First Time Mom?

Conversation I had today while at the immunizations clinic with my 2 month old:
"I can tell she's your first," said the random woman.
"Really?" I said.
"Yeah, it's obvious" She said assertively.
Then, trying to be as kind as possible with a splash of sass I said. "Well, you may need to work on your deductive reasoning, because she's my second."
Unamused or embarrassed or confused by her own incorrect assumption, she walked away.