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Exposure to video display terminals and risk of small-for-gestational-age birth.

Introduction

In view of the increasing use of video display terminals (VDTs) for work or pleasure, the potential relation between VDT use and pregnancy outcome has been a topic since the mid-1980s.

Some studies on animals and human cells have suggested that exposure to extremely low-frequency electromagnetic fields (EMFs) results in reproductive failure (Adey, 1981; Nielsen & Brandt, 1990). Extremely low frequency (45-60 Hz) EMFs were associated with spontaneous abortion after exposure to electrically heated beds, ceiling cable electric heating (Wertheimer & Leeper, 1989), or VDT use (Goldhaber, Polen, & Hiatt, 1988). Likewise, VDT use also has been associated, though inconsistently, with the risk of birth defects (Delgado, Leal, Monteagudo, & Gracia, 1982). Along the EMF spectrum, VDT-associated radiation has an exposure of about 0-5 V/m (for the electric field ) and 0.1-1 MegaT (for the magnetic field). These values are similar to those reported for electrically heated beds (Wertheimer & Leeper, 1989).

Laboratory studies have suggested that electric and magnetic fields may affect growth (Byus, Pieper, & Adey, 1987), DNA synthesis (Liboff, Williams, Strong, & Wistar, 1984), and the modulation of calcium binding to cell surface molecules (Blackman, Benane, House, & Joines, 1985). Consequently, it has been suggested that exposure to EMFs could result in adverse reproductive effects (Shaw & Croen, 1993).

Results are inconsistent in the association between VDT use and the risk of small-for-gestational-age (SGA) birth (Braken et al., 1995; Ericson & Kallen, 1986a; Ericson & Kallen 1986b; McDonald et al., 1988; Nielsen & Brandt, 1992; Nurminen & Kurppa, 1988). For example, an insignificant increase in risk of reduced birth weight was reported by Windham and co-authors (1990) in Santa Clara County, California, in users of VDT at home or at work (for more than 20 hours per week). A positive relation has been reported in other studies (McDonald et al., 1988; Nurminen & Kurppa, 1988). Another study, however, showed no association (Grajewski et al., 1997).

Analysis of the association between exposure to VDTs and SGA birth is of interest since many other EMF sources exist, such as television, electric typewriters, and electric blankets. The VDT-related EMFs are lower than those due to electric blanket or other common domestic appliances, but the common use of VDTs makes the analysis of their effect on reproduction of special interest.

To gain further information, we considered data from a case-control study on the association between VDT use in pregnancy and the risk of SGA birth. The specific utility of this study derives from the scarcity of data from southern Europe, since studies to date refer mostly to northern Europe or to the U.S. and Canada. Also of specific interest is the opportunity to analyze only cases of true SGA birth.

Methods

We conducted a case-control study on risk factors for SGA birth (Chiaffarino et al., 2006; Parazzini et al., 2005; Parazzini et al., 2007). Our cases were 555 women who delivered SGA babies identified at the Clinica Luigi Mangiagalli (the largest obstetric hospital in Milan, Italy) and the Obstetric and Gynecology Clinic of the University of Verona. An infant was defined as SGA if it weighed less than the 10th percentile in comparison with the birth weight expected for the same gestational age and sex, according to Italian standards (Parazzini, Cortinovis, Bortolus, Fedele, & Decarli, 1995).

The controls were women who gave birth at term (at least 37 weeks of gestation) to healthy infants of normal weight (i.e., between the 10th and 90th percentiles according to Italian standards) (Parazzini, Cortinovis, Bortolus, Fedele, & Decarli, 1995) on randomly selected days at the hospitals where cases had been identified. The interviewers surveyed the obstetric wards on days selected at random to interview controls whose age was comparable with cases. A total of 1,966 controls were interviewed.

Information was obtained on general sociodemographic habits; personal characteristics and habits; gynecological and obstetric history; and tobacco, alcohol, and coffee consumption in pregnancy. Exposure to VDT units during the first trimester of pregnancy was investigated in 347 cases and 1,519 controls (data on VDT exposure was not collected during the first phase of the study among cases and controls interviewed at the Clinica Mangiagalli). Their distribution according to general characteristics was largely similar to the total population included in the study. The definition of exposure was based on self-reporting and expressed in number of hours per week. Information was collected regarding the length of exposure, the beginning and the end of exposure during pregnancy, and the activity for which the VDT was used.

All information, apart from birth weight and week of gestation at birth, was obtained from personal interviews. The presence of hypertension in pregnancy was checked with clinical records. We defined gestational hypertension as diastolic pressure above 90 mmHg on at least two occasions 24 hours apart, without proteinuria.

We computed the odds ratios (OR) adjusted for age, as estimators of the relative risk of SGA birth, with their 95% approximate confidence intervals (CI) using the Mantel Haenzel procedure (Mantel & Haenszel, 1959). To account simultaneously for the effects of several potential confounding factors, we used unconditional multiple logistic regression, with maximum likelihood fitting, to obtain ORs and their corresponding 95% CI (Baker & Nelder, 1978). Factors included in the model are listed in the footnotes of the tables; the OR were considered statistically significant at the usual level p < .05. Included in the regression equations were terms for age and education, plus terms found significantly associated in this data set with the risk of SGA birth. These factors are listed in the footnotes to tables. Only multivariate estimates are presented in the tables.

Results

Table 1 shows the distribution of cases and controls according to age, gestational age at birth, and selected factors. The mean age at interview was 31 years for both cases and controls. The mean gestational age at birth was 37.4 weeks for cases and 39.6 weeks for controls. Participation rates were over 95% among cases and controls.

Smoking during pregnancy, history of intrauterine growth retardation (IUGR), and gestational hypertension increased the risk of SGA birth. More educated women and women who had previously given birth were at a lower risk of SGA birth.

No significant association emerged between marital status and risk of SGA birth.

The risk of SGA birth varied according to the occupational category considered. In comparison with professional and technical staff, the OR was 1.3 for office workers, 1.0 for blue collar workers, and 1.4 for housewives, students, or unemployed, but none of these risks was significant.

The distribution of cases and controls according to VDT use, length of use, and VDT-related activity is presented in Table 2. No association emerged with SGA births. Word processing was the main work done with VDTs for both cases and controls.

Discussion

This study found no association between VDT use and risk of SGA birth. These results are in general agreement with the limited published data.

In a study by McDonald and co-authors (1988) on about 25,000 women, the OR of low birth weight was 1.0 in women using VDT in comparison with nonusers. This study considered women who gave birth to babies weighing less than 2,500 g. Grajewski and co-authors (1997) found no relationship between SGA birth and VDT use in a large cohort study including more than 2,400 telephone operators who used VDTs at work. No association emerged in a study by Windham and co-authors (1990) of VDT use and risk of low birth weight, but when the analysis only considered babies with IUGR, the OR for heavy VDT use in pregnancy was 1.6, although this was not significant. In a study by Braken and co-authors (1993), a lower risk of low birth weight was suggested for the offspring of women who used VDTs at home or at work for less than 20 hours per week. In a more recent cohort study (European Longitudinal Study of Pregnancy and Childhood [ELSPAC]), nonsmoking women using VDT had babies with IUGR more often (OR 2.29, p < .005) than controls (Hruba, Kukla, & Tyrlik, 1999).

Potential limitations of this study should be considered. No direct measurements of magnetic fields were planned or done. We only considered exposure to VDT use, which was the main source of magnetic fields in this population, however. Recall bias is unlikely, because it implies that the women who experienced SGA birth would have underreported their exposure. This is in disagreement with the observation that generally any bias tends to overestimate the association, since cases may systematically remember their potential sources of exposure better (Goldhaber, Polen, & Hiatt, 1988; McDonald et al., 1995). In any case, in Italy the lay press has not drawn any particular attention to the potential risk of VDT exposure on reproductive outcome.

The absence of risk is related to the time spent working in front of the machine, but the highest nonionizing radiation is from the back and top of the unit (Goldhaber, Polen, & Hiatt, 1988). This suggests that exposure might be estimated better on the basis of the office seating arrangements and coworkers' use of machines, which make the office itself a source of exposure to EMFs. Direct measurements of extremely low frequency magnetic fields in the workplace might provide further information.

The choice as controls of women who delivered healthy infants only at term may have introduced some potential bias. No clear indications exist, however, of an association between VDT use and risk of preterm birth (Nurminen & Kurppa, 1988), and when we considered only cases delivered after 37 weeks of gestation, no differences emerged in the OR. In fact, the OR of SGA birth was 1.3 (95% CI: 1.0-1.7).

In Italy, women by law cannot work during the last two months of pregnancy. Thus we cannot properly analyze the effect of VDT exposure during this last period of pregnancy. No relation was observed, however, between VDT use for leisure and use by professional women who generally do not stop working during the last months of pregnancy (data not shown).

Conclusion

In conclusion, despite limitations in the evaluation of exposure in the present and previous published studies, the evidence from several countries with different patterns of VDT use gives further strong support to the lack of any association between VDT use and SGA birth.

Acknowledgements: We wish to thank Luigi Fedele for his contribution to the design and analysis of the study and Ivana Garimoldi for editorial assistance. The anaylsis was partially supported by Ricerca Corrente, Policlinico, Milan.

Pre-published digitally July 2009, National Environmental Health Association.

References

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FABIO PARAZZINI, MD

FRANCESCA CHIAFFARINO, SCD

LILIANE CHATENOUD, SCD

SONIA CIPRIANI, SCD

ELENA RICCI, SCD

VITO CHIANTERA, MD

RENATA BORTOLUS, MD

CRISTINA MAFFIOLETTI, MD

Corresponding Author: Fabio Parazzini, Researcher, Universita di Milano, Fondazione IRCCS Ospedale Maggiore Policlinico, II Clinica Ostetrica Ginecologica, Mangiagalli, Regina Elena, Milano, via Commenda 12, Milano, Italy 20122. E-mail: parazzini@marionegri.it.
TABLE 1
Distribution of Study Subjects According to Selected Factors (a)

                                     SGA (b)
Factors                               Birth         Controls

Age (y)
-25                                 60 (10.8)      180 (9.2)
25-29                              176 (31.7)      637 (32.4)
30-34                              207 (37.3)      767 (39.0)
[greater than or equal to] 35      112 (20.2)      382 (19.4)
Gestational week at birth,
  mean (range)                    37.4 (28-43)    39.6 (37-44)

Education (y)
[less than or equal to] 6           24 (4.3)        53 (2.7)
7-13                               415 (74.8)     1439 (73.2)
[greater than or equal to] 14      116 (20.9)      473 (24.1)

Marital status
Married                            541 (97.5)     1934 (98.4)
Unmarried                           14 (2.5)        32 (1.6)

Parity
0                                  404 (72.8)     1103 (56.1)
1                                  118 (21.3)      693 (35.3)
[greater than or equal to] 2        33 (6.0)       170 (8.7)

Smoking during the first trimester of pregnancy
No                                 396 (71.4)     1677 (85.3)
Yes                                159 (28.7)      289 (14.7)

Previous IUGR (f)
No                                 133 (88.1)      853 (98.8)
Yes                                 18 (11.9)       10 (1.2)

Hypertension in pregnancy
No                                 435 (78.4)     1863 (94.8)
Yes                                120 (21.6)      103 (5.2)

Occupation
Professional/technical              91 (16.4)      395 (20.1)
Office workers                     271 (48.8)      892 (45.4)
Blue collar workers                 63 (11.4)      230 (11.7)
Housewife or student/unemployed    130 (23.4)      449 (22.8)

                                        OR (c)
Factors                                (95% CI)

Age (y)
-25                                     NA (d)
25-29                                     NA
30-34                                     NA
[greater than or equal to] 35             NA
Gestational week at birth,
  mean (range)                            NA

Education (y)
[less than or equal to] 6               1+ (e)
7-13                                 0.6 (0.4-1.1)
[greater than or equal to] 14        0.6 (0.3-0.9)

Marital status
Married                                   1+
Unmarried                            1.5 (0.8-2.8)

Parity
0                                         1 +
1                                    0.4 (0.3-0.6)
[greater than or equal to] 2         0.5 (0.3-0.7)

Smoking during the first trimester of pregnancy
No                                        1 +
Yes                                  2.3 (1.9-2.9)

Previous IUGR (f)
No                                        1 +
Yes                                  9.3 (4.0-21.9)

Hypertension in pregnancy
No                                        1 +
Yes                                  5.0 (3.8-6.6)

Occupation
Professional/technical                    1+
Office workers                       1.3 (0.9-1.8)
Blue collar workers                  1.0 (0.6-1.6)
Housewife or student/unemployed      1.4 (1.0-2.0)

(a)  In some cases the sum does not add up the total because
of some missing values.

(b) SGA = Small for gestational age.

(c) OR= Odds ratios adjusted for age, education, parity,
smoking during the first trimester of pregnancy, gestational
hypertension, and history of intrauterine growth retardation.

(d) NA = Not applicable.

(e) + = Reference category.

(f) IUGR = Intrauterine growth retardation; parous women only.

TABLE 2
Risk of Small-for-Gestational-Age (SGA) Birth According to
VDT Exposure (1)

Factors                      SGA Birth    Controls      OR (95% CI)

                              No. (%)       No. (%)
Use of VDT
  Never                      204 (58.8)   971 (63.9)        1+
  Sometimes                  143 (41.2)   548 (36.1)   1.2 (1.0-1.6)
How often do you use it
(hours/week)?
  <11                         31 (8.9)    136 (9.4)    1.2 (0.8-1.8)
  11-20                       33 (9.5)    119 (8.2)    1.3 (0.8-2.0)
  [greater than or equal
    to] 20                    61 (17.6)   224 (15.5)   1.2 (0.9-1.7)
Main use of VDT
  Word processing            107 (30.8)   433 (28.5)   1.1 (0.9-1.5)
  Other                       36 (10.4)   115 (7.6)    1.4 (0.9-2.3)

(1) In some cases the sum does not add up the total because of
some missing values. + Reference category. OR = Odds ratios
adjusted for age, education, parity, smoking during the first
trimester of pregnancy, gestational hypertension, and history
of intrauterine growth retardation.
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Title Annotation:INTERNATIONAL PERSPECTIVES
Author:Parazzini, Fabio; Chiaffarino, Francesca; Chatenoud, Liliane; Cipriani, Sonia; Ricci, Elena; Chiante
Publication:Journal of Environmental Health
Article Type:Report
Geographic Code:1USA
Date:Jan 1, 2010
Words:3143
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