However, I do recognize that there is a total difference between the 24/7 biological support the pregnant woman provides for the embryo and the financial support that either parent may be legally required to provide after birth.
Pregnancy is 24/7 labor which, if treated in law as a form of labor, would violate labor laws - for starters, how many hours in a day or how many days in a row a person can be required to labor at a job, the fact that people have to be financially compensated for their labor by mutual employer-employee agreement, and the fact that numerous safety regulations have to be met by the employer, etc.
Furthermore, the person who has to pay child support, whether man or woman, can choose what type of labor he or she wants to engage in in order to make the money out of which he or she will provide for the child. It can be physical labor, sure, but it can also be essentially intellectual labor. The woman who is pregnant and doesn't want to be is likely to think that, if she merely had to work to pay child support, she could get a clean office job doing primarily intellectual work, go home at the end of a 7-8 hour day, have weekends off, and have sick leave and paid vacation. Well, after a child is born, that's a possibility for either the man or the woman who has to pay child support.
So pregnancy is not comparable to mere child support.