Ultrasound is one of the newer diagnostic tools used in the evaluation of pregnancy. Obstetricians use this modality to evaluate fetal anatomy, development, growth, and physiology. Diagnostic ultrasound use in obstetrics has been growing rapidly to become an integral part of prenatal care today. 1 In some countries, health policies now advocate routine ultrasound screening in the second trimester. 2–6 One prenatal ultrasound examination is recommended in countries such as the United Kingdom, 2 Denmark, 7 Norway, 8,9 and Australia. 4 In Germany, two-stage screening is recommended. 10 A survey conducted in France revealed that more than 90% of obstetricians perform two to three ultrasound examinations per pregnancy. 11 In Greece, three ultrasound examinations are recommended. 12 Although there are no standard recommendations for ultrasound examinations in the United States, the number of low-risk as well as high-risk pregnancies receiving at least one ultrasound nearly doubled from the beginning to the end of the 1980s. 1 The high proportion of exposure to prenatal ultrasound highlights the public health significance of routine ultrasound use.

Ultrasound exposure during pregnancy can occur from a variety of procedures including ultrasound imaging, conventional and color Doppler, and external electronic fetal monitoring. Ultrasonic imaging can determine gestational age, assess for congenital anomalies, and monitor fetal growth. Conventional and color-Doppler studies are used to evaluate fetal blood flow velocity and cardiovascular abnormalities and to measure blood flow through fetal arteries to diagnose potential risks for intrauterine growth restriction. External fetal monitoring provides information on fetal oxygenation and response to injury. Techniques such as the biophysical profile couple ultrasound imaging with external fetal heart rate monitoring to evaluate fetal well-being. Diagnostic and therapeutic procedures such as amniocentesis or cordocentesis are guided by ultrasound imaging. Some of these procedures require an extended time and follow-ups that increase both the duration and frequency of total ultrasound exposure in utero.

During the intrapartal period, ultrasound exposure can occur through external electronic fetal monitoring during labor to assess fetal well-being as well as ultrasonic imaging to evaluate fetal position and to rule out rupture of membranes through the evaluation of the amount of amniotic fluid present.

There are several perceived benefits to the use of ultrasound during pregnancy. 3,13,14 Ultrasound imaging is believed to improve both clinical care and obstetrical outcomes. It has expanded research in the field of fetal development. Ultrasound imaging has practically eliminated the need for fetal exposure to x-rays. Negative and positive psychosocial benefits have been suggested such as increased anxiety or increased bonding with the fetus during pregnancy. 15–17

The purpose of this paper is to summarize epidemiologic studies that suggested possible relations between prenatal ultrasound and adverse outcome.

Epidemiologic Studies of Ultrasound Safety in Pregnancy

A majority of epidemiologic studies tends to support the safety of diagnostic ultrasound during pregnancy. 3,17,18 There have been some reports suggesting that there may be a relation between ultrasound exposure during pregnancy and adverse outcome. Some of the reported effects include growth restriction, delayed speech, dyslexia, and non-right-handedness associated with ultrasound exposure.

Ultrasound and Birth Weight

Moore et al, 19 in a retrospective cohort study, compared the birth weights of 1598 exposed and 944 unexposed single livebirths. Exposure to more than one ultrasound procedure and first exposure during the third trimester were associated with a reduction in birth weight. These findings were adjusted for current and prior risk factors including smoking and prepregnancy weight. The authors note that their study cannot rule out the potential risk of ultrasound on birth weight but that maternal and fetal risk did play a major role in birth weight reduction.

Newnham et al20 conducted a randomized, controlled trial of 2,834 women with single pregnancies, half of whom were selected at random to receive ultrasound imaging and continuous-wave Doppler flow studies at 18, 24, 28, 34, and 38 weeks of gestation and half to receive single ultrasound imaging at 18 weeks. The authors reported significantly higher intrauterine growth restriction in the intensive group when expressed both as birth weight less than the tenth centile [relative risk (RR) = 1.35; 95% CI = 1.09–1.67;P = 0.006) and birth weight less than the third centile (RR = 1.65; 95% CI = 1.09–2.49;P = 0.020). The mean birth weight for the two groups did not differ significantly. A follow-up study confirmed these findings and indicated that the primary effect was on bone growth compared with soft tissue. Using linear modeling that adjusted for maternal size, obstetric history, and fetal factors, there was a strong trend for the intensive group to have smaller long-bone measures. The authors note that it was important to characterize these findings to obtain clues to a possible mechanism and direct future research. 21

Marinac-Dabic et al, 22 using data from the 1988 National Maternal and Infant Health Survey, 23 reported that there was a 2.0 times greater risk of low birth weight when there were four or more ultrasound exposures during pregnancy when compared with no ultrasound exposures.

Ultrasound and Neurologic Development

Kieler et al24 conducted a follow-up study of 8- to 9-year-old children of women who participated in a randomized, controlled trial on ultrasound screening during pregnancy in 1985–1987 in Sweden. The authors found no differences in non-right-handedness between children in the screening and nonscreening groups. However, among boys, association between ultrasound exposure in utero and non-right-handedness was reported (odds ratio = 1.33; 95% CI 1.02–1.74).

Campbell et al25 studied 72 children age 24–100 months who had undergone a formal speech-language evaluation and been found to have a delayed speech of unknown cause. For each case subject, two control subjects were matched for sex, date of birth, sibling birth order, and associated health problems. The study found that children with delayed speech had a higher rate of ultrasound exposure than the control group (RR = 2.8, 95% CI = 1.5–5.3;P = 0.001). These authors caution clinicians to discuss the potential for vulnerability of the fetus to noxious agents including ultrasound.

Salvesen et al, 26 in a follow-up study of 2,161 8- to 9-year-old singleton children of women who took part in two randomized, controlled trials in Norway, 1979–1981, reported higher odds of non-right-handedness among children who had been exposed in utero to ultrasound than among control children (odds ratio = 1.32; 95% CI = 1.02 −1.71). They noted that these may be chance findings but stress that future studies are necessary.

Salvesen and Eik-Nes 27 explored this finding further through meta-analysis of two follow-up studies of three randomized clinical trials of routine ultrasonography during pregnancy. A total of 4,715 children at age 8–9 were included in the analysis. A statistically significant difference in non-right-handedness among boys was reported.

Discussion

The prenatal period involves a time of rapid growth, particularly in highly vulnerable organs such as the brain and skeletal tissue. Exposure to environmental factors such as ionizing radiation, alcohol, chemicals, and certain drugs have been shown to have an increased risk of abnormal neurological development, physical deformities, and altered growth. High-intensity ultrasound may cause disruptions in normal cell growth and development through an increase in temperature or by cavitation. 28

Small epidemiologic studies have suggested possible adverse effects associated with diagnostic levels of ultrasound such as those used in the evaluation of pregnancy. Some of the reported effects include growth restriction, delayed speech, dyslexia and non-right-handedness. 19–22,24-27

The lack of statistical significance in these findings may be due to low power. The widely accepted use of ultrasound worldwide has hampered the ability to identify pregnant women who have not been exposed to ultrasound. This limits the control group size in many of the epidemiologic studies. Although many studies have reported no effect, small sample sizes, rare events, and confounding may explain some of the nonsignificant findings. In addition, ethical issues reduce the ability to test effects such as high doses, long exposures of ultrasound exposure, or early effects. Two studies on women immediately before an elective abortion have demonstrated morphologic changes in the plasma membrane and suborganelles under high doses of ultrasound exposure. 29,30 Such approaches are not viewed as ethical in many countries.

Pregnant women have come to expect at least one ultrasound during their pregnancy as a part of routine care, and thus, enrollment in studies in which they may not receive an ultrasound examination may be limited. This results in a shift in the research from comparison of ultrasound-exposed vs nonexposed to a comparison of less-exposed vs more-exposed study subjects, such as the approach taken by Newnham et al.20

A limited number of studies explored the psychological impact of the ultrasound procedure on the woman, her family, and her unborn child. Although there are several studies that evaluate the potential positive impact of ultrasound examinations such as increased bonding and improved family interactions, there is little research to evaluate the impact of false-positive findings of ultrasound examinations and ultrasound-guided procedures on the psychological well-being of the woman and her ability to bond with the infant or the effect of these procedures on the woman’s decision to abort the pregnancy if problems are detected.

Recent innovations in the use of Doppler flow studies, higher energy levels, smaller transducers with a more focused beam and longer times of exposure have created new areas of potential risk to the developing fetus. In addition, more procedures have been developed that are guided by ultrasound. Their long-term safety has not been fully evaluated. Concerns have been raised by the European Federation of Societies for Ultrasound in Medicine and Biology (EFSUMB), which recommended avoidance of Doppler ultrasonography and suggested that its effects may be most extreme during periods of organogenesis and bone formation and growth. Kossoff 31 noted that the EFSUMB had updated its recommendations in 1996 to encourage practitioners to avoid the use of maximum output-level energy beams.

The instructions and training regarding safe output levels vary widely across clinical settings. As a result, some of the clinicians who have been trained on the diagnostic use of the ultrasound do not receive adequate information concerning the potential bioeffects of ultrasound energy.

Most experts would agree that is considered prudent to expose patients to the least amount of ultrasound energy necessary to obtain diagnostic information as stated in the ALARA principle, which stands for “a s l ow a s r easonably a chievable.”28

Recommendations for the Future

Further research is needed to continue to evaluate the potential effects of ultrasound exposure during pregnancy. These studies should measure the acoustic output, exposure time, number of exposures per subject, and the timing during pregnancy that the exposure occurred, while controlling for potential confounding variables such as sociodemographic, medical, and obstetric risk factors. Controlled randomized clinical trials should be conducted where possible. Although ethical issues may impede the ability to conduct controlled trials by potentially denying a woman the prerogative of an ultrasound, observational epidemiologic studies on larger scales may provide the rationale and basis for conducting controlled trials. When a pure control group is not feasible, research can be directed to evaluating the frequency and timing of exposures during the pregnancy. We recommend that a new consensus development conference be held to gather the needed data and provide guidelines for future research needs as well as to respond to the rapid advances in this technology.

Ultrasound has been used for the past four decades and has progressed as an effective modality for many diagnostic and therapeutic procedures. It remains a unique tool for the practitioner but should be treated with the respect that it deserves. Until long-term effects can be evaluated across generations, caution should be exercised when using this modality during pregnancy. Further research that evaluates the potential effects is needed as this technology advances and becomes more diverse in both the mechanisms and use.