Genine, didn't they actually make mention of a premature birth in the movie? I definitely remember hearing it somewhere, I thought it was in the movie itself.
In any case, assuming that a human-Kryptonian pregnancy would last for nine months (and Tank is of course right - who knows how long the gestation period would actually be), even if Jason were as much as two months premature, that would still assume that Lois' and Richard's relationship progressed very swiftly. I could see things happening this way if the mind-wipe is there. But if it's not, she would have definitely rebounded into a relationship with Richard, and I would think that she would have at least considered the possibility that this baby could be Superman's.
Kathy
"Our thoughts form the universe. They always matter." - Babylon 5