Anyway, since causing someone to exist *isn't* a harm and since having consensual sex is *neither* illegal *nor* negligent, I certainly *don't* think that women should be forced to allow their embryos/fetus to use their bodies in order to survive.
I think the argument is that the negligent act was having unprotected sex. There is a duty of responsibility to care for ones' children (which could be imputed to the fetus); the breach is neglect in creating life without intending to bear it; the causation is negligently having unprotected sex despite the known risk of pregnancy; the damages would be loss of life.
If we are going off tort law then you'd be dealing with a fictional objectively reasonable person standard. A person who voluntarily consents to unprotected sex is assuming all foreseeable risks that accompany sex. That includes pregnancy, and you can't reasonably disclaim the scientific fact that sperm + egg + implantation = baby. It would be like you inviting a friend into your home, knowing that he has bird flu, and then later trying to sue when you catch his bird flu because you told him that he could enter but his germs had to stay outside and then said nothing when he entered. No one reasonably believes that is how things work. I think its safe to say that an objectively reasonable person knows that unprotected sex can lead to pregnancy. Thus a woman who claimed she did not consent to the pregnancy, only to unprotected sex, would be unreasonable, as a reasonable person knows that you cannot disclaim science. So it does not really matter than you think its somehow possible to pretend fact is not fact, because the court would apply an objective reasonableness standard and impute that knowledge to you.
Unjust enrichment is an equitable claim not a tort claim. That means a judge would have to balance the equities to determine if the fetus is unfairly withholding more than it is entitled to. Assuming again the fetus has some rights, then the mother would again owe a duty to care for her child, and gestation seems pretty obviously included in such care. That does not mean a judge might never find that it would be unfair for a woman to carry a child to term when weighing all factors (rape/health of mother/etc), but its a lot harder sell than the autonomy argument, which is the argument the pro choice people do best with.
