I've heard rumors, maybe that far back, of women getting mail ads for baby clothes a couple weeks before having a positive pregnancy test.

You are misremembering, what it you're referring to is the Target pregnancy prediction score which was advertised a lot in the early 2010s as the way "big data" was going to change the world because they figured pregnant women bought similar things

https://www.forbes.com/sites/kashmirhill/2012/02/16/how-targ...