ANNOTATIONSHowever, some recent studies8, l o seem to be reasonably reassuring. Nevertheless, until more is known about the possible harmful effects, it would seem advisable to reduce the number of scans carried out during pregnancy. This, of course, will apply more to scans in early pregnancy rather than t o measurements of the biparietal diameter. Assessments of the biparietal diameter are impossible before ten weeks since the fetal head is not formed and measurements are unsatisfactory before sixteen weeks. The most accurate time for maturity assessments is between twenty and thirty-four weeks. Accurate assessments of the duration of pregnancy can be made in the very early weeks, and such measurements would be of obvious value in doing serial estimations to assess fetal growth.Keeping in mind the possible inaccuracies, and limiting the number of scans done during pregnancy, ultrasonic measurement of the biparietal diameter is an extremely useful aid to the obstetrician. IAN MACGILLIVR.4Y . (1965) 'Fetal develoonient as deter-Gynaec. Brit. Cwlth, 71, 11. mined by ultrasonic pulse echo'techniques." Anivr.' J . Obsret. Gyitec., 92, 44. and complicated pregnancy.' Ann. Chir. Gynec, Fenn., 59, 71. diameter.'J. Ohstet. Gynaec. Brit. Cwlth, 76, 603. Gynec., 29, 842. 12, 114. detector.' Brit. nied. J . , 2, 92. Lancet, i, 11 33. 3 . Ojala, A., Ylostalo, P., Jouppila, P., Jarvinen, P. A. (1970) 'Fetal cephalometry by ultrasound in normal 4. Campbell, S. (1969) 'The prediction of fetal maturity by ultrasonic measurement of the biparietal 5. Hibbard, L. T., Anderson, G. V. (1967) 'Clinical applications of ultrasonic fetal cephalornetry.' Obster. 6. Kato, M . (1966) 'Visible mutation induced in Drosophila by ultrasonic vibration.' Bid/.