Federal officials are considering for the second time in a year whether to make morning-after birth control available without a prescription. The Food and Drug Administration may decide as soon as this week whether drug stores can sell the emergency contraception known as "Plan B" without a prescription to women age 16 and older. The pill can prevent pregnancy for up to 72 hours after sex. The sooner the pill is taken, the more effective it is. When it first rejected the idea last May, the FDA didn't dispute the safety of the drug. But officials questioned whether it would make teenagers more likely to have sex. Plan B supporters say the pill is a safe way to prevent thousands of unwanted pregnancies and the abortions that sometimes follow. But opponents worry that it would encourage risky sexual behavior.