Frequent prenatal ultrasound exposure could restrict fetal growth by a third.

Yet another recent controlled study, this one performed at the King Edward Memorial Hospital in Perth, Western Australia, again found that frequent ultrasound screening between 18 and 38 weeks does not improve pregnancy outcome, but in fact has an adverse effect.

It looked at 2834 women either given five scans compared to a single one at 18 weeks.

All subjects were given a scan in order to confirm the baby's gestational age.

Those following the intensive scan programme had an average reduction in birth weight of 25g.

"An adverse effect of frequent ultrasound examinations on fetal growth is biologically plausible," writes the Australian authors of the study.

"In monkeys, Tarantal and Hendrickx observed a significant reduction in mean birthweight after frequent prenatal ultrasound examinations".

The researchers were able to repeat these results in subsequent studies (Teratology 1989; 39: 137-47).

"Our findings suggest that five or more ultrasound imaging and Doppler flow studies between 18 and 38 weeks gestation, when compared with a single imaging study at 18 weeks gestation, increases the proportion of growth restricted fetuses by about one third," they concluded.

As Marc J N C Keirse of the Leiden University Hospital, the Netherlands, observed in an accompanying editorial: "Clearly it is prudent to limit such examinations to circumstances in which the information is likely to be useful." The Lancet, 9 Oct 1993.