OMG ... Rachel Zoe, who is best known for being super skinny ... and oh yeah, being a stylist to the stars, has a bun in the oven!

Zoe, who has been putting off getting pregnant with hubby of 12 years Rodger Berman because of her busy career, has been keeping the news under wraps, according to Us.

On September 9th, Rachel attended Karl Lagerfield's Chanel dinner wearing this carefully placed jacket ... hiding her little bump.

The 39-year-old, whose clients include Demi Moore and Kate Hudson, is "telling people she can't travel," adds the mag. At a recent Veuve Clicquot Polo Classic event in the Palisades, a witness saw Zoe only drinking water with lemon.

We have a feeling this bambino is going to have the best baby clothes in town!