Doc, it's wierd how well you know me at times or is it this is how most men handle similiar issues?
I'm starting to believe you and many others have dealt with this dilema before?
To procreate or not to procreate this is the question!
What most guys in your position do is nothing. In hopes that the problem will eventually go away, it doesn't get discussed so they end up giving in under pressure or to make their wife happy. Or.... their wife ends up having an 'accident'. Sadly, the more advanced in age she is the worse pressure is to keep the baby because despite the increased risk of birth defects, health issues and the guy just plain not wanted to be indentured into fatherhood again it's her last chance to have a baby.
It's a hard world, HM. I know quite a few guys who get caught out there. One dude I know was afraid to get the vasectomy but trusted his longtime girlfriend (who "was on the pill"). Poor, stupid bastard could even see the light at the end of the tunnel.... child support from his previous marriage was going to be done in a few years, Alimony was going to be lessened, had a new house that was going to be more affordable once the last kid hit college, etc... now his stupid ass is changing diapers! And he is totally miserable.
Your position is probably worse. A 40yr old woman that wants a baby would probably rather lie about changing her mind than invest the time necessary in finding another suitable father. Either that or you'll be dealing with nagging all the time or severe depression everytime she sees a baby.
Even worse... it's not an issue that can be addressed with condoms. Using a condom would mean saying "I don't trust you", another uncomfortable conversation.

You're definately screwed. At this point it's more or less a question of if you're screwed with or without a new baby. Either way, you're screwed.