Charles Mitchell of Evangelicals for Mitt notes a Los Angeles Times story reporting that Ronald Reagan became pro-life only in 1975, a year before his conservative challenge to Gerald Ford in the Republican primaries. It's a fair point if accurate, but I think some context is necessary.
Reagan's shift came a mere two years after Roe. Mitt Romney's came 32 years after the Supreme Court's abortion decision. Romney certainly had more information available to him earlier about fetal development, the number of post-Roe abortions, the percentage of those abortions that are elective, the impact of legalization, etc. than Reagan did. Maybe Romney's conversion is as sincere as Reagan's, but the two shifts do differ contextually in important ways. (Hat tip: Daniel Larison.)