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Objective To test a hypothesis of no association between ultrasound exposure in early fetal life and growth or impaired vision or hearing during childhood.
Design Follow up of eight to nine year old children born to women who participated in a randomised controlled trial on ultrasound screening during pregnancy.
Setting Nineteen antenatal care clinics run by three central hospitals in Sweden from 1985 to 1987.
Population and methods Of 4637 eligible singleton pregnancies, 3265 (71%) were followed up through a questionnaire sent to their mothers. Analyses were performed both according to randomised groups and to ultrasound exposure.
Main outcome measures Parents’ report of vision and hearing tests as recorded on child's record card. Parents’ report of their child's weight and height at 1, 4 and 7 years of age.
Results Reduced hearing was reported by 3.4% in the screening group compared with 3.5% in the nonscreening group (odds ratio [OR] 1.0; 95% confidence interval [CI] 0.67–1.41). The same prevalences were found when analysed according to ultrasound exposure (OR 1.0; 95% CI 0.67–1.42). Reduced vision was reported by 6.3% in the screening group compared with 7.8% in the nonscreening group (OR 0.8; 95% CI 0.60–1.03). Corresponding figures for ultrasound exposed and unexposed were 6.2% and 8.0%, respectively (OR 0.8; 95% CI 0.58–1.00). No statistically significant differences in body weight or height at 1, 4 or 7 years of age between screened and not screened children or between exposed and unexposed were found.
Conclusion This study found no association between ultrasound exposure in early fetal life and growth or impaired vision or hearing during childhood.
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The clinical value of routinely performed obstetric ultrasound is still being debated1,2. The safety of ultrasound should be an important part of this discussion especially as some studies3,4 have presented data that implicate long term harmful effects on children exposed in fetal life.
Ultrasound may damage human tissue by a rise in temperature or cavitation5. Local cell death or cell membrane damage, which in turn affects cell differentiation6, might be the result of ultrasound exposure. Mole5 implied that the relay system between the retina and visual cortex and the cochlea of the inner ear are obvious sites for irreparable damage because of one-to-one cell connections. Even the eye lens is a sensitive organ because of the lack of continuous blood circulation, so a rise in temperature caused by ultrasound can not be counteracted5. Consequently, minor visual or hearing losses could be possible results of ultrasound exposure in fetal life.
There are still controversies on how to interpret the association between ultrasound exposure in pregnancy and altered fetal weight found in both animal and human studies, as both decrease as well as increase in weight has been reported3,4,7. Altered growth during childhood might be expected if birthweight is affected by ultrasound.
Only two studies8,9 have focused on antenatal ultrasound exposure and subsequent vision or hearing and two10,11 on growth during childhood according to two reviews of epidemiological studies on ultrasound exposure3,4. None of these reports8–11 have found any associations between ultrasound and the studied outcomes. However, more studies in this field are needed as the two cohort studies9,10 had limitations because of small sample size10, imperfect matching9 and low response rate9. In the randomised follow up study by Salvesen et al.8,11 the scans were performed in gestational week 19 and 32. Nowadays there is a tendency to perform routine ultrasound earlier in pregnancy and to omit scans in the third trimester. No study has so far investigated the association between growth, vision and hearing during childhood and ultrasound scanning at about 15 weeks.
The aim of this study was to test a hypothesis of no difference in growth, vision and hearing during childhood among children in a follow up of a single stage randomised trial on ultrasound screening during gestational week 1512. Boys were studied separately as male fetuses are more vulnerable than female fetuses13.
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In the screening group 661 mothers did not respond or declined participation, leaving 1651 (71.5%) of 2312 screened children to be included in the follow up study. In the nonscreening group 711 did not respond or declined participation, leaving 1614 (69.4%) of 2325 children from the nonscreening group to be included. In early fetal life (before 19 weeks of gestation), 1704 children were exposed to ultrasound whereas 1561 were not.
The responders were older (27.7 years) and less often smokers (26%) compared with the nonresponders (27.0 years and 33% smokers; P < 0.01). There were no differences between responders and nonresponders in terms of parity, gender, gestational age and perinatal asphyxia. Family and social variables of the children followed up are presented in Table 2.
Table 2. Family and social variables for 8 and 9 year old children whose mothers participated in a randomised trial on ultrasound screening during pregnancy. Variables are shown both according to randomised groups and to ultrasound exposure. Values are given as n or n (%).
| ||Screening group (n = 1651)||Nonscreening group (n = 1614)||Ultrasound-exposed (n = 1704)||Unexposed (n = 1561)|
|Years of education (mother)|| || || || |
| No. who responded||1617||1586||1670||1533|
| <10 years||178 (11.0)||194 (12.2)||182 (10.9)||190 (12.4)|
| 10–12 years||849 (52.5)||764 (48.2)||867 (51.9)||746 (48.7)|
| College or university||590 (36.5)||628 (39.6)||621 (37.2)||597 (38.9)|
|Years of education (father)|| || || || |
| No. who responded||1583||1543||1638||1488|
| <10 years||286 (18.1)||270 (17.5)||293 (17.9)||263 (17.7)|
| 10–12 years||785 (49.6)||747 (48.4)||810 (49.5)||722 (48.5)|
| College or university||512 (32.3)||526 (34.1)||535 (32.7)||503 (33.8)|
|Family economy|| || || || |
| No. who responded||1609||1578||1663||1524|
| Very good||184 (11.4)||174 (11.0)||199 (12.0)||159 (10.4)|
| Good||707 (43.9)||689 (43.7)||729 (43.8)||667 (43.8)|
| Medium||657 (40.8)||644 (40.8)||671 (40.4)||630 (41.3)|
| Poor||61 (3.8)||71 (4.5)||64 (3.9)||68 (4.5)|
|Lived with both parents during childhood|| || || || |
| No. who responded||1610||1576||1663||1523|
| Lived with both parents during childhood||1362 (84.6)||1317 (83.6)||1413 (85.0)||1266 (83.1)|
The overall response rates for questions on vision, hearing, weight and height at ages 1, 4 and 7 years are shown in Table 1. There were no significant differences in response rates for the different items between th screening and nonscreening group nor between ultrasound-exposed and unexposed children.
The number of children with impaired vision and hearing together with odds ratios and 95% confidence intervals for comparisons between groups is presented in Table 3. The number of referrals for specialist examination is also shown in Table 3. When controlling for possible confounding factors such as maternal age, parity, education, smoking, gender, gestational age and perinatal asphyxia (Apgar score < 7 at 5 mins) the odds ratios remained practically unchanged.
Table 3. Prevalence of items regarding hearing and vision for 8 and 9 year old children whose mothers participated in a randomised trial on ultrasound screening during pregnancy. Results are shown both according to randomised groups and to ultrasound exposure. Values are given as n (%); comparisons are expressed as odds ratios (OR) with 95% confidence intervals (95% CI).
| ||Screening group (n = 1651)||Nonscreening group (n = 1614)||OR[95%CI]||Ultrasound exposed (n = 1704)||Unexposed (n = 1561)||OR[95%CI]|
|Reduced hearing||56 (3.4)||56 (3.5)||0.97 [0.67–1.41]||58 (3.4)||54 (3.5)||0.98 [0.67–1.42]|
|Referred to otorhinolaryngolist||152 (9.3)||172 (10.8)||0.84 [0.67–1.06]||159 (9.4)||165 (10.8)||0.86 [0.68–1.08]|
|Reduced vision||103 (6.3)||126 (7.8)||0.79 [0.60–1.03]||105 (6.2)||124 (8.0)||0.76 [0.58–1.00]|
|Use of spectacles||141 (8.6)||149 (9.3)||0.92 [0.72–1.17]||149 (8.8)||141 (9.1)||0.97 [0.76–1.23]|
|Referred to ophthalmologist||189 (11.5)||219 (13.7)||0.82 [0.67–1.01]||200 (11.8)||208 (13.4)||0.86 [0.70–1.06]|
Mean body weight and height from the health controls at 1, 4 and 7 years of age for all included children are shown in Table 4. The estimated differences in body weight and height at the exact ages of 1, 4 and 7 years, and shown in Table 4, were calculated using multiple regression analyses. No significant differences were found between screened and not screened or between exposed and unexposed children, respectively.
Table 4. Mean body weight and height from the health controls at 1, 4 and 7 years of age together with estimated differences in weight and height between screened and not screened children and between ultrasound exposed and unexposed, respectively. Negative values indicate higher values among not screened and unexposed compared with screened and exposed, respectively. No differences were significant. Values are given as mean and mean difference* [95% CI]. Scr = screened; Exp = exposed.
| ||Body weight (kg)||Body height (cm)|
|Age (years)||Body weight||Scr – not Scr||Exp – unExp||Body weight||Scr – not Scr||Exp – unExp|
|1 year||9.9||0.03 [−0.06 to 0.12]||0.06 [−0.03 to 0.15]||76.05||0 [−0.24 to 0.23]||-0.01 [−0.25 to 0.23]|
|4 years||17.55||-0.03 [−0.21 to 0.16]||0.01 [−0.18 to 0.19]||104.95||-0.01 [−0.36 to 0.35]||-0.03 [−0.39 to 0.32]|
|7 years||25.06||-0.04 [−0.41 to 0.32]||-0.01 [−0.38 to 0.35]||124.65||-0.05 [−0.57 to 0.47]||-0.04 [−0.55 to 0.48]|
The estimated difference in body weight at one year of age between exposed and unexposed children to smoking mothers was 129 g in favour of the exposed children. At four and seven years of age the unexposed children had the highest mean body weight. The mean difference at four years was 156 g and at seven years 547 g. None of these differences was statistically significant. Separate comparisons for boys made on vision, hearing, mean body weight and height showed only minor and nonsignificant differences (results not shown).
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No association between exposure to ultrasound in early fetal life and impaired vision or hearing among eight or nine year old children could be found in this study. Neither were there any statistically significant differences in body weight or height at 1, 4 or 7 years of age between children from the screening and nonscreening group nor between exposed or unexposed. The results are in agreement with previous studies8–11.
A major limitation of validity for a study like this in which mailed questionnaires were used is nonresponse bias. The overall response rate here was 71%, which is in accordance with studies using this approach17,18. No difference in response rate between the screening and nonscreening group nor between exposed and unexposed children was observed. The responders were older and less often smokers compared with the nonresponders (P < 0.01). There were, however no differences between responders and nonresponders in terms of parity, gender, gestational age and perinatal asphyxia. The response rate for questions on hearing and vision corresponded well to the overall response rate, but the response rate for questions on weight and height was somewhat lower. The two lowest rates were recorded for questions on body weight and height at seven years of age.
Although the original study was a randomised trial, we chose to analyse also according to ultrasound exposure as it was possible adverse effects of ultrasound that were of interest19.
Of the women in the nonscreening group, 4.1% had an ultrasound scan before 19 weeks of gestation and 1.3% of the women in the screening group had no scan. Children who were exposed to ultrasound later in fetal life were not considered as ultrasound-exposed as the aim was to investigate possible adverse effects of routine ultrasound scanning in the second trimester. Analyses performed according to exposure did not change the results and the odds ratios remained practically unchanged after controlling for possible confounders.
The 75 g higher birthweight (P= 0.01) of screened compared with not screened children to mothers who smoked found in the original study12 could be observed as a nonsignificant difference only up to one year of age, which is in agreement with the findings by Salvesen et al.11. If ultrasound exposure caused reduction in smoking during pregnancy12, the effect on growth only lasted for a short period in childhood. Nonetheless, growth in childhood is affected by many factors, of which smoke exposure in fetal life plays only a minor part20. This study could not support the idea of ultrasound initiating improved growth in childhood as suggested by Salvesen et al.11, but it is important to note that nonresponse bias might have been introduced because smoking women had a lower response rate.
In this study, no direct testing of the children was performed as vision and hearing among the participating children were evaluated by parent assessments. The parents’ report should be based on the results from tests used in the Swedish child and school health care systems14,15. The Swedish screening program for vision and hearing impairment at child and school health care centres has a very high standard, with an attendance and detection rate above 95%14. Although the possibilities for detecting disabling vision or hearing by these screening procedures are very good14, minor visual or hearing losses such as those hypothetically caused by ultrasound exposure in fetal life might not be revealed. Another important factor to consider is that power calculations showed that an increase in the prevalence of impaired vision or hearing of less than 57% could not be detected even with a sample size of 2000 in each group, and our final sample sizes contained only around 1600 children in each group.
Impaired vision was reported by 6% to 8%, which is more than expected but in agreement with the figures reported by the parents in the study by Salvesen et al.8. As about 9% reported use of spectacles which are used not only to treat impaired vision but also refractive errors and squinting, it is possible that many parents included these conditions when answering the question about impaired vision. We do not believe that this misclassification of impaired vision by the parents affects the results. A lower prevalence of squinting or refractive errors among the ultrasound exposed children compared with the unexposed might, in theory, conceal a potentially higher prevalence of impaired vision, but such an effect of ultrasound is unlikely.
The prevalence of impaired hearing found in this study for ultrasound exposed (3.4%) and unexposed (3.5%) is in agreement with the expected prevalence of 4%15 and with the findings by Salvesen8. Persistent middle ear effusion is the most common cause of reduced hearing in children15,21 but there is no reason to suspect an uneven distribution of middle ear effusion between the groups as there is no theoretical correlation between this condition and ultrasound exposure in fetal life.
As male fetuses probably are more vulnerable than female fetuses13, and both impaired vision and hearing are more common in males14,22, boys were analysed separately, but no differences were found.