR.
o Due to single line of sight, slightly greater intensity than B-mode which both covers a larger area and is generally moving (intensity = power/area = watts/cm2).
o M-mode is low power/intensity, with very low risk of significant fetal heating.
Colour/power Doppler ultrasound technology
o Used to identify blood vessels. Colour / power Doppler overlays a colour box onto B-mode to identify moving reflectors, usually red blood cells in blood vessels.
o Intermediate energy/power/intensity and risk of fetal heating (though see next).
Spectral/Pulsed Wave (PW) Doppler ultrasound technology
o Used by experts for ‘flow studies’ on vessels e.g. umbilical and middle cerebral arteries
o Capable of high intensity with high energy/power held stationary down a single line of sight. Usually the highest intensity of all diagnostic ultrasound although, depending on settings, there can be considerable overlap in intensity/heating potential between colour/power Doppler and spectral/PW Doppler.
o Spectral/PW Doppler can pose significant heating risk to the fetal tissues it passes through in just a few minutes.
o See previous page for Spectral/PW Doppler use being restricted to appropriately-trained staff
Continuous Wave (CW) Doppler ultrasound technology
o Used in CTG machines and hand-held Dopplers (+ other uses not relevant here).
o Due to their particular physical properties (narrow bandwidth, air-backed transducer), CTG/handheld Dopplers use Very Low power/intensity technology, with a negligible risk of significant fetal heating, meaning they can safely be used for extended periods.
Very Important Safety Distinction Between PW and some CW Doppler Technologies:
While they ‘sound’ the same to the human ear, the Doppler technology employed in spectral/pulsed wave (PW) Doppler has massively greater capacity to heat the fetus than the very low power continuous wave (CW) Doppler technology used for CTG/handheld Doppler.
Ultrasound unavoidably heats the tissues it passes through.
o Diagnostic U/S, used appropriately, mostly doesn’t heat tissues much, although, as above, colour, power and spectral/PW Doppler do have significant tissue-heating capacity
o Therapeutic U/S heats tissues well above physiological levels – for example physiotherapy U/S for injured muscles/tendons and even more so, kidney/gall stone lithotripsy & high intensity focused ultrasound for tumour destruction.
Before 1992, due to safety concerns, the power output of diagnostic U/S machines for obstetric/fetal studies was restricted. That is, the machine took responsibility for fetal safety. From 1992, in order to obtain better quality images, the power output / intensity for machines used in obstetric studies was permitted to greatly increase.
The person undertaking the U/S now carries full responsibility for ensuring fetal safety. This person is expected to be fully trained in ultrasound safety.
To aid the user, there are now on-screen output display standards (ODS) indicators - thermal index (TI) and mechanical index (MI) – located top right of screen & saved permanently on every image.
o MI mostly relates to risk of harm involving tissues containing air, where bubbles can form and collapse releasing pressure waves into surrounding tissues. It is not generally a concern in obstetric scanning but can be a concern in the newborn. Keep MI < 0.7 CHECK
TI is extremely important in pregnancy scanning. It indicates the ratio of the machine output power to the power required to raise the temperature of insonated tissue by 1° C. Therefore, the TI estimates the temperature rise during a scan, based on machine physics & assumptions about the patient. It does not (cannot) actually measure the temperature rise.
o Studies show the TI may underestimate heating by up to a factor of 2 i.e., TI = 1.0 could correspond to a worst-case temperature elevation of 2 ° C.
o < 1.5 ° C is thought a likely safe increase above normal body temperature in pregnancy. Therefore, authorities advise keeping TI < 1.0 (even with Doppler) and ideally TI < 0.7.
o There are 3 TI in current use:
§ TIS (soft tissue) - no bone in the field, used in first trimester < 10 weeks
§ TIB (bone) – used ≥ 10 weeks when fetal bones mineralise
§ TIC (cranial) – not used in obstetrics, of relevance to newborn head scan
· The additional heating vulnerability associated with some fetal ultrasound examinations
o The first trimester/early second trimester fetus is probably more vulnerable than later due to the period of critical organogenesis, small fetal size and poor heat dissipation due to the as-yet-poorly-developed feto-placental circulation.
§ Colour, power and Spectral/PW Doppler should be avoided in the first trimester
§ The exception is where it is used for the screening and diagnosis of congenital abnormalities or aneuploidy by appropriately trained staff within the WIMFM.
o Tissues near bone (brain, spinal cord) are particularly vulnerable because bone absorbs much of the ultrasound beam, converting sound energy to heat, and conducting it to adjacent tissues. The TIB index recognises the additional risks of tissue heating near bone.
o Finally, staff must be constantly aware that vulnerability to heating exists across the whole of pregnancy and in all fetal tissues (see Animal Studies near the end of the document).
Keeping TI < 1.0 (and ideally TI < 0.7)
· Only undertake indicated ultrasound studies.
· Limit the study duration and dwell time to minimise fetal tissue heating
o Keep the study as short as needed to obtain necessary information and meet safe training requirements.
o Limit the dwell time - time spent focusing the beam on one area or fixed position.
o Do not rest the ultrasound probe on the woman when not undertaking real-time image acquisition i.e., if discussing an image frozen on the screen.
· Special heating issues
o In a febrile patient, take even greater care about additional heating.
o Do not use any probe (especially vaginal probe) that appears to be self-heating.
· Use the appropriate Obstetric Pre-Set Ultrasound Program
o Use obstetric pre-sets, not the gynae or abdo pre-sets which may have higher power.
o Use the appropriate obstetric pre-set for GA, so appropriate TI (TIS or TIB) is shown
· In demonstrating fetal heart motion, do not use Spectral/PW Doppler. Instead either
o show the woman the heart beating on the screen (B-mode)
o measure the fetal heart rate (FHR) with M-mode
· Continually monitor the TI and keep it < 1.0 and ideally < 0.7
o Ensure TI < 0.7 before you start the scan (obstetric scan pre-sets should ideally already be set at < 0.7)
§ While using B mode, the TI should remain < 0.7. For interest, take note of the small TI changes that can occur when you alter depth, width, number/location of focus, tissue harmonic imaging (THI), write zoom, other.
§ If briefly using colour/power Doppler during AFI, must keep TI < 1.0. For TI 0.7 – 1.0, do the study quickly and avoid placing the Doppler colour/power box over the fetus as much as possible (learn how to do this adjustment). Those without formal ultrasound credentials or completed MFM term must not scan with TI > 1.0.
o Trained experts in scanning (sonographers, MFM consultants/fellows, accredited registrars under their direct supervision) may occasionally need to undertake brief spectral/PW Doppler imaging or special colour/power Doppler studies with TI 1 – 2.5 (never above 3). Both the TI and study duration must be governed by ALARA principles.
ALARA – as low as reasonably achievable for both TI / MI and scan duration
Source: BMUS 2010 – Guidelines for safe use of ultrasound diagnostic equipment.
In considering the safety of ultrasound, we need to examine Human Epidemiological, Animal Study and In Vitro studies. Data from humans is, of necessity, observational not experimental, as it is not feasible to undertake or withhold ultrasound in clinical practice for purely research reasons.
Human Epidemiological Data
The limited epidemiological literature about ultrasound safety in the second trimester of pregnancy has not found an association between ultrasound and adverse perinatal events, neurological & cognitive development, autism, dyslexia, speech development, childhood cancer or adult mental health disorders.
· It is unclear if there is a small association with non-righthandedness and, if so, the mechanism.
· Importantly, much of the epidemiological data comparing fetal exposure versus no exposure dates from pre-1992 when ultrasound machine power was considerably lower.
· While more recent epidemiological evidence is also reassuring, there is a relative paucity of it, especially given fetuses in high-income-countries are often exposed to multiple scans.
For example, little data exists around the impacts of:
· Studies in the first trimester
· Study duration
· Modes used during the ultrasound (B-mode v Doppler) and the duration of mode use
· Average and range of TI recorded during the ultrasound
· Cumulative effects of repeated ultrasounds
Ultrasound authorities warn that subtle fetal effects are possible, cumulative effects are possible and the absence of evidence of harm does not equate to evidence of absence of harm. Prudent use is required.
Animal Studies – Early Pregnancy
· Pre-implantation and early post-implantation embryos exposed to low levels of ultrasound intensity (B-mode) do not appear to be adversely affected but those significantly exposed to very high levels of intensity (Doppler) may be affected.
Animal – Later Pregnancy
Intensity levels associated with B-mode appear safe but spectral/PW Doppler studies can be more concerning. The following studies were cited by RCOG 2015 and/or the WFUMB Webinar 2018.
· The brains of fetal chicks in late gestation were exposed to B-mode for 5 or 10 minutes or Spectral/PW Doppler for 1, 2, 3, 4, 5 minutes. Learning and memory function were assessed on day 2 post hatch. B-mode ultrasound exposure did not affect the chicks’ ability to learn and develop memory. However, following 4 and 5 minutes of PW Doppler, the chicks had significantly poorer short-, intermediate- and long-term memory (Schneider-Kolsky 2009).
· The ductus venosus in the livers of fetal rats in late gestation was exposed to PW Doppler for 3, 10, 15, 20, 60, 300, 600 seconds. Intensity level (ISPTA.3) at the focal zone was 140 mW/cm2, well below the FDA permitted fetal ultrasound maximum of 720 mW/cm2. For all exposures ≥ 15 seconds, cell death was identifiable; the apoptotic index correlated with the exposure time. Recovery from the damage was demonstrated by 12 hours after exposure (Pellicer 2011).
In Vitro Ultrasound Phantom Studies
· An ISUOG test phantom has been constructed to simulate the human maternal-fetal environment. It records temperature changes during scanning via sensors embedded in tissues mimicking the skin surface, sub-surface, deeper tissues, amniotic fluid and fetal bone/tissue interface.
· A recent ISUOG Phantom study using one brand (unnamed) of ultrasound machine and four transducers suggested that, under normal use conditions, temperature increases were highest at the transducer-skin interface with increases 1.4 – 9.5° C (note that initial ‘skin’ temperature was 10 degrees lower than internal temperature). Lower temperature rises of 0.1 – 1.0° C were observed in deeper tissue and at the bone interface, within the accepted safe range. Most heating occurred within in the first few minutes, then stabilised. Doppler modes generated the highest temperature increases. Increasing output power significantly increased heating in B-mode. (Smith, Lees 2021).
· BMUS 2010. Guidelines for the safe use of diagnostic ultrasound equipment. British Medical Ultrasound Society. Ultrasound 2010;18:52-59.
· FDA 2013. Cibull SL, Harris GR, Nell DM. Trends in diagnostic acoustic output from data reported to the USA Food and Drug Administration (FDA) for device indications that include fetal applications. J Ultrasound Med 2013;32:1921-32.
· Gill R. The Physics and Technology of Diagnostic Ultrasound (textbook). Second Edition. 2020.
· Moderiano M, McEvoy M et al. Safety of ultrasound exposure: knowledge, attitudes and practices of Australasian sonographers. J Diag Med Sonography 2018;34(5):357-367.
· Pellicer B, Herraiz S et al. Ultrasound bioeffects in rats: quantification of cellular damage in the fetal liver after pulsed Doppler imaging. Ultrasound Obstet Gynecol 2011;37:643-648.
· Schneider-Kolsky ME, Ayobi Z et al. Ultrasound exposure of the fetal chick brain: effects on learning and memory. Int J Devel Neurosci 2009;27:677-683.
· Smith SF, Miloro P, Axell R, ter Haar G, Lees CC. In vitro characterisation of ultrasound-induced heating effects in the mother and fetus: a clinical perspective. Ultrasound 2021;29(2):73-82.
· WFUMB 2013. Policy and Statements on Safety of Ultrasound. Ultrasound in Med & Biol 2013;39(5):926-929.
· WFUMB Webinar Recording on Safety and Bioeffects 2018. YouTube. Accessed 6.6.21.
For beginner scanning tips, see 12-minute Medical Aid Films based on ISUOG lectures:
The principles of ultrasound imaging (see also ISUOG Basic Training Lectures 1, 2)
The basic steps of an obstetric ultrasound examination (ISUOG Lecture 9, The 6 steps)
AFI – Amniotic Fluid Index
CMFM - Certificate of Maternal Fetal Medicine
COGU – RANZCOG Certificate of O & G Ultrasound; DDU – Diploma of Diagnostic Ultrasound
CTG - Cardiotocograph
CW – Continuous Wave
FHM / FHR – Fetal Heart Motion/Rate
ISPTA – Spatial Peak Temporal Average Intensity
ISUOG – International Society of Ultrasound in Obstetrics and Gynaecology
JMO – Junior Medical Officer
MI – Mechanical Index
MW – Midwife
mW/cm2 – milliWatts per square centimetre (intensity measurement)
PW – Pulsed Wave
TI – TIB, TIS – Thermal Index Bone, Thermal Index Soft Tissue
U/S – Ultrasound
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