Estimated by whom? I believe that number is used by abortion advocates who want to inflate the numbers so as to promote the illusion that abortion is a common method of birth control used by many women. They use that illusion to try to convince more women to have abortions.
However, if you look at the numbers, the 50% estimate just doesn't hold up. Rough and dirty estimate, ignoring confounders such as number of women in childbearing age, statistical analyses, and so forth:
So, the number of women who have had abortions is roughly (50 million / 150 million) x 0.5 = 17%.
That's entirely possible. Maybe I remembering a number from Russia or something.