function onPlayerReadyVidible(e){'undefined'!=typeof HPTrack&&HPTrack.Vid.Vidible_track(e)}!function(e,i){if(e.vdb_Player){if('object'==typeof commercial_video){var a='',o='m.fwsitesection='+commercial_video.site_and_category;if(a+=o,commercial_video['package']){var c='&m.fwkeyvalues=sponsorship%3D'+commercial_video['package'];a+=c}e.setAttribute('vdb_params',a)}i(e.vdb_Player)}else{var t=arguments.callee;setTimeout(function(){t(e,i)},0)}}(document.getElementById('vidible_1'),onPlayerReadyVidible); More than half of American teens have had sex by age 18, but teenage pregnancy and birth rates extended their 2-1/2-decade decline because of increased contraceptive use, according to a U.S. government study released on Thursday. Most of the 55 percent of teens who have had sex by 18 used some type of protection, typically a condom, the study of more than 4,000 teenagers by the U.S.