Facebook has offered users a new way to inform their friends and relatives en masse that they're "expecting a baby."

On Monday, the Menlo Park social-networking giant added that category to the life events that people can post to their timeline, a list that already included engagements, marriages and new pets.

It's a voluntary decision, and obviously it's up to each individual or couple how they want to inform the world about a pregnancy. But expectant parents should keep one thing in mind before selecting this option: Facebook's new feature is less about informing friends and family than about informing the company's advertisers.

Facebook routinely uses the information that people share about themselves to match up ads likely to fit their interests and demographics.

"When Facebook offers the ability to announce a 'new life event' such as a pregnancy, they are building a digital dossier about you so you can be the target of focused advertising," said John Simpson, privacy project director for Consumer Watchdog, in an e-mail. "Pregnant women are a particularly lucrative target."

Desirable demographic

In fact, early notice of pending motherhood is considered the holy grail of retail advertising.

Consumers form brand loyalties and shopping habits surprisingly early in life, and are incredibly reluctant to change them. But a pregnancy is that rare life event when everything gets tossed up in the air, as new time demands, life stresses and product needs can overwhelm old habits.

"There is almost no more profitable, product-hungry, price-insensitive group in existence" than expectant parents, wrote New York Times reporter Charles Duhigg in his fascinating new book, "The Power of Habit."

Advertisers are only too happy to take advantage of the anxiety that comes with being an expectant mother. In February, research firm eMarketer published a survey helpfully pointing out to its clients that new or expecting moms will spend more than they can afford to get the best items for their kids. That's particularly true for products related to child safety, such as infant seats and baby formula.

The report's fairly offensive title: "Anxious and Overspenders: Two Words That Define New Moms."

Target-ing mothers

Duhigg's book lays out the incredible lengths that Target stores have gone to in an effort to identify and reach these consumers ahead of other retailers. Combing their vast databases, the company's statisticians identified shopping patterns that can signal not just that a woman is pregnant, but how far along she is.

They start loading up on unscented lotions at the beginning of the second trimester, vitamins before the 20-week mark, and hand sanitizers and washcloths just before the delivery date, Duhigg reported.

Target's sophisticated software spots this behavior and responds by sending out ads and coupons for diapers, cribs, maternity clothes and the like.

How accurate are the company's predictions?

Duhigg cited an anecdote where a father berated a Minnesota Target manager after his high school daughter started receiving coupons for baby clothes. A few days later, after a conversation with his daughter, he called back to apologize. Turns out Target knew the young woman was pregnant before her father did.

'You are the product'

But Target quickly discovered a big problem. Many women found it creepy and invasive to receive ads for baby goods from a retailer they had never informed of their pregnancy. Go figure.

Target addressed the issue by watering down its mailers with unrelated and untargeted items like lawnmowers, making the baby plugs a little less blatant.

Facebook seems to have found its own way around this challenge, by encouraging people to inform advertisers about their pregnancy themselves, under the guise of sharing their happy news with friends.

"Life Events are easy and expressive ways to mark significant moments in your life," Facebook's Matthew Dierker wrote in a blog post.

Of course, so are conversations with your friends and relatives.

"When you use services provided by online giants like Google and Facebook, you are not their customer," Simpson said. "You are the product, and are being sold to the highest bidder."

James Temple is a San Francisco Chronicle columnist. E-mail: [email protected] Twitter: @jtemple