"It is reasonable to assume that if the Founders had wished to make so specific a change in the common practice of the times,...."
Wrong, because you still don't have a grasp on the reason for the Constitution.
Its function is not to tell citizens what they can do...it is to tell government what it can do....anything not covered by the enumerated powers is beyond the scope of the federal government.
The first fallacy of your argument is that those who wrote the Constitution were modern conservatives. They weren't. They opposed the conservative King of England and taxation without representation. So that's your first fail.
I bolded the last section, because extended your arugment logically, therefore the government cannot restrict abortion in any way because they are not mandated by the Constitution to deal with this issue. The government has no right to impede, restrict, or hinder a woman's right to determine what happens to her own body.
Last but not least, nothing in the Constitution recognizes that life begins at conception. In fact, the unborn are not even mentioned, so any idea that the Constitution protects life from the moment of conception is simply you projecting your own ideas into it.