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All Woman
The truth about ultrasound scanning
Is there more to it than 'is it a girl or boy'?
BY DR NADINE JOHNSON
Monday, June 29, 2009
ULTRASOUND has been used in obstetrics to look at babies for over 30 years. The word 'ultrasound' refers to sound waves at a frequency greater than the human ear can appreciate. The ultrasound probe that is placed on your belly sends out these sound waves which are reflected from your baby back to the machine. The computer in the machine then forms an image of your baby which you see on the screen. No harmful effects from ultrasound to babies have been found. Unlike X-rays, no radiation is used. Sound waves can increase body temperature but this is significant only for prolonged exposure times.
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Beyond determining the sex of your baby, ultrasound is used throughout the pregnancy by your obstetrician to better manage your pregnancy.
Many women do not remember their last menstrual period which is what doctors usually use to determine how many weeks pregnant you are. The most accurate time to determine the baby's age with ultrasound is during the first trimester (up to 13 weeks). Other uses during this time are to confirm your baby is alive and well especially if you have been bleeding, and to identify twin pregnancy and ectopic pregnancy (pregnancy outside of the uterus).
As women age, their ovaries which makes the eggs needed to form babies are more likely to make eggs with abnormal chromosomes (genes). The most common chromosomal problem that can occur is Down's syndrome (trisomy 21). By measuring the thickness of the skin at the back of the baby's neck between 11 and 13 weeks of pregnancy, we are able to tell if the baby is at increased risk of having Down's syndrome as these babies have a greater measurement than normal. This is called nuchal translucency measurement.
Swelling from fluid at the neck (nuchal translucency)
If you can only afford to have one ultrasound examination during your pregnancy, then the most useful would be during the second trimester between 18 to 20 weeks of pregnancy. At that time it is still possible to tell the age of your pregnancy with reasonable accuracy but in addition we are able to look at the baby's anatomy (structure). This is called the anomaly scan. Three out of every 100 pregnancies will have a baby in which a body part is malformed.
This could be a major or minor malformation of the face, brain, heart, lungs, abdomen or limbs. Most major abnormalities will be visible, however, minor ones may not be visible. Diagnosis is important while you are pregnant in order to make special preparations for the baby's birth.
At 22 - 24 weeks of pregnancy, ultrasound can be used to predict two conditions which can cause medical problems for both the mother and the baby. Using a specialised technique called Doppler to measure the blood flow to the womb, it is possible to predict an increased risk of developing preeclampsia (high blood pressure and protein in the urine) in high-risk women. We can also use ultrasound to measure the cervix (neck of the womb) at this time. Women with a short cervix are at increased risk of delivering their baby preterm (long before the due date). A cerclage (stitch in the cervix) may help to prevent preterm delivery. In patients who have had recurrent miscarriages because of a suspected weak cervix, measuring the cervix frequently from as early as 14 weeks can be done to see if the cervix is truly weak at which time a cerclage can be done.
During the third trimester (after 28 weeks) ultrasound is useful to find the placenta (afterbirth). If this is close to the cervix (placenta praevia) then delivery by caesarean section is needed to prevent excessive bleeding at delivery. Measurement of the baby's growth is important to ensure that the baby is not growth restricted (too small) or macrosomic (too large). If we are concerned about your baby's welfare (activity) then a biophysical profile will be ordered to assess this. A healthy baby will have a normal assessment. We can also use Doppler to measure the blood flow to the baby from the placenta as an additional assessment of your baby's health.
These are the more common uses of ultrasound in pregnancy. Finally, you may have heard or read about three dimensional (3D) and four dimensional ultrasound (4D).
This technology provides a more realistic and readily recognisable image to parents than the more commonly used two-dimensional (2D) images which can be difficult to appreciate if you have not been trained to recognise them. Their greatest advantage is that they aid in parental bonding to the baby. 3-D US may also provide additional help in diagnosing abnormalities of the spine, face, feet and heart.
However, for routine ultrasound imaging in pregnancy conventional 2D ultrasound pictures is more than adequate.
Therefore, ultrasound scanning during pregnancy has many indications apart from knowing the gender or getting a picture. More importantly, it can provide additional information that will aid your doctor to ensure a healthy mother and baby.
Dr Nadine Johnson is a lecturer / maternal - foetal medicine specialist at the University Hospital of the West Indies (specialist in high-risk pregnancies).
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