Is "投个好胎" a proper set phrase/idiom? What does it mean? I'm trying to understand it by finding the meaning behind each word but failed miserably.

What I got from MDBG:   

投 (tóu): to cast / to send / to throw oneself (into the river etc) / to seek refuge / to place oneself into the hands of

个 (gè): individual / this / that / size / classifier for people or objects in general

好 (hǎo): good / well / proper / good to / easy to / very / so / (suffix indicating completion or readiness) / (of two people) close / on intimate terms / (after a personal pronoun) hello/ to be fond of / to have a tendency to / to be prone to

胎 (tāi): fetus / litter / tire / abbr. of 輪胎｜轮胎

"Casting/investing/putting in a good baby or perhaps tire" just doesn't make any sense...

Does 好胎 here perhaps mean good karma?
投 (v); plunge into

胎 (n); pregnancy

"投胎" (plunge into pregnancy) is a compound word. It means "to reincarnate"

Olden time Chinese believed after people died and became ghosts, some of them were allowed to reincarnate and reborn into the living world 

"投个胎" break up the compound word "投胎" and treating 投 as a verb; 胎 as a noun. When 胎 is a noun, we can add classifier '个' (a)  before it to indicate indefinite 

Similar phrase: 投幾次胎 (to reincarnate a few times)

different from "投个胎" (to randomly reborn), we can have “投个好胎", which mean "to reborn into a good family"