40 weeks, and being dismissed

Teri: "They say she's six weeks.  That can't be.  Nic wasn't home until x."

Me: "I think they determine that from the last period, not the date of conception."

Teri: "What are you talking about?  It has to be from the date of conception."

Umm, no, it doesn't.  And it isn't.

Why don't you just say, "Oh, my way of thinking may not be adding up, but your explanation can't be right . . . "