Ultrasound during pregnancy is used to monitor various aspects of development. In fact, one of the most exciting developments in modern obstetrics is the use of high-frequency sound waves to produce ‘pictures’ of thewithin the uterus. This method was pioneered by the British obstetrician, Professor Ian Donald, and is based on the principle that when high-frequency sound waves strike a solid object, the waves can be picked up by a suitable receiver and converted into pictorial form.
The history of ultrasound is interesting because it was first applied to the detection of submarines, by surface warships, during World War I. The modern diagnostic ultrasound machine is a development of these earlier uses, so, not for the first time, a wartime invention has been put to peaceful use.
THE USE OF ULTRASOUND IN VERY EARLY PREGNANCY
An ultrasound picture of the developing embryo can be obtained by about five to six weeks after the last menstrual period. By eight weeks, a clearly defined fetal echo can be shown on the ultrasound screen. Ultrasound is most often used in cases of threatened abortion in early pregnancy to determine if the embryo is continuing to grow and develop. It can also be used at this stage in pregnancy if there is reason to think that the baby may be abnormal.
ULTRASOUND EXAMINATION AFTER THE FIRST TWELVE WEEKS
Ultrasound examination is becoming more and more widely used and it seems likely that, in future, all pregnant women will have at least one ultrasound examination during the course of a pregnancy. At present, ultrasound examination is used in the following conditions.
The ultrasound picture of twins is characteristic and a diagnosis can be made very early. In most cases twins will not be suspected before sixteen to twenty weeks, even later in some cases, and an ultrasound scan, at this time, will quickly establish the diagnosis. Triplets, quads and even quins can also be diagnosed.
When a woman is unsure of the date of her last menstrual period, or has irregular periods, it is difficult to estimate when the baby is due. In this event, an ultrasound scan will measure the diameter of the fetal head and will give a good indication of the maturity of the fetus and hence the expected date of delivery.
This examination is most accurate when done at about twenty-eight to thirty weeks. The measurement of the length of the fetus, crown-rump length, may also be used, as may the circumference of the fetal chest, to give an indication of the likely birthday.
Successive ultrasound scans, at weekly intervals during the last ten weeks of pregnancy, to measure the fetal head diameter give a useful indication of how well the fetus is growing. Slow growth suggests that the fetus is not receiving all the oxygen and nourishment that it should from the placenta.
This situation sometimes calls for early delivery of the fetus to avoid the risk of a more severe lack of oxygen as the pregnancy progresses. A baby that is smaller than it should be is said to be ‘small for dates’. Ultrasound scans are frequently used to detect the ‘small for dates’ situation and to measure subsequent fetal growth as the pregnancy progresses.
An ultrasound scan shows clearly which way round the fetus is and enables a breech presentation to be diagnosed.
The Position of the Placenta
The placenta is usually placed in the upper part of the uterus where it will not interfere with the fetus as it passes down through the cervix in labor. If the placenta is placed low down in the uterus, it may obstruct the passage of the fetus and, becoming separated, cause vaginal bleeding. This is the condition known as placenta praevia. It can be detected by an ultrasound scan which will demonstrate clearly where the placenta is placed in the uterus.
Major fetal abnormalities, especially those involving the head, too large a head-hydrocephalus, or failure of development of the upper part of the head – anencephalous and spine – spina bifida, can be detected by ultrasound scans. If the diagnosis is made sufficiently early in pregnancy, the pregnant woman can be offered a termination. Other abnormalities can be diagnosed by amniocentesis or fetoscopy.