NEW YORK — Teen motherhood has gained a bit of celebrity allure with the pregnancies of Jamie Lynn Spears and Bristol Palin, but front-line professionals see a starkly different reality involving poverty, lost opportunities and a cost to taxpayers in the billions of dollars annually. At minimum, the public cost of births to teens 17 and younger is $7.6 billion a year, according to research presented Thursday at a national forum in Chicago. The calculation includes both the lower taxes that these often impoverished families contribute and the extra social services they require. "Teen births do have substantial, widespread negative...