According to the CDC, about 46% of women who had abortions in 2004, had had at least one prior abortion. Find it in the CDC's MMWR report for November 23, 2007, Table 13. Look at the bottom of the first column, where is shows that 53.7% of all American women who had abortions in 2004, had zero previous abortions. Subtract that from 100% and get approximately 46% of women who have had at least one prior abortion.
Hat Tip: Chris Gacek, of the Family REsearch Council.