This is a very, very rare instance where his crack addled brain is more correct than you.

A fetus develops and grows inside a placenta. Which is inside the uterus, which oddly enough, is inside a woman. No placenta, no baby. No baby, no placenta. So yes, to say your mother had you in inside her placenta is correct. It's just not how most people think about it.

So write it down on a calendar, the brain damaged idiot he is, was technically more correcter than you in this instance.