Fetal Ultrasound - General Information

Ultrasound is the modality of choice when imaging the pregnancy and fetus.  It is noninvasive, safe due to absence of radiation, low in cost and has widespread availability.  The technique has high accuracy and superior spatial resolution, allowing real time, color Doppler, multiplanar and 3-4 dimensional capabilities.

The first prenatal ultrasound is usually done in the first trimester, with the purpose of confirming the pregnancy as well as to date the pregnancy.  In addition, it is able to help establish the location and size of the fetus, as well as the number in the case of multiple gestations.  It can be performed as part of the first trimester ultrasound genetic screening, as well as to screen for any anomalies of the uterus or cervix. In the first trimester, use of spectral Doppler is discouraged as it delivers greater energy to the fetus than simple 2-dimensional or M-mode imaging.

The next prenatal ultrasound is routinely done in the second trimester, between 16 and 22 weeks gestation, with the aim of evaluating in greater detail the fetal anatomy and identify any birth defects.  In addition, it is possible to evaluate and follow fetal growth, and examine the placenta as well as the quantity of amniotic fluid.  In the event that a fetal anomaly is detected, ultrasound may also be used to guide certain diagnostic procedures such as amniocentesis and chorionic villus sampling which may identify the cause. 

An additional ultrasound in the third trimester is not standard practice.  However, it may be done in the following instances, to name a few: monitoring of fetal growth, assessing evolution of any detected fetal anomaly, or determine fetal position before delivery, as well as to confirm presence of a normal fetal heartbeat if the fetal movements become reduced or absent.

Throughout pregnancy, ultrasound may be used to determine the cause of unexplained vaginal bleeding or other complications.  It may also be used to guide certain procedures that are performed in an attempt to improve the outcome of a fetus with a given problem, e.g. amnioreduction/amnioinfusion, antenatal shunt placement, or ablation in the context of Twin-Twin Transfusion Syndrome.

Details on recognized indications of ultrasound for antenatal imaging are available at:

Fetal ultrasound should be done only for valid and recognized medical indications, and not for creation of “keepsake” videos or pictures, or determination of fetal gender outside of a medical context as is done in certain cultures.

 

 

 

 

1. Operator dependent
2. Small field of view
3. Limited soft tissue contrast
4. Beam attenuation by adipose tissue
5. Difficult in the presence of low amniotic fluid
6. May be hampered by fetal position
7. Limited cerebral detail, especially later in gestation with ossification of the skull.



 

 

An example of an Obstetrical Ultrasound checklist for technologists or reporting physicians

There are many superb fetal ultrasound resources developed over many decades by smart and dedicated medical professionals from multiple disciplines; it is not our intention to assemble them all here. You are encouraged to review the following documents for detailed information on suggested components of thorough ultrasound evaluation of a fetus:


General safety

Studies performed since ultrasound was instituted for application on humans in the late 1940’s have all failed to demonstrate any harmful effects on human subjects in the postnatal period. More intensive research studies were performed specifically on pregnant rats exposed to the maximum power of diagnostic ultrasound for up to 3 days after fertilization also failed to demonstrate any conclusive harmful effects on mitotic chromosomes and DNA damage.

Additional details on research done to study the safety of ultrasound is available at: http://www.ob-ultrasound.net/history2.html

The AIUM position statement indicates that, if exposures are kept as low as reasonably possible, and if performed by a qualified health professional, exposure received from modern diagnostic equipment in the standard clinical setting, ultrasound does not pose a risk.

http://www.aium.org/officialStatements/34

(AIUM Statement on Prudent Use and Clinical Safety)

No teratogenic effects have been documented to date on any study.  Although some studies have reported subtle effects of exposure to diagnostic ultrasound during pregnancy, including lower birthweight, delayed speech, dyslexia and left-handedness, other studies could not independently corroborate these conclusions. 

An extensive longitudinal study performed by John Newnham et al, and published in the Lancet in 2004, followed over 2700 children that were subjected to 5 ultrasounds vs just one from 18 weeks to 38 weeks.  The exams consisted of the routine exam and uterine artery Dopplers.  The children were followed up to age 8.  In their study, they demonstrated no teratogenic effects of ultrasound.  Although a slight reduction in birth weight was noted in the multiple-ultrasound group, there was no significant difference in the size of the children between this group and the controls by age 1 year or thereafter.

Thus, there appears to be insufficient evidence to confirm any long-term harmful link from ultrasound when using levels reached in standard clinical practice, and this is reflected in the AIUM position statement:

http://www.aium.org/officialStatements/16

(AIUM Statement – Conclusions Regarding Epidemiology for Obstetric Ultrasound)

Theoretical risk of biological effects of heat

The first concern in ultrasound is the theoretical risk of tissue damage caused by heat if the temperature of the tissue increases.  The increase in temperature is dependent on the frequency, size of probe, pulse duration, self-heating of the probe, exposure time and tissue properties, among others.

The Thermal Index (TI) is the ratio of attenuated acoustic power at a given point /attenuated acoustic power required to raise the temperature of that point by 1 degree Celsius. It provides a real-time display of the probability that an exposure in progress could induce thermal injury in the subject.  In the average clinical setting, the soft tissue thermal index (TIS) and the bone thermal index (TIB), attempt to estimate the maximum temperature increase. 

Adult tissues are more resistant to temperature increases induced by extensive exposure to ultrasound than fetal or neonatal tissues.  Exposure of adults for duration of up to 50 hours of ultrasound, resulting in increase in body temperature by up to 1.5 degrees Celsius have not shown any  significant adverse effects.  Thus, most clinical ultrasound exams fall far short of any such risk.

In the prenatal period, adverse outcome is a possibility at any point in the pregnancy, but the more apparent effects in animal studies have been noted during the organogenesis period, corresponding to the first trimester in humans. Furthermore, in antenatal sonograms, the ultrasound wave passes through layers of fluid, which may cause the TI to be underestimated by up to a factor of 2.  For this reason, ultrasound exposure in the first trimester are likely still safe, but should really be kept as short as possible.

Additional information is available at:

http://www.aium.org/officialStatements/17

(AIUM Statement on Mammalian Biological Effects of Heat)

Theoretical risk of acoustic damage

Reports of increased fetal activity during ultrasound exams prompted research, performed by Arukumaran et al, to see if the ultrasound wave could be perceived by the fetus.  Although ultrasound waves, by definition, are not normally audible, it appears that the ultrasound wave in the womb may result in a faint vibration which the fetus might hear.  These studies estimate the sound heard by the fetus would correspond to a noise level of around 84dB, or the equivalent of city traffic heard from inside a car.  The level at which sustained hearing loss may result after extended exposure is in the 90-95dB range.  Therefore it appears that acoustic damage to the fetus is unlikely during routine clinical obstetric ultrasound exams.

Full article available here.

Doppler safety

Temperature and vibration increases become progressively greater from grey-scale/ B-mode to color Doppler to Spectral Doppler.  Potential for adverse thermal effects increases with the duration of the exposure at a specific location. For this reason, use of spectral Doppler in the first trimester should be avoided, and used only when there is a clear benefit.  Even then, the exposure should be kept to a minimum, to keep the thermal index under 1.0 (minimal risk). 

When documenting fetal heart beat and rate, M-mode is preferred because of the lower acoustic intensity delivered to the fetus.

http://www.aium.org/officialStatements/43

(AIUM Statement on Measurement of Fetal Heart Rate)

When using Doppler to evaluate maternal structures such as uterine arteries, there is no harm to the fetus as long as the fetus lies outside of the path of the ultrasound wave.

Detailed recommendations of safe use of Doppler in the first trimester are available at:

http://www.aium.org/officialStatements/42

 (AIUM Statement on the Safe use of Doppler Ultrasound during 11-14 week scans, or earlier in pregnancy)

Intravenous contrast for obstetric ultrasound

Two groups have reported use of intravenous (microbubble/microsphere) contrast in obstetric ultrasound, for characterization of uteroplacental blood flow, and fetofetal transfusion in monochorionic twin gestations.  In both cases, no adverse effects were noted.  Additional theoretical applications include evaluation of placenta accreta.

According to research published by Jun Murotsuki from Japan  in 2007, no mutagenic effects were noted in vitro thus far, and in vivo testing in animals has not shown mutagenic or teratogenic effect of these agents. However, there is greater risk of mutagenic effects in the first trimester and consequently these agents should be avoided during this period.  Because research in this domain is still very limited, these agents should not be utilized even in the second and third trimester unless there is distinct benefit clearly outweighing the risks.

For further reading here

Summary

Ultrasound has evolved immensely since its inception and is now standard of care in antenatal care, with use in first and second trimesters primarily.

There is no concrete evidence to indicate that there are any harmful long-term effects of ultrasound on the fetus, in the exposures usually sustained during standard modern obstetric ultrasound exams, as long as they are carried out for a valid medical reason and by a properly trained health care specialist. 

Other than attention to minimizing the duration of the ultrasound exam in general, and avoidance of Spectral Doppler in the first trimester, no other specific measures are warranted.  

There are starting to be reports of use of intravenous (microbubble/microsphere) contrast in obstetric imaging, with encouraging results, but caution should still be exerted.  Specifically, these agents are still to be avoided in the first trimester because of their mutagenic potential.

 

Recommended Readings

  • Benacerraf BR. Ultrasound of Fetal Syndromes. Second Edition. Churchill Livingstone, 2008.
  • Callen PC.  Ultrasonography in Obstetrics and Gynecology. Fifth Edition. WB Saunders and Co, 2008.
  • Sanders RC.  Structural Fetal Abnormalities: The Total Picture. Second Edition. Elsevier Health Sciences, 2002.
  • Woodward PJ.  Diagnostic Imaging: Obstetrics. Amirsys, 2005.
  • Woodward PJ. Expertddx: Obstetrics. Amirsys, 2009
  • Fundamental and Advance Fetal Imaging: Ultrasound and MRI. Editors Beth Kline-Fath, Dorothy Bulas and Ray Bahado-Singh; Wolters Kluwer, (2015).

Websites

Fetal Imaging Index

Fetal Ultrasound Images