Your beliefs about who has to care for your children are very very modern. Remember that children as young as 6 used to work in the mines and weaving factories. Fully responsible for their own safety, food and getting paid.

So your interdependence arguement goes out the window.

As for termination at 4 weeks or 40 - mechanically there are differences. Legally and CONSTITUTIONALLY there are none. In neither case does The Government have any ceded power that allows it to regulate the woman's body or to violate the 13th Amendment and compel her into involuntary servitude of carrying to term or delivering.

If you had a magic teleportation device that could teleport the fetus out of the host woman's womb with no direct affect on her body, then The Government could compell that procedure - because it doesn't act on the host woman. But other than that, The Government's ability to regulate and act simply doesn't exist.