METHOD

The Medical Birth Registry of Norway comprises all Norwegian births with at least 16 weeks of gestation. Notification is compulsory, and is performed by midwives within the first week after birth. Any diagnosis available at that time should be reported. We used the registry as our source of ascertainment. Since the registry only captures diagnoses within the first week after birth, ascertainment is assumed to be poorer for internal defects such as cardiac defects than for external defects like spina bifida with an estimated ascertainment of 80%.⁹

The birth defects included in the analyses were: central nervous system (CNS) defects, cardiac defects, respiratory system defects, oesophageal defects, and clubfoot. These outcomes were selected based on results of earlier investigations regarding magnetic fields and reproductive outcomes.^6,7 Children could be registered with up to three different types of malformations. We did not consider multiple defects as a separate category. However, when considering spina bifida and hydrocephalus, those with a recorded anencephalus were not included. Similarly, children with hydrocephalus in combination with spina bifida were not counted as hydrocephalus cases.

The Norwegian person identification number is recorded for child and mother of all births in the registry. Through the mothers’ personal identification numbers, Statistics Norway identified all mothers registered in The Medical Birth Registry of Norway who had lived in a corridor around each power line, broad enough to include all potentially exposed houses, on 1 January 1980 or later. The corridor ranged from 25 m on each side of a 25 kV line to 300 m on each side of a 420 kV line. In the period 1986 to 1997, exact new addresses were updated on 1 January annually. Due to the lack of exact address information in the period 1981 to 1985, births during this period were excluded. A woman entered the cohort the first year she was registered in a residence within the corridor and left when she moved out. The child was regarded as exposed if the mother lived in an exposed residence for more than half the pregnancy.

For every child in the corridor registered in The Medical Birth Registry of Norway with one of the defects to be included in the analyses, born in 1980 or between 1 January 1986 and 31 August 1997, two controls matched for sex, year of birth, and municipality were selected randomly. The distances between residence and power line were measured on maps, mainly of scale 1:5000. Maps have earlier been shown to be better correlated with actual distances than GIS based distances⁸ as used in our previous publication.⁷ Information on currents of each power line was obtained from the different power companies. By use of a computer program (Teslaw) developed at SINTEF Energy, Norway, estimations of magnetic fields in the residences under study were performed. The program presents the result as µT root mean square magnetic field strength. This is the sum of the vectors for the individual conductor in a given situation integrated over one period. The calculations took account of height of towers, distance between phases, ordering of phases, distance between the power line and the house, and average load on the power line. Distance to the power lines was defined as the distance from the closest corner of the house to the mid-point between the outer phases of a line. A residence was regarded as exposed if the estimated magnetic field was 0.1 μT or above.

Odds ratios (OR) for the exposed compared to the unexposed with 95% confidence intervals (CI) were obtained from conditional logistic regression for 1 to 2 matched data adjusting for mother’s age and highest family education. The analyses were carried out for CNS defects, heart defects, respiratory defects, clubfoot, and oesophageal defects. Significance levels were set to 5% for each hypothesis test, and no attempts were made to adjust for multiple comparisons. The statistical software used was EGRET for Windows 2.0.31.¹⁰

RESULTS

Of a total of 744 324 births in Norway in the period of investigation, 128 680 were within the corridor from whom we found 465 cases and selected 930 controls.

The highest ORs were found for hydrocephalus and cardiac defects (table 1). Lowest ORs were found for spina bifida and oesophageal defects.

DISCUSSION

This investigation showed no significant increased or decreased risk for the defects evaluated.

Comparing these results with our findings in the earlier residential cohort study,⁷ the excess risk of oesophageal defects and the reduced risks of cardiac and respiratory defects was not reproduced in the present study. Neither do the present results support earlier findings regarding maternal occupational exposure and birth defects.⁶

For spina bifida, oesophageal defects, and clubfoot there is a shift from increased to decreased risk, in the present study compared to the previous residential cohort investigation, although only the risk of oesophageal defect was significant in the previous study. For hydrocehalus and cardiac defects we observed a shift from decreased to increased risk, significant only for cardiac defects in the previous study. For respiratory system defects, the reduced risk was closer to unity compared to the previous study. The distance measurements in this study are more accurate⁸ than in the previous study. We therefore consider the results in this study more reliable.

Information of the mothers’ addresses only once a year in the study period was one important limitation in this study. Another critical problem was the limited number of cases and subsequent limited statistical power.

Residential exposure based on calculated magnetic fields from power lines only with no personal measurements, may have introduced some exposure misclassification, but a previous dosimeter study among children living close to a power line in Norway, showed that the magnetic fields from the line is the major source of exposure.¹¹ This should be the case for adults too. In comparison with other countries, like Sweden, the contribution of ground currents to magnetic fields in homes is minor in Norway because of a different grounding system.

The Norwegian birth registry only includes birth defects identified at the maternity ward in the hospital during the first week after birth. The majority of birth defects are probably detected shortly after birth. However, defects diagnosed later, such as cardiac defects, are less likely to be detected and might therefore be seriously underreported in the registry.

In the present population based, nested case-control study, we took advantage of the population registration system in Norway. Furthermore, by defining the study population as adults who had lived in geographical areas crossed by high voltage power lines, we could assume these lines to be the main source of exposure. The design made it possible to control for factors associated with area of residence and socioeconomic status.

In conclusion, this study does not support the hypothesis that residential exposure to electromagnetic fields from power lines causes any of the investigated outcomes.