Women who pass on the happy news of their pregnancy to loved ones on the site can be bombarded with promotions for baby clothes, nappies, prams, and toys as a result of the policy.
The move has now been condemned by parenting experts who warn it could make expectant mothers feel as though they are being 'cyber-stalked' by big business, the Daily Mail reports.
According to the paper, it has also fuelled concerns over Facebook's willingness to exploit the private information of its one billion users to make a profit.
The US social networking site set up a 'Life Events' section on users' pages in the summer of last year to allow them to tell friends about major news, which allows to announce details of their pregnancy, including predicted due date and - based on scans - the gender, the paper said.
However, it has been learnt that the firm has secretly changed this policy and is selling access to information posted by expectant mothers on the Life Events section to advertisers.
This is potentially lucrative for Facebook, which was floated on Wall Street last year and is looking for ways to cash in on the treasure trove of personal information it holds, the paper added.
The founder of the Netmums.com website for parents said many people would be alarmed by the development.
"While many women willingly give away personal details about their pregnancies to marketing firms, they do this knowingly and usually in return for free product samples or discount vouchers," said Siobhan Freegard.
Facebook, however, has denied selling personal information directly to advertisers.