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All your pregnancy questions answered

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Doppler Ultrasound

Blood flow characteristics in the fetal blood vessels can be assessed with Doppler 'flow velocity waveforms'. Diminished flow, particularly in the diastolic phase of a pulse cycle is associated with compromise in the fetus. Various ratios of the systolic to diastolic flow are used as a measure of this compromise. The blood vessels commonly interrogated include the umbilical artery, the aorta, the middle cerebral arteries and the uterine arcuate arteries.

The use of color flow mapping can clearly depict the flow of blood in fetal blood vessels in a realtime scan, the direction of the flow being represented by different colors. 'Color' doppler is particularly indispensible in the diagnosis and assessment of congenital heart abnormalities. Doppler USG is also useful for evaluating a growth retarded fetus and establishing when the fetus is at risk and needs to be delivered.

How many scans do you need?

There is no hard and fast rule as to the number of scans a woman should have during her pregnancy. A scan is ordered when an abnormality is suspected on clinical grounds. Otherwise a scan is generally booked at about 7 weeks to confirm pregnancy, exclude ectopic or molar pregnancies, confirm cardiac pulsation and measure the crown-rump length for dating.

A second scan is performed at 18 to 20 weeks to look for congenital malformations, exclude multiple pregnancies and to verify dates and growth. Placental position is also determined. Many centers are now performing an earlier screening scan at around 13-14 weeks to measure the fetal nuchal translucency and to evaluate the fetal nasal bone to aid in the diagnosis of Down Syndrome.

A third scan may sometimes be done at around 34 weeks to evaluate fetal size and assess fetal growth. Placental position is further verified.

How safe is a scan for my baby?

It has been over 40 years since ultrasound was first used on pregnant women. Unlike X-rays, ionizing irradiation is not present and embryotoxic effects associated with such irradiation should not be relevant. The use of high intensity ultrasound is associated with the effects of "cavitation" and "heating" which can be present with prolonged insonation in laboratory situations.

Harmful effects in cells of experimental animals or humans however have not been demonstrated in the large amount of studies that have so far appeared in the medical literature purporting to the use of diagnostic ultrasound in the clinical setting. Apparent ill-effects such as low birthweight, speech and hearing problems, brain damage and non-right-handedness reported in small studies have not been confirmed or substantiated in larger studies from Europe. The complexity of some of the studies have made the observations difficult to interpret. Every now and then ill effects of ultrasound on the fetus appears as a news item in papers and magazines. Continuous vigilance is necessary particularly in areas of concern such as the use of pulsed Doppler in the first trimester.

The greatest risks arising from the use of ultrasound are the possible over- and under- diagnosis brought about by inadequately trained staff, often working in relative isolation and using poor equipment.