Do Obamacare's covered contraceptives cause abortion?

On 9/26/2013, U.S. Bishops asked Congress to support their opposition to the contraceptive coverage in Obamacare, even to the point of keeping the government shut down and letting the U.S. default on its debt because, they say, the covered contraceptives cause abortion. Are they right? See or listen to the investigative report from NPR.