First trimester ultrasound scans and left-handedness.

Abstract

To the Editor: We recently reported an association of prenatal ultrasound exposure in gestational weeks 17 and 32 with left-handedness. 1 Teratogenic substances often act during limited periods of development, and we speculated whether the second trimester (17-week) scan rather than the later (32-week) scan caused the increase in prevalence of left-handedness. During recent years, first trimester scans have been used more commonly. Hypothetically first trimester scans may be less harmful than second trimester scans, as the inborn ability of the fetal brain to repair damage or to replace dead cells can act over a longer period. 2 This letter reports an analysis of whether first trimester scans increase the risk of left-handedness. One of the hospitals excluded from the previous study 1 was the university hospital in Linköping. The main reasons for excluding Linköping were their policy with first trimester scans and limited written documentation on their scanning policy. 3 In the initial study we found it crucial to apply this stringent approach concerning documentation on ultrasound programs for the hospitals included. However, because our data might disclose adverse effects from first trimester scans and we have now supplementary information on the scanning program in Linköping from the physician who organized it (Anders Selbing, personal communication, 2001), such stringency might be disregarded. Routine scans were started in Linköping on January 1, 1978, and from that point in time almost all pregnant women were scanned. A Diasonograph NE 4200 static scanner (Nuclear Enterprises Ltd., Edinburgh, UK) was used. As the scans were generally performed in gestational week 12, men born in Linköping from January through July 1978 were considered to have not been exposed to ultrasound, while men born from August through December 1978 were considered to have been exposed. In Sweden, all males around the age of 18 are enrolled to be tested for military service. Until 1998 a majority of the enrollees were tested as to whether they shoot left-handed or right-handed; these data on left-handedness were obtained through the National Service Register. In the analyses we compared left-handedness among men born in Linköping with men born in 48 hospitals that did not practice ultrasound scanning before 1980. 1 Only men born as singletons in 1978 and enrolled for military service testing in 1996 to 1997 were included. In these additional analyses we found an increased risk of left-handedness (odds ratio [OR] = 1.4; 95% confidence interval [CI] = 1.0–2.1) among men born in Linköping in August – December 1978 (ie, those presumably exposed to a first trimester ultrasound scan) but not among those born there in January – July 1978 (OR = 0.9; 95% CI = 0.6–1.3) (Table 1). The results are in agreement with previous studies on ultrasound exposure and left-handedness. 1,4,5 It is, however, important to note that the results are based on a relatively small cohort and should be confirmed in future studies. As there is now a tendency to perform ultrasound scans earlier in pregnancy we find it important to publish our results indicating that first trimester scans, similar to second trimester scans, may increase the risk of left-handedness. Table 1: Prevalence and Relative Risk of Left-handedness According to Ultrasound Exposure and Time of Birth among Men Born in Sweden, 1978Helle Kieler Sven Cnattingius Juni Palmgren Bengt Haglund Ove Axelsson