WASHINGTON (AP) — The maker of an unproven drug intended to prevent premature births says it will voluntarily remove the product from the United States. The late Tuesday announcement comes after regulators planned to move ahead with a long-delayed effort to force it from the market. More than four years ago, Switzerland-based Covis Pharma’s drug, Makena, failed to show a benefit in helping mothers carry their pregnancies to term. The injectable drug has become an example of the challenges the Food and Drug Administration faces to withdraw a medication when the manufacturer won’t do so voluntarily. The timing of the drug’s removal remains uncertain.