By: 1 March 2008


Most pregnant women in the UK are now offered at least one ultrasound examination as a routine part of their antenatal care. The number of scans per pregnancy is likely to increase to at least two in the near future in light of the NICE guideline on antenatal care which suggests that all women should have at least two scans: a dating scan between 10 and 13 weeks to accurately determine the gestation of the pregnancy and detect multiple pregnancies, and anomaly scan between 18 and 20 weeks to detect structural abnormalities. The number of routine scans can easily increase to four if a nuchal translucency measurement is included in the assessment of Down’s risk and if the placenta is found to be covering the internal os at the 20-week scan, necessitating re-scanning at 36 weeks to establish the placenta site.

Apart for its medical indications, routine obstetric ultrasonography has also become a social experience and an expectation in our society. The obstetric ultrasound examination is attractive in a manner that is atypical of other medical procedures. Expectant mothers generally view the ultrasound scan of their fetus as a positive experience. This has led to ever increasing demands for ultrasound scans without clear clinical indications to the point that patients do not ask their obstetrician if they need a scan, but rather ask when it will be scheduled or repeated. A recent study showed that 87% of patients show up for their scan accompanied by their partners or other family members1 and many expect to be given a scan image as a memento of their experience. This frequently put healthcare professionals under pressure to request an ultrasound scan as a matter of indulgence, rather than of need.

“Diagnostic” ultrasound scanning during pregnancy has been proven to be beneficial to outcome or obstetric management as it has the potential to show many unsuspected problems, including erroneous menstrual dates, twins, placenta praevia, and devastating anomalies of the fetus. However, large studies have shown that “routine” scanning in residual low risk pregnant women does not decrease perinatal morbidity and mortality, nor does it result in fewer unnecessary obstetric interventions2,3.

Advocates of routine scanning claim that “seeing the baby” on a real time scan precipitates maternal-fetal attachment, and relieves anxiety. However, there is no experimental evidence to support this hypothesis, and it is likely that the attenuated anxiety and stress following ultrasound examination are likely to reflect increased anxiety in anticipation of the procedure rather than reassurance. Also, ultrasound findings of uncertain significance, like the so-called soft markers for Down syndrome, can be a source of great prenatal anxiety for pregnant women4. Furthermore, although it is widely believed that ultrasound may promote the adoption of health recommended behaviours, no trial to date has shown significant decreases in maternal smoking attributable to maternal visualisation of the fetus5.

A recent study has shown that most women request an ultrasound scan to check for fetal abnormalities, for their own reassurance and to check for appropriate fetal growth6. However, ultrasound scanning is only a diagnostic tool with its own limitations, and it is important that women and their partners should be well informed about the specific purposes of ultrasound scans and what they can and cannot achieve. For instance, the recent NICE guideline on antenatal care concluded that there is no evidence to support routine scanning for growth post 24 weeks. Similarly, the sensitivity of ultrasound scanning for the identification of external structural abnormalities, such as neural tube and abdominal wall defects is extremely high (over 90%), but the detection rate of serious internal structural abnormalities, such as congenital heart disease or congenital diaphragmatic hernia, remains at or below 50%7. Also, although anomalies are an important cause of perinatal mortality, the diagnosis of a lethal anomaly does not change the eventual outcome, a lethal anomaly will be lethal whether the death occurs after early termination in utero or after birth. Nonetheless, for both lethal and less serious anomalies, antenatal diagnosis offers parents and healthcare providers the important benefits of being able to consider options about whether to proceed to delivery, whether to deliver at a high-risk centre, and what therapies to consider subsequently.

An issue that is commonly overlooked is that of the safety of ultrasound scanning. Despite its safety record so far, antenatal scanning may have subtle but rare long-term consequences. Indeed, there has been considerable debate in both the lay and medical press about the safety of ultrasound scanning. Studies on animal models have suggested that ultrasound scanning may have an effect in central nervous system development8, and a study on human fetuses has suggested that routine ultrasound scanning may be associated with non-right handedness9. Recently, an editorial has highlighted the disturbing complexity of what we don’t know about ultrasound scanning, and reminds its readers that ultrasound is a form of energy that causes heat elevation and mechanical effects (albeit not necessarily harmful effects) in the tissues through which it travels, and that there may be issues if ultrasound is not performed by well-trained individuals for medical indications10. This should always be borne in mind when requesting an ultrasound scan, because even though malformations may not occur, teratogenesis is not only structural.

The baby’s perception of being scanned is another area of controversy. It is widely believed that babies do not perceive being scanned to a great extent. However, it is a common observation that babies, who are being scanned because the mother complains of reduced fetal movements, suddenly become impressively active when an ultrasound probe is applied11. Interestingly, a recent article has suggested that ultrasound scanning is generally perceived by the fetus as a sound similar to the highest notes on a piano, but if the probe is aimed right at the fetus’ ear the sound is as loud as that made by a subway train coming into the station12. Because of these uncertainties, most medical authorities support the ALARA principle for ultrasound scanning (as low as reasonably achievable, i.e. perform the scan for the shortest time possible and with the lowest output possible to permit adequate diagnostic acuity).

It may be that antenatal scanning could be seen as a benign hospital-based entertainment event, but definitive evidence for its safety in the context of multiple scans is not available and until then clinicians must be seen to have exercised restraint and discipline in its use.

As with every medical intervention, ultrasound scanning has a considerable cost and therefore has to be rationed on clinical grounds in order to be a cost-effective intervention. A recent UK study showed wide variations in the cost of ultrasound scans (ranging from £18 to £204) and called for further studies on the cost and cost-effectiveness of routine ultrasound scanning13.

A guide to who needs a scan
At Southmead hospital, a reminder poster (shown) is displayed in all the clinical areas in the Womens’ Health Department for the benefit of both clinical staff and patients alike. This is distributed to all new doctors and midwives at induction to the Trust, and even though it can be overridden at the doctor’s judgement, it provides staff with a consistent evidence based advice for their practice.

This paper is not intended to suggest that ultrasound is a dangerous diagnostic modality, but to highlight the uncertainties and limitations of ultrasound scanning and act as a reminder to healthcare providers to exercise their clinical judgement before they request/perform such a scan. As with any clinical investigations it is important to ask “Is this scan really necessary?”


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