It should be against the law for a woman to deceive her spouse/significant other about the paternity of an expecting or existing child. In most cases the mother receives monetary gain from the man she's deceived about the pregnancy.
Cases exist where a man who's been the victim of this fraud has signed the birth certificate under false pretenses and still must pay child support. This should be committing a fraud and the penalties should be equal to misdemeanor/felony charges depending on dollar amounts.
Before a man signs the birth certificate, the mother (the only person that knows 100%) should have to sign an affidavit stating who's the father of that child. No woman should be apposed to declaring who the actual father of her baby is, unless she has something to hide.