Saying that chemical contamination in the city’s water supply led to miscarriages and infant deaths, 25 women have sued Chesapeake, Va., and almost 170 more plan to do so. According to a growing number of studies, the chlorine commonly used to purify drinking water can cause birth defects and miscarriages when it mixes with organic matter, such as fertilizer in surface water. In what has become a test case for the nation, the Chesapeake women are arguing that the city did not adequately warn them when toxins in their tap water reached harmful proportions — sometimes almost 10 times higher than established danger levels. The women are seeking nearly $1 billion in damage, but chemical and water industry reps say there is no conclusive evidence that chemicals in tap water damage fetal development.