A month later, Jeri said, "Tom, something's wrong. I didn't change back."
"Do you want to change back, love?"
"I'll always love you but that doesn't change whether I want to change back. Something's wrong..."
Tom went back to the Physical Sexual Transmorphing book once again, and studied it as best he could. Of course, there was something he had missed--he was too new at this and really didn't know to look up every single quirk of the spell ahead of time.
"It seems," he explained to Jeri, "that you're pregnant. If you're pregnant you can't change back, because a man can't be pregnant."
"But I want to be a man!"
"Well...."