Original Articles: PDF OnlyMagnus Per; Jaakkola, Jouni J.K.; Skrondal, Anders; Alexander, Jan; Becker, Georg; Krogh, Truls; Dybing, ErikEpidemiology: September 1999 - p 513-517 Free Abstract Chlorination of drinking water that contains organic compounds leads to the formation of by-products, some of which have been shown to have mutagenic or carcinogenic effects. As yet, too little is known about the possible teratogenic effects on the human fetus. We linked the Norwegian water work registry, containing 1994 data on chlorination practice and color (an indicator for natural organic matter), with the Medical Birth Registry for 1993–1995. The proportion of the population exposed to chlorination and a weighted mean color number in drinking water was computed for each municipality. Among 141,077 births, 2,608 (1.8%) had birth defects. In a comparison between exposed (high color; chlorination) and reference groups (low color; no chlorination), the adjusted odds ratio was 1.14 (0.99–1.31) for any malformation, 1.26 (0.61–2.62) for neural tube defects, and 1.99 (1.10–3.57) for urinary tract defects. This study provides further evidence of the role of chlorination of humic water as a potential cause of birth defects, in a country with relatively low levels of chlorination byproducts. © 1999 Lippincott Williams & Wilkins, Inc.