When is it done:
Ultrasound or sonar scans are routinely done at any time during pregnancy. When screening specifically for chromosomal abnormalities, the best time for the scan is between 11 and 14 weeks. For structural defects, better views are obtained later on in the pregnancy.
This depends on the experience of the sonologist and on the condition being looked for.
Ultrasound imaging has been increasingly used in obstetrical care since the 1970’s.
The ultrasound uses high frequency sound waves which are bounced off the baby. A computer then translates the echoing sounds into video images which reveal the baby's shape, position and movements.
Mothers look forward to the ultrasound examination because it gives them a first glimpse of their baby. At the same time, doctors and midwives use the ultrasound to gather a wealth of information about the baby and the pregnancy.
The following are some of the uses of the ultrasound:
- Assessing the viability of a pregnancy - The ultrasound has the ability to quickly and accurately establish whether a foetus is dead or alive and to predict whether a pregnancy is likely to continue after a threatened miscarriage. It allows early identification of a failed pregnancy before the patient experiences any signs that indicate a miscarriage.
- Assessing the number of foetuses and the type of twins - With ultrasound, multiple pregnancies can be detected at a stage where they are not clinically suspected. This may improve the management of such pregnancies. Ultrasound is the only way to determine whether the twins are identical or not. This is very important since both types of twins require a different approach in the care and management.
- Dating a pregnancy, checking the heartbeat and measuring growth - Using an ultrasound, the gestational sac can be seen from five to six weeks and the foetus by week six or seven. As soon as the foetus can be visualised, it can be measured and its viability confirmed by the detection of heart movements. The use of ultrasound to assess the duration of the gestation is based on the fact that in early pregnancy, foetal growth is rapid and there is little biological variation in size. In the first trimester, the measurement of the foetal crown–rump length is usually accurate within seven days. After 13 weeks, this measurement becomes less reliable and other methods (measuring the head, the tummy and the thigh bone) are used. Measurement of the foetus after 30 weeks is of no value for assessing the duration of pregnancy since at this advanced stage, there is too much variation between big and small babies of the same gestational age.
- Checking for foetal structural abnormalities directly. A large number of foetal abnormalities can now be detected by modern ultrasound imaging. A good sonologist can detect abnormalities such as a cleft lip and palate, spina bifida, major limb or brain abnormalities and any severe abnormalities of the heart or kidneys. The presence of one defect may suggest the presence of others, which are more difficult to detect with ultrasound. Another test such as an amniocentesis may be used if the findings suggest the presence of a chromosomal abnormality. The early detection of severe abnormalities in the developing foetus offers parents the option to consider termination of the pregnancy. Identification of a treatable condition may lead to intervention that can save the baby's life. Identification of other types of abnormalities may have the advantage that the paediatrician is forewarned and prepared to treat a baby that may be unwell at birth.
- Locating the placenta - The ultrasound is the best available method for locating placental position and to rule out placenta praevia (= obstructing the birth canal). It is also important prior to chorionic villus sampling and amniocentesis.
Some hospitals are equipped with 3D ultrasound machines. With the three-dimensional scan, a computer rapidly stores a series of images to create a life-like picture of the baby and doctors are able to see a cross-section of any part of the body. The 3D scan is usually considered an extravagance rather than a necessity and its use in the detection of abnormalities is limited.
The main purpose of scanning in late pregnancy is to identify a foetus with intrauterine growth restriction which may benefit from elective delivery.
(Ilse Pauw, Health24)