Ultimately, however, the law imposes obligations on parents post birth and most would not argue against them. Is it unreasonable, on that basis, to extend it for 16-20 weeks pre birth?
Yes, it is extremely unreasonable. Because post-birth there's a baby, an autonomous, if dependent, unit, able to breathe, eat, and excrete.
Pre-birth there's a foetus, and to live it requires the use of my body, including my digestive, respiratory, nervous and circulatory systems. And this use impedes and often damages, possibly irreparably my own use of these, my bodily systems and functions.
I get to say who uses my body and for what. No one else.