BALTIMORE  One sister is 14; the other is 9. They are a vibrant pair: the older girl is high-spirited but responsible, a solid student and a devoted helper at home; her sister loves to read and watch cooking shows, and she recently scored well above average on citywide standardized tests.

There would be nothing remarkable about these two happy, normal girls if it were not for their mother’s history. Yvette H., now 38, admits that she used cocaine (along with heroin and alcohol) while she was pregnant with each girl. “A drug addict,” she now says ruefully, “isn’t really concerned about the baby she’s carrying.”

When the use of crack cocaine became a nationwide epidemic in the 1980s and ’90s, there were widespread fears that prenatal exposure to the drug would produce a generation of severely damaged children. Newspapers carried headlines like “Cocaine: A Vicious Assault on a Child,” “Crack’s Toll Among Babies: A Joyless View” and “Studies: Future Bleak for Crack Babies.”

But now researchers are systematically following children who were exposed to cocaine before birth, and their findings suggest that the encouraging stories of Ms. H.’s daughters are anything but unusual. So far, these scientists say, the long-term effects of such exposure on children’s brain development and behavior appear relatively small.