It's much more than just the things you buy for the new baby. Having a child is a watershed moment in your life from a marketing perspective: 1. Your life is different. You used to self-herd [1] yourself to one set of brands and businesses before you had a child. That is now broken. Marketers have a small window of opportunity to get you hooked on new brands and businesses. Once they do, you'll self-herd to them for a very long time, if not for ever. 2. You are tired and you don't make the best decisions in that situation. This leaves you especially susceptible to marketing, which, because of #1, will have a long-lasting impact.

That being said, the article is mostly bullshit. My wife recently had a baby. We didn't keep it a secret in any way on Facebook, Google or by our purchase behavior. We didn't get hit by any kind of directed marketing (except for retargeting ads online) until after we had the child and the record of birth is part of the public county records.

[1] http://www.stubbornmule.net/2008/05/self-herding/

But this only applies for the first child, when the parents are the most nervous about living up to expectations. By the second or third child, they're like: "I get onesies at Goodwill and garage sales since they're cheaper and they'll only be in it for a couple of months before the knees are worn-out, or they've outgrown it." and "Pack-n-play? Those are expensive - I just take a blanket along for them to play on. Takes less room in the car and easier to clean."

I think it's clear that this is about gathering data, not selling anything.