cancer. Some of the studies evaluating exposures that might increase the risk of birth defects, congenital anomalies, and low birthweight were reviewed in volume 1 (NRC, 1991). One of these sets of studies, performed by the California Department of Health Services (CDHS), was updated in a special issue of Epidemiology (Swan et al., 1992; Deane et al., 1992; Zierler, 1992).

CDHS investigated cardiac defects among babies born in Santa Clara County in 1981 to September 1, 1982. The investigators compared the rates of cardiac defects in the study area (suspected of water contamination) with the rates for the rest of the county. Investigators found an increased risk of cardiac defects in the study area (RR = 2.6, p = 0.01). The results could not be due to recall bias, because the information is documented by charts. However, the investigators also compared the times when the birth defects occurred with when the water contamination occurred and concluded that the timing could not link them. Investigators mention other possible exposures, such as air contaminants and other contaminants in the water, that might have increased the rates of cardiac defects.

In a second study, CDHS compared the Los Paseos census tract, which received water from one contaminated well, with a control census tract that had demographic characteristics similar to those of Los Paseos. Women who had been pregnant in 1980 and/or 1981 were contacted by mail, telephone, or home visit and interviewed to determine rates of spontaneous abortion, congenital anomalies, and low birthweight, as well as various risk factors. After adjustment for confounding variables, the Las Paseos spontaneous abortion rate was 2.3 times that in the control community (odds ratio, OR = 2.3, 95% confidence interval, CI = , 1.3-4.2). Drinking tap water was also found to be associated with spontaneous abortions in both the control area and case area. The congenital anomaly rate was 3 times that of the control area (OR = 3.1, 95% CI = 1.1-10.4). However, the authors commented that the observed pattern is not consistent with a single teratogen, but rather with several teratogens affecting the fetus at different times during gestation. They did not find any low birthweight babies in the Los Paseos area. Tests for chemicals in the control area did not show chemical contamination. The investigators concluded that the exposure data were insufficient to determine whether the leak into the contaminated well was a cause of the increased rates of spontaneous abortions and congenital malformations.

The relation between environmental exposures and birth defects remains important but difficult to study. An innovative study from New York state used existing data to detect some associations between congenital malformations (all types pooled) and residential proximity to hazardous-waste sites (Geschwind et al., 1992). This study examined unusu-

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