Science! It’s time for science! And, in this case, we mean science that you keep to yourself and certainly never bring up in an argument.
It turns out that, despite your wife or girlfriend’s complaints, it’s not her pill-form birth control that has caused her recent weight gain. That would be food and/or laziness.
Again, we remind you to never ever ever EVER use this newly acquired knowledge except to quietly reassure yourself when alone that you were right…. But not too loudly, because she may have bugged your office/masturbatorium.