After a short amount of time afterward the pregnancy has been announced, yes.
Other contracts do not work like this, though. Again, it seems to me that if the consent and contractual rights and obligations go further than simply the intercourse and attach to the fetus, then the two parties are, at the time of coitus, signing up with the mutual understanding that both are signing up. I see no other way the contract theory can get off the ground in this argument.
Further, you state below that you just see it as cruel to force a woman to carry against her will; but, where does cruelty come in to play in the contract argument? First, cruelty ought not play a part; and, second, seems like it would not be against her will if she implicitly or explicitly consents to the contract.
As part of the contract, he would be able to opt out within a short amount of time after the pregnancy is announced (if he has not already opted out before the sexual act).
Is this understood by both parties ahead of time? Seems like there are plenty of reasons to think it is not understood and must, then, be made explicit in order to hold.
Take the case of a man deciding to have intercourse with a woman sans condom. It is reasonable to believe that he should know, from a simple biological standpoint, that he is risking pregnancy. Thus, it is reasonable for the woman to believe he recognizes and accepts said risk. Why should this man be allowed to "opt out" thus saddling the woman with either the physical cost of having an abortion (and, yes, there are physical costs involved in said procedure, to include the possibility of not being able to conceive and carry in the future) or following through with the pregnancy and having to support the child on her own?
I don't think an "implied contract" can excuse the man from such obligations and responsibilities.
I also want to clarify that it is not the physical costs, in itself, that would render the contract null. It is the foreseeable knowledge of the physical costs (just as it would be the foreseeable knowledge of the cruelty) that make the stance of "implied consent" for these things unreasonable.
Implied consent can only be argued for where it would be reasonable to expect an individual to accept the consequences of their action/deed/etc. If such expectations are not reasonable, then explicit consent is needed for a contract theory.