The safety of visual display units (VDUs) in terms of reproductive outcomes has been questioned since the widespread introduction of VDUs in the work environment during the 1970s. Concern for adverse pregnancy outcomes was first raised as a result of numerous reports of apparent clusters of spontaneous abortion or congenital malformations among pregnant VDU operators (Blackwell and Chang 1988). While these reported clusters were determined to be no more than what could be expected by chance, given the widespread use of VDUs in the modern workplace (Bergqvist 1986), epidemiologic studies were undertaken to explore this question further.

From the published studies reviewed here, a safe conclusion would be that, in general, working with VDUs does not appear to be associated with an excess risk of adverse pregnancy outcomes. However, this generalized conclusion applies to VDUs as they are typically found and used in offices by female workers. If, however, for some technical reason, there existed a small proportion of VDUs which did induce a strong magnetic field, then this general conclusion of safety could not be applied to that special situation since it is unlikely that the published studies would have had the statistical ability to detect such an effect. In order to be able to have generalizable statements of safety, it is essential that future studies be carried out on the risk of adverse pregnancy outcomes associated with VDUs using more refined exposure measures.

The most frequently studied reproductive outcomes have been:

• Spontaneous abortion (10 studies): usually defined as a hospitalized unintentional cessation of pregnancy occurring before 20 weeks of gestation.

• Congenital malformation (8 studies): many different types were assessed, but in general, they were diagnosed at birth.

• Other outcomes (8 studies) such as low birthweight (under 2,500 g), very low birthweight (under 1,500 g), and fecundability (time to pregnancy from cessation of birth control use) have also been assessed. See table 1.

Table 1. VDU use as a factor in adverse pregnancy outcomes

OR = Odds ratio. CI = Confidence Interval. RR = Relative risk. NR = Value not reported. NS = Not statistically significant.

Discussion 

Evaluations of reported clusters of adverse pregnancy outcomes and VDU use have concluded that there was a high probability that these clusters occurred by chance (Bergqvist 1986). In addition, the results of the few epidemiologic studies which have assessed the relation between VDU use and adverse pregnancy outcomes have, on the whole, not shown a statistically significant increased risk.

In this review, out of ten studies of spontaneous abortion, only two found a statistically significant increased risk for VDU exposure (Goldhaber, Polen and Hiatt 1988; Lindbohm et al. 1992). None of the eight studies on congenital malformations showed an excess risk associated with VDU exposure. Of the eight studies which looked at other adverse pregnancy outcomes, one has found a statistically significant association between waiting time to pregnancy and VDU use (Brandt and Nielsen 1992).

Although there are no major differences between the three studies with positive findings and those with negative ones, improvements in exposure assessment may have increased the chances of finding a significant risk. Though not exclusive to the positive studies, these three studies attempted to divide the workers into different levels of exposure. If there is a factor inherent in VDU use which predisposes a woman to adverse pregnancy outcomes, the dose received by the worker may influence the outcome. In addition, the results of the studies by Lindbohm et al. (1992) and Schnorr et al. (1991) suggest that only a small proportion of the VDUs may be responsible for increasing the risk of spontaneous abortion among users. If this is the case, failure to identify these VDUs will introduce a bias that could lead to underestimating the risk of spontaneous abortion among VDU users.

Other factors associated with work on VDUs, such as stress and ergonomic constraints, have been suggested as possible risk factors for adverse pregnancy outcomes (McDonald et al. 1988; Brandt and Nielsen 1992). Failure of many studies to control for these possible confounders may have lead to unreliable results.

While it may be biologically plausible that exposure to high levels of extremely low frequency magnetic fields through some VDUs carries an increased risk for adverse pregnancy outcomes (Bergqvist 1986), only two studies have attempted to measure these (Schnorr et al. 1991; Lindbohm et al. 1992). Extremely low frequency magnetic fields are present in any environment where electricity is used. A contribution of these fields to adverse pregnancy outcomes could only be detected if there was a variation, in time or in space, of these fields. While VDUs contribute to the overall levels of magnetic fields in the workplace, only a small percentage of the VDUs are thought to have a strong influence on the magnetic fields measured in the working environment (Lindbohm et al. 1992). Only a fraction of the women working with VDUs are thought to be exposed to levels of magnetic radiation above that which is normally encountered in the working environment (Lindbohm et al. 1992). The lack of precision in exposure assessment encountered in counting all VDU users as “exposed” weakens the ability of a study to detect the influence of magnetic fields from VDUs on adverse pregnancy outcomes.

In some studies, women who are not gainfully employed represented a large proportion of the comparison groups for women exposed to VDUs. In this comparison, certain selective processes may have affected the results (Infante-Rivard et al. 1993); for instance, women with severe diseases are selected out of the workforce, leaving healthier women more likely to have favourable reproductive outcomes in the workforce. On the other hand, an “unhealthy pregnant worker effect” is also possible, since women who have children may stop work, whereas those without children and who experience pregnancy loss may continue working. A suggested strategy to estimate the magnitude of this bias is to do separate analyses with and without women not gainfully employed.

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