Weighing the Risks: What You Should Know about Ultrasound

By Sarah Buckley
Issue 102, September/October 2000

Ultrasonography was originally developed during World War II to detect enemy submarines. Its use in medicine was pioneered in Glasgow, Scotland, by Dr. Ian Donald, who first used ultrasound to look at abdominal tumors, and later babies in utero in the mid-1950s.¹ The use of ultrasound in pregnancy spread quickly.

In westernized healthcare systems, ultrasound, which may be offered to a pregnant woman either to investigate a possible problem at any stage of pregnancy or as a routine scan at around 18 weeks, has become almost universal in pregnancy. In Australia, where I live, 99 percent of pregnant women have at least one scan, paid for in part by our federal government, through Medicare. In fact, from 1997 to 1998 Medicare paid out AU$39 million for obstetric scans, compared to AU$54 million for all other obstetric Medicare costs. In the US, the American College of Obstetrics and Gynecology (ACOG) estimates that 60 to 70 percent of pregnant women are scanned, despite an official statement from ACOG that recommends against routine ultrasound.² At a cost of roughly $300 per procedure, this represents a cost of approximately $70 to $80 million each year in the US.

Besides routine scans, ultrasound can be prescribed to investigate problems such as bleeding in early pregnancy. Later in pregnancy, ultrasound can be used when a baby is not growing, or when breech or twin births are suspected. In such cases, the information gained from ultrasound can be very useful in decision-making, and generally most professionals support the use of ultrasound in this context.

It is such use of routine prenatal ultrasound (RPU) that is more controversial, as this practice involves scanning all pregnant women in the hope of improving the outcome for some mothers and babies. RPU seeks to gain four main types of information:

Estimated due date. Dating a pregnancy is most accurate at early stages, when babies vary the least in size. By contrast, at 18 to 20 weeks the expected date of delivery is only accurate to within a week either way. Some studies have suggested, however, that an early examination or a woman's own estimation of her due date can be as accurate as RPU.^3,4
Unsuspected physical abnormalities. While many women are reassured by a normal scan, in fact RPU detects only between 17 percent and 85 percent of the one in 50 babies that have major abnormalities at birth.^5,6 A recent study from Brisbane, Australia, showed that ultrasound at a major women's hospital missed about 40 percent of abnormalities, many of which are difficult or impossible to detect.⁷ The major causes of intellectual disability, such as cerebral palsy and Down syndrome, or heart and kidney abnormalities, are unlikely to be picked up on a routine scan.

There is also a small chance that the diagnosis of an abnormality is false positive. In some instances, normal babies have been aborted because of false-positive diagnoses.⁸ A United Kingdom survey found that one in 200 babies aborted for major abnormalities were wrongly diagnosed.⁹

In addition to false positives, there are also uncertain cases in which the ultrasound image cannot be easily interpreted, and the outcome for the baby is not known. In one study involving babies at higher risk of abnormalities, almost 10 percent of scans were uncertain.¹⁰ This can create immense anxiety for the woman and her family that may not be allayed by the birth of a normal baby: in the same study, mothers with questionable diagnoses still had associated anxiety three months after the child's birth. Uncertain findings also lead to repeated and/or prolonged scans, increasing the expense, inconvenience, and dosage of ultrasound.

In some cases of uncertainty, further tests such as amniocentesis are recommended. In such situations, there may be up to two weeks wait for results, during which time a mother must consider whether or not she will terminate the pregnancy if an abnormality is found. Even mothers who receive reassuring news have felt that this process has interfered with their relationship with their babies.¹¹

Location of the placenta. A very low-lying placenta (placenta previa) puts mother and baby at risk of severe bleeding, and usually necessitates a cesarean section. However, 19 out of 20 women who have placenta previa detected on RPU will be needlessly worried, as the placenta will effectively move upwards as the pregnancy progresses.¹² Furthermore, detection of placenta previa by RPU has not been found to be safer than detection in labor.¹³

Multiple fetuses. Ultrasound can detect the presence of more than one baby at an early stage of pregnancy, but this knowledge affords no documented health advantages for mother or babies, and, without RPU, almost all multiple pregnancies are discovered before birth.¹⁴