The national birth rate dropped for the third straight year, with declines for most ages and all races, according to a federal report released Thursday. Teens and women in their early 20s had the most dramatic dip to the lowest rates since record-keeping began in the 1940s. Experts suspected the economy drove down birth rates in 2008 and 2009 as women put off having children. With the 2010 figures, suspicion has turned into certainty.