My understanding is that he wrote an law review “note” defending/promoting abortion.
That "note" was published a month before he became president of the Review.
"Politico asked months ago if Mr. Obama had ever written anything for the Harvard Law Review as a student. The Obama campaign responded narrowly, with a Clintonesque statement that "as the president of the Law Review, Obama didn't write articles, he edited and reviewed them." This month it turned out Mr. Obama had written an article -- but it was published a month before he became president."