RR didn't have the evidence before him that we have today. Nor did he have the luxury of applying much improved life support technology to the problem of supporting the alive unborn scheduled for termination. Is that answer sufficient?
If Ronald Reagan didn't have the 'evidence' that we have, then surely, Bush has a twisted mind because he favors an exception in cases of rape, incest and for the life of the mother?
BTW, your 'evidence' seems to be that life begins at conception because (1) a seperate DNA is created and (2) the cell begins multiplying. However, DNA was discovered long before Ronald Reagan entered the politican arena. Why did he continue to favor exceptions for the life of the mother, and for cases of rape and incest?