The value of ultrasound in pregnancy

What is ultrasound?

Ultrasound is a high frequency sound which is beyond the hearing range of humans. It uses frequencies in a pulsed fashion from the probe, which is applied on to the abdomen or into the vagina to produce the image.

The frequency of the sound wave varies with the probe used – transvaginal ultrasound has a higher frequency than the trans-abdominal one.

For optimum imaging, a gel has to be applied to the scanned area. The image is then visualised on a screen in black and white with varying shades of grey. 3D imaging utilises 2D imaging in three planes, and there is an artificially coloured image which is a reconstructed image of the volume of the structure image. 4D imaging is a real-time 3D, with time being the fourth dimension. It allows a person to view a moving structure, particularly the fetus.

Is it safe to use in pregnancy?

Although there have been no adverse effects reported on the use of ultrasound for obstetrical diagnosis, safety is always a concern.

The main fear is the heating or thermal effect, which can cause damage to susceptible organs. There is also concern that prolonged exposure may harm a developing fetus.

“Prolonged use of Doppler ultrasound with colour flow mapping to visualise the heart and produce the sound of the heartbeat, should be restricted.”

That is why prolonged use of Doppler ultrasound with colour flow mapping to visualise the heart and produce the sound of the heartbeat, should be restricted. Basically, ultrasound should be done only if advised by the doctor, not because you have the desire to see the baby. Currently, no adverse effects have been reported with the use of 2D/3D/4D ultrasound in obstetrics.

When can it be used in pregnancy?

In early pregnancy (less than 12 weeks), ultrasound plays a key role in assessing the viability, growth and number of fetuses. It is also used to detect intrauterine pregnancy and, indirectly, ectopic pregnancy (pregnancy outside the uterine cavity e.g. tubal pregnancy).

At 11 to 13 weeks and six days, it can be used to detect or suspect chromosomal abnormalities like Down’s syndrome by looking at the skin at the back of the neck (nuchal translucency) and the presence of a nasal bone.

In the second trimester, the detailed scan for fetal structural anomaly is usually done between 18 to 22 weeks of gestation.

Beyond this, ultrasound is not really required except when fetal growth abnormalities are suspected, in which case blood flow studies of various vessels in the fetus can be done as well to determine when is the best time to deliver.

In cases where an abnormality is suspected, an invasive procedure like amniocentesis or placental biopsy (chorionic villus sampling) – if required – is performed under ultrasound guidance.

Can a fetal abnormality be missed by ultrasound?

Yes, this is possible even in experienced hands. The most common is heart abnormalities which can be very subtle and is detected only after the baby is born.

Some abnormalities like ventriculomegaly (water in the brain) can develop suddenly even if the 18 to 22 weeks scan was normal. Certain abnormalities like a cleft palate at the back of the throat cannot be detected by ultrasound.

Is 3D screening necessary?

For routine fetal abnormality, 3D or 4D screening is not necessary as most of the time, the reconstructed image is for the parents to see what their unborn baby looks like.

In special circumstances, there are special modifications to the software that allows better visualisation of the fetus.

Abnormalities detected via 3D screening helps the parents iden-tify the problem to seek further counselling on therapy. It also helps them bond with the unborn child. – The HEALTH

Dr Raman Subramaniam is the Fetal Medicine and Gynaecology Centre, Kuala Lumpur consultant obstetrician and gynaecologist. He can be contacted at raman_132 at hotmail.com.

 

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