One or more ultrsound checks are now routine in pregnancy. The Royal College of Radiologists guidelines say these are useful because they provide information about the expected date of delivery and multiple pregnancies, but they admit the scientific basis remains controversial.2

The Midwives Information and Research Service argues that ultrasound should not be routine. Midwife Anne Viccars says women should have the benefits explained and be made aware that it is a screening test for abnormalities. There may be no point in having ultrasound if the woman would go ahead with the birth anyway.

The test carries a false-positive risk, according to a large study which showed that one in 200 aborted foetuses had no major defect, contrary to the scan result.6 There have never been controlled studies on ultrasound safety. But Peter Twining, consultant radiologist at Nottingham University Hospital, casts doubt on the one in 200 figure and believes false positives are much rarer.

Overwhelming evidence shows that ultrasound is safe although it is still prudent to keep its use to a minimum. One scan at 18-20 weeks is the optimum unless more are clinically indicated.